How to restore a Scubapro Mark VII Honker

[rsingler]

A Honker? You can pick one up on eBay for $40 on a good day.

Why would you even want to? Well if you love vintage equipment, this one is a classic! And if you have feet-down trim problems, then all you need is 3 ½ lb of balancing brass up on the neck of your tank to help you get horizontal! Perhaps you’re a distracted photographer, who can make use of what this regulator was designed for: a warning “honk” when you reach anything from 200-400 psi tank pressure, from the days when SPG’s were rare.

The problem is, this reg is no longer serviced by Scubapro. Many parts are no longer available. And all that’s out there on Frogkick and other schematic websites is a parts breakdown.

This thread will lead you through the Mark VII in detail, dispelling old wives’ tales, explaining how it works, along with how to service it, how to restore it, and where you can source parts that are close enough to original specification to enable creation of a demonstration regulator that is within the original manufacturer's specification. Where other ScubaBoarders chime in with valuable info, I’ll update the thread to include or correct information.

DISCLAIMER: Restoring a “no longer supported” regulator is a risky endeavor that should not be used to prepare an old specimen for your actual use in the water, much less to give to a friend. This procedure is designed for preparation of a mantle-top specimen about which you can brag to your friends,
“This antique could actually be used!”, even if you shouldn’t.
‘Nuff said.​

The Scubapro Mark VII “Honker” is a follow-on to the venerable Mark V balanced piston first stage regulator. It was introduced around 1971 and went through multiple iterations for over 25 years until at least December, 1997, when Schematic Revision "S" appeared. This regulator was essentially a Mark V coupled with a “sonic oscillator” that used an unstable brass cylinder and low tank pressure air flow to vibrate against the reg body, producing an audible “honk” that could be heard for a significant distance underwater when tank pressures dropped below 250-400 psi. Even though SPG’s were available at the time, the regulator continued to sell as a "safety feature". Indeed, a 1978 warning advised owners not to rely on the "honk" but to utilize an SPG.

The Mark V regulator design inside the Mk VII continues to be one of the best performing balanced regulators out there. Its large piston head resulted in brisk lockup, and only the invention of the blunt-nose piston for the cone seat of the Mk10 regulator (which became the Mk10+) ended its reign. With the smaller piston head of the Mk10+ however, lockup was occasionally problematic, and the Mk 5 and Mk 7 continued in regular use until large-head pistons returned with the Mk20, and then Mk25 series (still in production today).

The Mark VII body had at least three iterations, including an early move from a 2475 psi yoke with the same 3/8-24 threading in each of the yoke, LP port and HP port, to a 7/16-20 threaded yoke rated to 3000 psi (???), and finally a heavy yoke with a 9/16” thread wing nut eventually succeeded by the familiar knob style used in other Scubapro regulators.
The first and third iterations are pictured below:

Interestingly, the Revision Q schematic from September, 1989 shows all four yoke knobs together with a heavy yoke body and 3000 psi specification. No warning is given regarding earlier models with 2475 psi yokes. The 1994 schematic includes a note specifying that only the 9/16” knob and yoke is rated to 3000 psi, while 3/8” and 7/16” are limited to 2475. In contrast, a schematic from 1976 (Revision C) erroneously shows a thin legged yoke with 3000 psi stamped on the body, with a 7/16” winged yoke screw. The 1994 schematic superceded this apparent error.

Ambient pressure ports into the regulator began with three holes, although as early as 1984 they were replaced by two SPEC-style dimples with five tiny perforations in each dimple, to better retain silicone grease. This persisted to 1997 in Revision S. See second picture above.

The Honker never had more than two LP ports and one HP port, but eBay Honkers typically come with a variety of port multipliers or tees.

From the outside, here is the component breakdown:

On the end with the single 3/8” or 7/16” HP port in the center, we see the high pressure compartment of the Mk5 regulator on the left. The high pressure port was changed from 3/8” to 7/16” according to a 1988 schematic, although it probably occurred much earlier. On the right, a large unvented cap with an o-ring seals a low pressure compartment that contains the oscillator assembly.

The HP seat carrier originally held a flat seat, though it will also accommodate any of the Mk10 cone seats.

On the other end, we see two LP ports, with a flat head screw fitting in between.

The left hand port is machined directly into the body, and you should see “AUDIO” stamped alongside. This port is the outflow of the oscillator chamber, and should always be where the primary second stage is plugged in. The LP port on the right is in the center of a large pin spanner cap, which covers the Mk5 piston head. This is a “high flow” port, coming straight off the base of the piston. However, using it will bypass the oscillator safety feature. It can be considered for either an octo, a BCD inflator hose, or both as a tee was commonly mounted here.

