Reassembling my SP MK2

[lhpdiver]

While I wait for my service kits to arrive from Singapore... I have disassembled and cleaned my MK2. It was a bear getting the yoke nut loose. I thought I'd share some observations and ask a few questions at this point.

- The thing that bothers me the most is the condition of the piston. On the outer edge along the upper side of the plunger (?) the chrome is worn away and it is a little rough to the touch. Interestingly the stem (?) of the piston is a little worn at the same relative position as well. Should I take some fine sand paper and sand it down ?

- I've picked up some cristo-lube. In addition to the o-rings which I'll give a light coating to, should I hit anything else ?

- Is it critical that I use a torque wrench on the yoke nut ?

- Why is it that the cap does not need to be torqued ?

- I need a part. From the schematic it's a "spacer" part number 10044021. There wasn't one there when I got the reg. Do I need to walk into a ScubaPro dealer (with schematic) and say "I need one of these" ?

Thanks

 

[redacted]

I wouldn't touch anything with sand paper or any other abrasive at this point. As long as the roughness on the piston is not in the o-ring grooves, it should not be a problem. If it is in the grooves, you'll just have to wait and see if it causes leakage into the ambient chamber or accellerated o-ring wear. I always use a torque wrench and torque to 22 ft-lb but I don't think it is critical. Too much torque will destroy stuff like with the Mk20 problem. Depending on the spec you find, I think too much is over 27 ft-lb. Way too little will let it leak but that is probably something less than 10 ft-lb. Whill too little torque may not leak, it can allow you to fasten the 1st to a tank valve and then turn the body loosening the yoke nut. Be careful not to do that and you can probably get away with a guesstimate. The cap is just not sensetive to these problems. Snug it up and you should be fine. That spacer is probably an IP adjustment washer (my old schemats doesn't show that part number). I'd assemble without it and see what the IP is. If it is too low you will need to find some washers. I've made them out of thin nylon sheets but would not recommend it.

I usually also hit threaded surfaces with a little lube.

 

[DA Aquamaster]

I'll second Awaps's comment not to touch the piston with any abrasive. On the Mk 2, the piston stem o-ring is mounted on the piston and the mating surface is in the reg body. It's that surface that is important from a seal standpoint and if it is pitted, you need to replace the reg body.

Similarly on the piston head, what is important is the condition of the groove. The piston head o-ring then seals on the wall of the cap and that is again an important surface. If that surface is pitted, you need to replace the cap.

Ideally you would replace the piston and if it were a customer reg, I'd do that, in large part because any roughness can create a place for ice formation to occur in very cold water (probably not a big concern in Singapore.) But on the other hand, if it is just missing a little chrome in a non critical area and it will be used in warm water, it might work just fine for you. (With the big disclaimer here, that it's a guestimate as I have not seen the part and that the ultimate responsibility for deciding to use it or not is yours.)

Coating the piston stem, the exposed portion of the piston head and the mainspring with crystolube will go a long way toward preventing corrosion or salt deposition on these parts and will also help greatly in preventing freeze ups in cold water.

You will want to lightly coat the smooth surface inside the cap to provide some corrosion protection as well as some initial lubrication when installing the cap over the piston head. You will also want to lubricate the piston stem o-ring and groove quite well as inadequate lubrication can cause vibration and resonance.

Lightly coating the threads on the cap and yoke retainer will also help prevent any corrosion from locking these parts together.

Over torquing the yoke nut does not have the same consequences on the Mk 2 as it would on a Mk 20 as the o-ring seals on the side of the retainer, not the bottom and the retainer does not bottom out in the body like it would on a Mk 10, 15, 20 or 25. But with that said, over torquing can combine with a little corrosion or salt deposition to make it extremely hard to get the retainer undone at the next annual service. I also agre with Awap that 22 ft pounds is ideal - enough to keep it from loosening if mishandled while attached to a tank, yet not so much as to make disassembly difficult a year and a hundred dives or so later.

In any case, corrosion inside the ambient chamber on the piston and cap is normally a sign of inadequate soaking/rinsing of these parts after diving. The large holes in the cap are to promote water and heat exchange but also to make it easier to thoroughly rinse this area. If the reg is properly rinsed, corrosion and salt deposition won't occur.

Newer Mk 2 pistons have a plastic guide that fits over the piston stem and inside the spring to keep the piston centered. I am not exactly sure what you are describing but it is perhaps possible that the piston is running a bit off square relative to the body and also that the top of the piston head may be striking the top of the compression chamber/cap. A tech would have to look at it to tell for sure.

A Mk 2 should be adjusted using up to 3 shims to achieve an IP of 145 psi at 3000 psi. This ensures the IP will remain above 120 psi at 300 to 500 psi. If the IP is lower at 3000 psi, the IP will fall out of the desired range at lower tank pressures and the flow rate will be adversely affected. If 3 shims is not enough to achieve 145 psi, a new spring is needed. Too many shims will overcompress the spring, reduce the spring's working range and create the potential for a broken spring - one of the few failures that can potentially result in a failure to deliver air. So I do not recommend using too many shims nor do I recommend making your own shims as they may exceed the specified thickness.

The Mk 2 is probably one of the easiest first stages to service. However, my overall impression in this case is that given there are potential problems with missing shim(s), a worn piston and a possible piston alignment problem, it is well worth taking it in to an SP tech for annual service and possible parts replacement. The parts and service are cheap when you consider it is life support equipment and the possible consequences of a failure.