Between the two ports is a small plug with an adjustment slot for a flat-bladed screwdriver. This plug is sealed by an o-ring, and although the o-ring specified appears to be a duro 70 seal, it should be noted that this plug seals a HP mechanism used to fix the final position of the oscillator. This will be discussed further below. The safe range of adjustment is probably less than one full turn of the plug. For reasons discussed below, it is probably safest to leave the plug fully screwed in, if you can get a honk with that setting. Additional recommended modifications will also be covered later.

The tank mount is a standard yoke fitting, of either 2475 psi or 3000 psi rating. The rating is quickly distinguished by the thickness of the yoke. The tank screw went through three iterations, from a 3/8-24 to a 7/16-20 to the current 9/16-18 thread size. The inlet is protected by a sintered metal filter below a star retainer, and the flat filter used earlier rests on an o-ring, while later versions were bored to accept the more common conical filter currently used in the Mk25, without an oring spacer.

The sheer size of the reg body (it weighs 3lb 7oz!) also caused a change in the handle of the then-popular J-valve, which was too long to slide past the regulator when a diver attempted to pull it into the “reserve” position.

That problem prompted the required addition of one of two warning decals.
This one read:

WHEN USING THE MARK VII REGULATOR ON A RESERVE VALVE,
BE SURE THE LEVER ARM WILL CLEAR THE REGULATOR WHEN PULLED.
IF NOT, THE LEVER ARM MUST BE "DOWN" WHEN ATTACHING A REGULATOR.​

Finding a Honker with intact decals is a rare find indeed!

Two to three ambient pressure holes in the body are found on the regulator side (the left side when viewed from the HP port end; or the right side when viewed from the LP outlets). See the second photo above.
Let’s now look at the internals in more detail.

(continued in next post)

 

[rsingler]

As noted above, the basic regulator is a Mark V. The other half of the huge brass body is occupied by the “oscillator” mechanism, or “honker”. Here is a basic schematic. I must give full credit to former ScubaBoard member awap, the original artist of this graphic, which I have adapted using Microsoft Publisher to demonstrate the function of the oscillator.

There are myriad discussions of the Mk5 in other threads, and we will not spend much time on the regulator here. It should be noted that the Mk5 (and presumably the Mk7) piston was replaced with a piston with a large shoulder at the base of the piston shaft. I do not have a Service Bulletin that specifies this for the either the Mk5 or Mk7. However, sometime between Mk7 Rev B in 1974 and Rev H in 1984, the piston 101-4 was replaced with the 10.101.003. However, from Mk5 schematics as late as 1981, piston 10.101.003 was not yet a shouldered piston.

Sometime between 1981 and 1984, the piston was modified to add the shoulder. However, the piston part number did not change as late as the Mk7 Revision S in 1994, or the Mk5 Revision V in 1997. EDIT: The reason for the piston replacement was the small risk of the knife edge becoming embedded in the (then) flat seat. The conical seats had not been introduced yet, and the shoulder provided a stop beyond which the piston could not travel, preventing it from burying itself in the seat. There was never a piston recall, despite rumors to the contrary.

Looking at the oscillator side in more detail, we see that the oscillator is a two part brass cylinder with a large o-ring near one end, and two small air passages.

It is NOT designed for disassembly, and the two parts are threaded together and then locked with small deformations in the threads between the two pieces.

At least one older thread on Scubaboard notes that the oscillator should not be submerged, “to avoid damaging an internal mechanism.” As can be seen from the pictures below of a disassembled oscillator, there is in fact no “mechanism” inside.

Whether the two halves were designed for ease of machining, or for a subtle purpose related to oscillation which I have not been able to determine, there will be no harm from removing corrosion by submergence in an ultrasonic cleaner, as long as the oscillator is dried internally by prolonged flushing with dry air through the side pinhole, which will then exit via the oring land on the flat end. Because of the size of the hole, submergence is not recommended unless extensive cleaning of brass corrosion is required. Even, then, it may be impossible to remove corrosion without disassembly.

The following diagrams show the operation of the oscillator and its supporting tuning components.
Below we see the oscillator assembly schematic. Note specifically the High Pressure input, to a small piston called the HP Stem, which protrudes into the oscillator chamber when sufficient pressure exists. This holds the oscillator away from the Intermediate Pressure outflow passage, allowing low pressure air to flow to the second stage.

It would appear that the oscillator is designed to be unstable. The outside thick o-ring is found near the outflow end of the chamber, while the tensioning spring is at the opposite end. Being pressed toward the LP outflow by spring force, the pivot point that holds the oscillator away from the LP outlet is not centered, but deliberately asymmetric with regard to the center of gravity of the oscillator. When High Pressure (tank pressure) is insufficient to force the small piston downward, it retracts partially, and the asymmetric loading of the oscillator, together with airflow to the second stage, causes it to vibrate against the body of the first stage.

Note however, that if the HP piston (controlled by the slotted screw on the hose side of the regulator) is retracted too far, with low tank pressure the oscillator can collapse competely against the outflow tract for the second stage, and airflow will be almost completely blocked. This is why I do not recommend using the slotted screw cap as an adjustment mechanism, but rather leaving it fully screwed in.

Note that a tiny amount of flow can pass by the large (loose) oring around the oscillator, and down the central bore of the oscillator, but this will be insufficient to support breathing. Therefore, your octo must go on the other LP port (the port in the center of the large pin cap).

From the manual:
NOTE: It is recommended that when installing an octopus regulator or utility adapter on the Mark VII (Cat. No. 10-107-000) 1st stage, the non-audio low pressure port be used. This is for added safety in the very rare event that the audio portion of the Mark VII 1st stage fails. It is also recommended that additional low pressure hoses or accessories such as; utility adapters, air moisturizers or air heaters not be attached to the audio port, since these items may cause the audio alarm to not function properly.

Read more at: http://vintagescuba.proboards.com/thread/2604#ixzz5wiXKRvZS

(continued on next post)

 

[rsingler]

Attached to this post are ten useful documents regarding the Mk VII.

The _Technical Manual is the original owners manual for the Mk V and VII and 108/109, among other regulators.

Four schematics are attached from Revision B in 1974 to Revision S in 1997.
If anyone has the original Revision A, it would be great to see!

Two recall documents regarding the oscillator o-ring are attached. This issue will be discussed in detail in the tuning section to follow. Unfortunately, the original specifications for the o-ring are not known to me, but the consequences of minor size and durometer differences are discussed.

The Warnings and warning decals required shortly after introduction of the regulator are attached.

Perhaps best of all is an NEDU document from 1971 outlining the performance of the Mk VII under severe conditions. It's a great read.

 

[rsingler]

To supplement the servicing posts below, I have attached an "O-ring Crosswalk" document that may be of use.
The original 1971 o-ring specifications attached to the XXX-XX numbering system are not known to me. However, by the 1980's Scubapro had gone to its now standard 01-050-xxx numbering system, and specifications for many of those o-rings are known.

As the second page of the attached document will show, the Mark 7 underwent serial substitution of o-rings over a period of about 15 years. The specifications for many, but not all, of these orings are known. That said, at the last Scubapro Service Technician's seminar, it was emphasized that the Mark 7 is no longer serviceable, and that its o-rings are are special Parker items no longer available. Thus, the warning/disclaimer in Post #1 remains important to keep in mind. Though the o-ring "equivalents" listed have enabled a regulator restoration that in many cases appears to meet manufacturer specifications for Intermediate Pressure and sound-making, I would reiterate that the regulator should not be used in actual diving. Remember also that the unreinforced piston was introduced more than 30 years ago, and replacements are no longer available.

However, in preparation for restoration of your mantelpiece show regulator, the substitutions listed, and their McMaster-Carr part numbers will enable you to work on your Mark VII without a "service kit". Three parts are conjectural: the oscillator o-ring specification is unknown. The previous recall, coupled with my personal experience, suggests that some 2-117 o-rings commonly seen on "annotated" schematics will in some cases NOT allow oscillation. Note especially that the brown Viton 2-117 from McMaster Carr will not oscillate. A workaround that delivers more consistent results is listed on that page. After considerable experimentation with multiple Viton, Nitrile and EPDM o-rings, I believe that the EPDM 2-117 delivers the most consistent oscillator resonance.

The second part not found on the original schematic is an extra "shim" for the HP stem. The spring force on this stem determines the maximum "honking pressure" for the oscillator. Spring fatigue has been found to produce sound onset at a pressure below the specified 250 psi. Shimming this spring with a PTFE shim almost identical in size to the existing spring pad will raise the tension on this HP stem much like a shim on a main piston spring. This part addition is NOT authorized by Scubapro, but may raise the "honking pressure" to within specification.

Finally, the balance of the oscillator assembly itself is critical to sound-making. As above, it is possible to shim the oscillator spring to produce sound when all other repairs have been made. A shim suggestion is made on the Crosswalk, if needed. Once again, this part addition is NOT authorized by Scubapro.

Steps for disassembly, cleaning, reassembly and tuning will follow in the next posts.

NOTE: the crosswalk document was edited and replaced with Version 3 on 12/4/19.

 

[rsingler]

Disassembly and cleaning of the Mark VII:

Disassembly of the Mk VII is straightforward. Part numbers referred to below match the numbers in the Schematic_198807_RevP11.pdf file in post #3 above.
Remove the star retainer with a blunt hooked pick and remove the flat or cone filter. If present (flat filter model only) remove the 01.050.144 oring [40] from the base of the tank inlet in the body.

Starting from the HP side (the end with the center HP port), remove the HP port plug (if any) or the 7/16" adapter if the HP port is the old style 3/8" port. Next, remove the seat retainer [17] on the left with a small pin face spanner. Use of a pin spanner with finger pressure on the tool will reduce the chance of spalling the pin holes and scratching your chrome.

Next, (protecting yourself from a flying part) express the seat with an air gun with rubber tip, applied to the seat retainer hole. Alternatively, if the seat is stuck fast, screw a wood screw into the seat and pull it out. Remove the o-ring 01.050.157 [18] from the seat retainer. Carefully reach inside the reg body with a plastic pick and remove the oring [20] which may be stuck deep in the high pressure compartment. Do not use a metal instrument, and be careful to avoid touching the knife edge of the piston.

Turning to the opposite end, remove the right-hand large cover (with a center LP port) [34] with a pin spanner. Due to the depth of this cover, the piston head will be buried deep inside the regulator.

It is generally too deep to reach without scoring the piston land on the reg body. In order to remove the piston, piston spring and shims, return to the HP seat side, and carefully place a piston bullet inside the shaft of the piston. Using the bullet to press on the piston, and making sure to keep it aligned with the bore in the reg body, carefully push the piston out. Using this technique will avoid the tendency to “rock” the shaft of the piston just before it slides out of the body, which will prevent scoring of the shaft or knife edge.

Leaving the bullet in place to protect the knife edge, remove the spring and shims, and count the number of shims used.

It is typical to find one at each end of the main piston spring. If you do not see two, inspect the bore of the reg body, as one shim may be stuck deep in the recess of the reg body. It can often be removed with a blast of air (at which point it will promptly disappear in your shop). Alternatively, picking it from the body should be done gingerly with a straight brass or plastic pick. If you do not plan to clean your reg body, it is not necessary to remove this shim.

Leaving the piston bullet in the piston, wrap it carefully with a cloth and set it aside. Protecting the piston shaft and knife edge from nicks and scratches is critical. Consider inspecting the knife edge with loupes or a $20 USB microscope camera attached to your PC or phone. If scratches or nicks are found, consider reviewing How to Restore a Knife Edge Piston

Now it is necessary to remove the o-ring 01.050.136 or .138 [29] from the depth of the high pressure portion of the reg body.

Consider consulting this thread: How to use a Double Hook Pick. Use of a straight brass pick to spear and extract the o-ring is an easy way to scratch this high pressure land. A scratch may result in slow bubbling of high pressure tank air past the piston shaft o-ring and out from the ambient pressure holes of the regulator. The scratch may cause early failure of this key high pressure o-ring. That said, use of a double hook pick is also fraught with danger to your equipment, as its sharp steel point will scratch both brass and chrome.
Burying the round back of the double hook in the land, in front of the oring, shift the handle of the pick slightly and rotate the hook until it appears on the other side (or in the middle of) the o-ring. Carefully pull the o-ring out, not allowing the sharp pick to score the side of the reg body.

Now turning to the opposite side, remove the large oscillator “retainer” cap [2] with a small pin face spanner. Remove the large o-ring 01.050.184 from the cap, and set the parts aside. Under the cap is a spring [4] for the oscillator. Remove this and set it aside. Grasping the oscillator end with a pair of pliers,

pull it straight out of the reg body. It should hold a large thick o-ring on the side of the deep end, and an oring buried in the flat side of the cylinder on the deep end. Removing this oring is most easily done by using your air gun with a rubber tip, placing it against the tiny hole in the side of the brass oscillator cylinder. This will pop the edge up from the land,

where it can be caught without damaging the land. If you choose to dig the o-ring out, keep in mind that this one is typically dried and flattened, and it is easy to score the land with a metal pick.
The o-ring on the side of the oscillator barrel can easily be removed with a straight pick. How dry and cracked it is may give you an idea how long it’s been since your Mk 7 was serviced. If the o-ring is marked with a gray stripe instead of a white one, it’s been since 1988!

The final bit of disassembly is to remove the stem retainer cap [28+29] with a flat bladed screwdriver. After removing the plug and its o-ring, you'll notice the head of the HP stem deep inside. Resist the temptation to try to hook it, as there is a critical smooth o-ring land in the reg body level with the top of the HP stem.

Instead, push the stem out of the body from the opposite side, inside the oscillator bore, using a thin wooden dowel. Along with the stem and spring, you should find a brass spring pad above an oring (which may be buried deep in the hole).

This completes disassembly.

If you don’t own an ultrasonic cleaner, removing verdigris corrosion from the parts is a straightforward matter of elbow grease with Dawn “Free & Gentle” type detergent (hypoallergenic with minimal scent and no color) and a toothbrush. Follow this with a soak in 50% household vinegar and warm water, and then one more scrub with Dawn if necessary. Thorough rinsing with clean water should follow, allowing your parts to air dry. If you have high-mineral-content water, the final rinse might be best in distilled water before drying, to reduce mineral deposits inside the reg.

 

[rsingler]

Proper performance of the Mk VII is a function of just three things:
1) Condition of sealing surfaces (o-ring lands, piston shaft, knife edge)
2) Condition of removable parts (piston integrity, HP seat and o-rings)
3) Balance of operating components:
a) balance of piston spring pressure vs HP seat (Intermediate Pressure)
b) balance of HP stem spring compression vs tank pressure (honk onset pressure)
c) balance of oscillator components for resonance (spring compression, oscillator o-ring friction, oscillator compartment wall condition)

The next two posts will deal with 1) inspecting and restoring the sealing surfaces, and 2) replacing parts with partial regulator reassembly.
Items 3a-3c will be covered in the Final Assembly and Tuning post to follow.

As discussed ad nauseum in the double-hook pick thread and elsewhere, a ham-fisted scratch of a sealing surface as a result of misusing a sharp steel instrument may turn your antique regulator into a doorstop. At the last DEMA convention where I attended regulator technician recertification classes for five different brands, between 10% and 33% of the “technician” audience in each class admitted to never having opened a regulator before the class, in response to the same question from each trainer! And you wonder why your reg set doesn’t perform properly after just picking it up at your LDS…

All sealing surfaces of the regulator should be inspected and possibly restored before reassembly.

To do this properly, your equipment should include either a binocular microscope

$80-$200 on eBay,

or a USB microscope

$16-$400 on eBay,

and Micromesh in grits from 2,400 to 10,000.

$16/pack on eBay

For the piston knife edge and shaft, as noted above consult How to Restore a Knife Edge Piston
You are aiming for a sharp piston knife edge and a scratch-free shaft.

It’s probably not possible to inspect the depth of the land where the piston shaft HP oring rides

You just have to hope that a prior technician didn’t scratch it deeply, while digging out a dried-up hard 90-durometer o-ring. Diagnosis of this problem, when it is subtle, can be done in a bathtub with a small pony cylinder:
After pressurizing your restored regulator, you’ll measure IP and listen/look for air leak.
An IP creep indicates a problem at the knife edge/HP seat interface.
Bubbling from the ambient holes indicates a leak elsewhere:
a) High pressure air can leak past the shaft o-ring on either the shaft side or the land side without causing IP creep.
b) Intermediate pressure air can leak past the piston head o-ring on either the piston head side or the piston land side.

If the leak rate is slow enough, you can submerge your reg with an ambient hole uppermost in the tub. Crack the tank valve with the reg tilted so the piston head (the large retainer cap with the LP hose coming from it) is higher than the HP seat side, at perhaps a 30-degree angle. Position the reg in the tub so one of the ambient holes is uppermost (usually with the tank lying on its side in the tub. Carefully watch the ambient hole. If the leak is from the HP side at the piston shaft, you should see bubbles appear quickly, as they leak past the piston shaft and immediately encounter an ambient hole. If on the other hand, the leak is from the piston head side, it will take awhile for enough air to accumulate in the slightly higher piston head compartment, and when bubbles finally appear, they will appear on the piston head side of the hole. Of course, if the leak is large enough that air is pouring out, this subtle test won’t work, but then your reg should have had a visible cause for the problem in the beginning.

Hopefully, there will be no leak, because you’ve restored all the accessible lands, and there is no scratch in the bottom of an inaccessible o-ring land. But if there is, start with replacing the appropriate oring. At the time of replacement, inspect the affected area for imperfections that might be polished out.

Let’s look at each of the areas you’ll need to address:

The piston land starts at the base of the threads in the piston compartment and extends to the level of the shim ledge deep in the reg body. Inspect this area for loss of chrome (likely at this age), and linear corrosion where the ambient side of the piston head was exposed to seawater and floating sand. See Piston or diaphragm for more information on this topic.

In this picture,

we see a reasonably well-preserved piston land in a late model Honker, before detergent washing and ultrasonic cleaning. It’s likely that this reg was slightly flooded via the LP hoses. You can see some rust on the “bottom side” of the piston land. Since the Mk VII is commonly stored with its broad base down and the yoke up, any moisture will then collect on the “base side” of any chamber. There is also a thin blue-green line of verdigris where the piston o-ring sat. While most of the verdigris was removed in the ultrasonic, there was residual land roughness to touch.

With your finger on top of a small rectangle of Micromesh, you can polish away this roughness and any residual corrosion, and then inspect for any pitting beneath.

As you can see, most of the rust is gone, and with your finger, you can feel that the land for the piston o-ring is now smooth. Note that the blue-green line now appears brown, as the chrome under the corrosion was eaten away, and you have oxidized brass now showing. Additional polishing will turn this area bright brass color, but taking off that much metal is probably unnecessary. For this reg, brief passes with 6,000 grit and then 8,000 grit were all that was necessary.

(continued in next post)

 

[rsingler]

The other compartment of interest is the oscillator compartment on the other side. It is only accessible from one end, of course. Again, on the "bottom side" of the compartment, verdigris is noted. This corrosion will in many cases prevent resonance of the oscillator.

Using the same technique of finger pressure on a small square of Micromesh wiped around the interior, the light dusting of verdigris was removed.

You should work to have a completely smooth oscillator chamber, as any irregularities will decrease the chance that your oscillator will resonate freely at the proper tank pressure.
Inspect the oscillator cylinder for smoothness. It is not necessary to Micromesh it unless corrosion has made the outer surface uneven.

Finally, inspect the land just under the cap of the HP Stem (the cap with the flat head screw slot). It should be smooth, but is not really accessible for polishing.

Note the short vertical scoring on the far side of this land in the photo. The land was probably scratched by the thin tip of the HP Stem, because the stem was not pulled out or inserted carefully.
If scratched, using a 70 duro o-ring instead of a 90 duro may aid in sealing, if there is any leak from that fitting. It should be noted, however, that it is a high pressure cap. The specified original Scubapro o-ring appears to have been a 70 duro, which is surprising for a high pressure seal. The attached o-ring crosswalk in post #3 shows my 90 duro recommendation for this cap, although the seal deep in the recess for the HP Stem has been left at 70 duro.

The regulator is now ready for initial assembly. If you are not familiar with a particular regulator, I find it helpful to lay all the parts out next to their o-rings, and place all the new replacement items next to them.

Once the match between the new and old parts has been made, removing all the old parts will help ensure that nothing is forgotten on reassembly. Nothing should remain on the mat at the end.

Part numbers referred to below match the numbers in the Schematic_198807_RevP11.pdf file in post #3 above.
I begin by starting with the Mark V portion. The obvious first step is to install the HP oring. There are dozens of ScubaBoard threads on the construction of homemade dowel jigs for insertion of this oring in the Mk V, Mk VII and Mk10, but frankly, nothing works as well as one of the custom designed Mark V o-ring insertion tools.

With the tool in place, the small black domed rod sits just at the level of the oring land. This dome ensures that the o-ring spreads into the land when pushed down into the bore by the other half of the tool.

The second half of the tool is just a passageway for a well-lubricated o-ring to be pushed into position, and a brass rod to squeeze it into place.

Once the o-ring is in place, passing a lubricated piston bullet through the bore ensures that the new o-ring is well seated in the land. If this step is skipped, you risk shaving the edge of the o-ring with the piston knife edge when you first install it.

Lightly lube the piston head o-ring 01.050.177 [3], and slide it into place on the piston head [32]. Assemble the piston complex, consisting of piston with o-ring and bullet, first shim and spring.

Add the second shim that will contact the other end of the spring by placing it into the regulator body instead of balanced on the top of the spring. It is convenient to use a dot of lube that temporarily sticks it deep in the reg body.

Lightly lube the bullet and piston shaft, and holding the reg body above it, pass the piston complex upward into the regulator body, holding it vertically so the spring stays centered. As you feel the bullet pass through the bore, put pressure with your fingers on each end of the piston complex (both the piston head and bullet tip) to ensure that the joint between the two parts stays tight. This will reduce any tendency for the piston knife edge to snag the HP o-ring as it slides past.

Once the piston is inserted into the body, lube and attach the o-ring 01.050.184 [33] to the piston head retainer cap [34] and screw it into place, tightening it with a pin spanner.

Insert the new seat into the HP seat retainer, and add o-ring 01.050.157 [18] to the retainer. Insert o-ring 01.050.147 [20] into the HP end of the reg body, and use a 1/8" dowel to tamp it symmetrically into position. Alternatively, as recommended by Scubapro, you can use a 3/8" socket as a tool to press this oring flat onto its land. Take care to avoid contacting the knife edge with any tool.

Screw the HP seat retainer into the body, tightening it with a pin spanner. The reassembly of the regulator portion of the Mark VII is now complete.

 

[rsingler]

So now it’s time to cut to the chase: how do I make my Honker “honk”? As described briefly in post #2 above, the oscillator is a metal cylinder which is centrally spring-loaded on one end, and held away from the LP outflow tract by an opposing pressure loaded pin (HP Stem) which is off center. Therefore, at pressures above 400 psi, say, the oscillator is pinned in a slightly canted position against the wall of the regulator. In fact, the earliest models of the oscillator were slightly bell shaped, probably to allow that off-center position to nestle at any likely angle against the reg body.

This is sheer conjecture, but I’d bet it was only manufacturing cost that dictated the eventual shift to a straight sided cylinder from a bell. That then means only that the shallow rim of the oscillator, rather than its side, makes contact with the reg body, which is probably insignificant in practice.

The LP air inflow point is also off center, and adjacent to the HP pin.

Therefore, it generates its force to one side of the pivot point formed by the HP stem, possibly resulting in a swirling motion of LP air at the end of the oscillator. This airflow begins the vibrating process.

In any case, in order to tune your Mark VII, you need to balance four factors:
I) intermediate pressure coming from the “Mark V” side of the first stage, which affects the velocity and mass of air jetting into the oscillator chamber;
II) friction of the oscillator against the side of the chamber;
III) spring pressure behind the oscillator; and
IV) projection of the HP Stem into the oscillator chamber.

Fortunately, one factor is effectively irrelevant for tuning, and the other three can be adjusted without disassembling the “Mark V” side. I wish I could tell you, “Here’s the formula; here’s how you do it.” But unfortunately, each factor above influences the others, so changing one element may require multiple tests with another element. So there’s a bit of trial and error to it, and without a gas bench providing easily adjustable “tank pressure” between 200 and 500 psi, it’s workload intensive.

Factor I: Intermediate Pressure
This is the easy one. Pick what you want and tune to that IP. Manufacturer specification is for IP to be between 125 and 145 psi at both 500 and 3000 psi. In my experience, the IP change with tank pressure is less than 5 psi.
Intermediate pressure can be adjusted after service either by changing the number of piston spring shims or by changing the seat. The service instructions have suggested that up to three shims are allowable. Given that the Mk 5 service manual in later iterations allowed up to five shims, I think there’s a bit of latitude if IP is low. I’d have no hesitation adding up to three more shims. I’m assuming that we are talking about 0.010” thick shims, which were typical for this regulator. You can cut them yourself with large hole punches out of 0.010” polyethylene sheet. McMaster Carr sells a 20mm punch for the OD (PN 3418A2) ($47.27), and a 0.50” punch for the ID (PN 3427A15) ($19.65). There is no latitude here if IP is high, however. Removing shims means that there will be metal to metal contact between the reg body and spring, or spring and piston. This accelerates corrosion, and makes spring action slightly less consistent, as the spring base may bind against the metal of the piston head or reg body.

The easier approach is to just use Mk 10/10+ cone seats. Courtesy of @couv , here are the three seats supplied in the Mk10 service kit (still available).

Without disassembling the reg, you can adjust up to 10 psi simply by unscrewing the HP seat retainer and substituting the seat. In order, the lowest IP is obtained with the “-II” seat, mid is the “-I” seat, and max is the “+” seat. This is obvious upon looking at the seats, since the protruding cone of the “-II” seat means that the piston will have to rise very little (compressing the main spring very little) before sealing against the cone. Conversely, the flattest “+” seat requires that the piston travel further, compressing the main spring more, yielding a higher IP.

Once you have settled on a given IP, and confirmed that there is no IP creep, you can return to tuning the oscillator.

(continued in next post)

 

[rsingler]

Factor II: Oscillator cylinder friction
Assuming that you have polished the wall of the oscillator chamber and confirmed that the cylinder itself is smooth, you only need to apply a thin wipe of lube to the inner wall of the chamber. Do not lubricate excessively. A tiny amount of friction is necessary to start resonant vibration. The thick o-ring at the deep end of the oscillator determines how tightly the cylinder fits. Basically, after removing the retainer cap and spring, you should be able to grab the end of the cylinder with your fingers, and with a little effort, pull it straight out without a tool. This is NOT a tight fit like a typical o-ring air seal. It is important that the cylinder is able to “wiggle” inside the chamber. Similarly, pushing the oscillator back into the chamber should generate almost no resistance to one finger. The oscillator should not fall out when the chamber is inverted, however.

The short answer to making this work is “a 2-117 EPDM 70 durometer o-ring.” This has been the most successful of seven o-rings I have experimented with. But it may not work consistently in every reg.

Other possibilities include:
Nitrile (Buna-N) 2-117 durometer 70
Chemical Resistant Black Viton Fluoroelastomer 2-117 durometer 75
EPDM metric 2.4 x 20.8mm durometer 70

O-rings that frequently did not work included:
Brown Viton 2-117 durometer 75
EPDM metric 2.4 x 20.3 mm durometer 70

Factor III: Oscillator Spring Pressure
The Mark VII was designed with precise pressure on the oscillator in mind. Once the HP stem withdrew due to lowered tank pressure, the oscillator would “wobble” with the HP stem end acting as a pivot, and light spring force on the oscillator allowing it to vibrate. Too light a force, and the vibration is “warbling” and intermittent. Too heavy a force and the oscillator is pinned in place and does not vibrate at all. Only at that magic medium pressure does one hear the irritating honk that tells you that you are very low on air.

With the old springs inside these regs, many are not generating the force they originally did upon compression. Therefore it is most common to get a bit of a honk, but not one that is consistent. To increase the spring force, it is simple to add a shim between the spring and the oscillator. The amount of shimming is typically between 0.01” and 0.03”.

The O-ring Crosswalk in Post #3 above lists 316SS shims from .010” to .09” in thickness along with their McMaster Carr part numbers. You can also hand cut 0.010” HDPE shims with 5/8” and 3/8” hole punches.

When a trial gives only a half-hearted “toot” instead of a honk, just unscrew the oscillator retaining cap [2], remove the spring and drop in a shim. Replace the spring and cap and try again.
Note that this is NOT an authorized Scubapro service procedure. On the other hand, there is only one new old stock 01.020.106 oscillator spring left in my shop.

Factor IV: HP Stem Projection
The HP Stem is contained in a tiny high pressure compartment supplied directly with tank pressure. When this compartment is pressurized, the HP stem naturally wants to squirt out the hole in the bottom and protrude against the oscillator. Without a spring, it would hold the oscillator well away from the LP air outflow tract and there would be no obstruction to breathing. At the other extreme, if it were to stay fully retracted, the oscillator cap spring would push the oscillator against the outflow and air would be almost completely shut off, except for the central pinhole and whatever leaked around the thick (non-sealing) oscillator o-ring.

But the HP Stem is spring loaded, so it takes a measurable amount of pressure to push the stem pin into the oscillator chamber and hold the oscillator away from the outflow. That spring force was calibrated by Scubapro to yield an open passageway above 400 psi, with sound generated by oscillator vibration at a maximum of 250-400 psi. However, these 30-year-old springs have in many cases become weaker, and honking may not become audible until tank pressure has dropped below 250 psi. In this case, the spring needs to be augmented, and just as with IP adjustment, the easiest thing to do is shim the spring.

This picture shows the HP Stem assembly with a single PTFE shim mounted above the spring. For size comparison a shim is shown below the stem along with the original brass springpad, and below it the McMaster Carr substitute. The duro 90 O-ring is mounted on the slotted cap and the Duro 70 stem O-ring that goes deep in the compartment is shown to the right.

As seen above, McMaster Carr sells PTFE shims of almost the exact dimensions of the brass spring pad that lies over the deep oring seal in the HP Stem compartment. Adding one shim to the top end of the spring is all that may be necessary to raise the onset of honking from a tank pressure of 200 psi, to one of 300 psi or more. If you cannot obtain a honk above 200 psi with a shim at the top end of the HP Stem spring, I would consider adding a second shim below the spring, just above the spring pad.

After removing the Stem Retainer [28], the oscillator retainer cap [2], spring and oscillator, you can easily push out the HP stem with its spring and brass spring pad. Add the PTFE shim and reinsert. It’s as easy as that…except that you may now have moved the oscillator back slightly with the HP Stem at the pressure you want, which compresses the oscillator spring more. You may now have to remove or thin your oscillator spring shim.

Conversely, if you decided to try addressing Factor IV before Factor III, putting in a HP Stem shim empirically before obtaining a honk, you may end up with too high an honk onset pressure. Once you shim the oscillator to generate that honk, and find you have to remove the HP stem shim because the onset pressure is now 500 psi, your honk may disappear because you have allowed the pivot point to retract, placing less load on the oscillator spring, until you add more shim thickness to the oscillator instead.

As you can see, it’s a balance of factors that often has you reassembling the oscillator compartment four times, until you find the sweet spot. Most frustrating is when you obtain a honk between 250 and 400 psi, and it then completely disappears while you are playing with your new toy! What has likely happened is that the o-ring around the oscillator has settled in, and friction has dropped. All that is usually needed is just a little more force behind the oscillator spring, in the form of an additional 0.010” shim. Note also that for the same reason, excessive lube in the oscillator chamber will not help things.

As a final note, your reg may be missing its brass spring pad for the HP Stem spring. If that is the case, an acceptable alternative brass shim from McMaster is listed on the O-ring Crosswalk.

You're done!
IP is set and does not creep.

Oscillator components are cleaned and given new o-rings, +/- an oscillator spring shim.
Your honk begins at no lower than 250 psi, and is resonant and firm. It disappears above 400 psi.
Life is good.

 

[Scubagaskets]

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