 

[lhpdiver]

I wouldn't touch anything with sand paper or any other abrasive at this point. As long as the roughness on the piston is not in the o-ring grooves, it should not be a problem. If it is in the grooves, you'll just have to wait and see if it causes leakage into the ambient chamber or accellerated o-ring wear. I always use a torque wrench and torque to 22 ft-lb but I don't think it is critical. Too much torque will destroy stuff like with the Mk20 problem. Depending on the spec you find, I think too much is over 27 ft-lb. Way too little will let it leak but that is probably something less than 10 ft-lb. Whill too little torque may not leak, it can allow you to fasten the 1st to a tank valve and then turn the body loosening the yoke nut. Be careful not to do that and you can probably get away with a guesstimate. The cap is just not sensetive to these problems. Snug it up and you should be fine. That spacer is probably an IP adjustment washer (my old schemats doesn't show that part number). I'd assemble without it and see what the IP is. If it is too low you will need to find some washers. I've made them out of thin nylon sheets but would not recommend it.

I usually also hit threaded surfaces with a little lube.

Thanks awap - might practice putting this thing back together while waiting on the parts kits.

 

[lhpdiver]

The Mk 2 is probably one of the easiest first stages to service. However, my overall impression in this case is that given there are potential problems with missing shim(s), a worn piston and a possible piston alignment problem, it is well worth taking it in to an SP tech for annual service and possible parts replacement. The parts and service are cheap when you consider it is life support equipment and the possible consequences of a failure.

Thank you. Warning noted. Aside from having a working reg when I'm done the main purpose of this exercise is to understand how this thing works and how to service it. When I'm done with the MK2 maybe I'll try my MK25 next. My motivation is not to save a buck. Heck - all these tools and such have cost many years of service costs.

As for your concerns.
No missing shim(s). There were/are 3.
The "spacer'" that is missing sits between the yoke and the body. Its black rubber.
I've attached 2 photos of the wear on the piston. It seems the chrome has flaked off or worn down. What do you think ?

Thanks again. This is great stuff.

 

[DA Aquamaster]

From the pattern and location of the lost chrome, I'd say it's due to corrosion and should not pose a problem (although if parts were available, I'd still replace it). My guess is there was standing salt water in the ambient chamber for an extended period of time while the reg was stored on it's side.

In any case coat the parts lightly with crystolube to slow any further corrosion.

I also can relate to your wanting to better understand how things work. I got started as a tech after years of DIY reg work and the development of a very good understanding of what did what. From a homebuilder/aviation perspective, I see a great deal of benefit in knowing how things work and being able to do some fairly advanced maintenence yourself.

Check with your local dive shops. Finding a potential tech who is a diver, knows one end of a torque wrench from the other and who has both an interest in and understanding of regulators is often hard to find so many shops will take you up on an offer to do part time work for them, especially if you do it for shop credit rather than cash. What you get is free training, a lot more experience and often most importantly, access to parts for your own regs.

 

[redacted]

Thank you. Warning noted. Aside from having a working reg when I'm done the main purpose of this exercise is to understand how this thing works and how to service it. When I'm done with the MK2 maybe I'll try my MK25 next. My motivation is not to save a buck. Heck - all these tools and such have cost many years of service costs.

As for your concerns.
No missing shim(s). There were/are 3.
The "spacer'" that is missing sits between the yoke and the body. Its black rubber.
I've attached 2 photos of the wear on the piston. It seems the chrome has flaked off or worn down. What do you think ?

Thanks again. This is great stuff.

I found that spacer in a newer schematic of a Mk2+ that I have on my work computer. I don't believe it was on the '96 schematic I looked at last night but I'll check again this evening. Looks to me like its function is just to reduce wobble in the yoke. Probably only significant when you are mounting the Mk2 on a tank - might help keep the o-ring mating surface line up with the tank o-ring. Once it is set-up, that spacer should be of no consequence. But it may be a useful need if you want to go searching for a DIY supportive LDS once you get that reg working. Nine out of 10 may say no, so just keep looking for that 10th one. There are shops that would rather have you spending your service kit money in their shop than spending it on ebay.

Edit: Neither my 1997 schematic nor my 2 1994 Mk2s have that spacer.

 

[lhpdiver]

I found that spacer in a newer schematic of a Mk2+ that I have on my work computer. I don't believe it was on the '96 schematic I looked at last night but I'll check again this evening. Looks to me like its function is just to reduce wobble in the yoke. Probably only significant when you are mounting the Mk2 on a tank - might help keep the o-ring mating surface line up with the tank o-ring. Once it is set-up, that spacer should be of no consequence. But it may be a useful need if you want to go searching for a DIY supportive LDS once you get that reg working. Nine out of 10 may say no, so just keep looking for that 10th one. There are shops that would rather have you spending your service kit money in their shop than spending it on ebay.

Edit: Neither my 1997 schematic nor my 2 1994 Mk2s have that spacer.

I got the schematic off this link (which you had given me earlier this year)
http://www.deepsouthdivers.org/

 

[lhpdiver]

Just received my parts kits from Singapore (one week - not bad) and there is a new part which I didn't have before and which I don't see on the schematic. Was wondering if you guys know what it is, where it goes etc. (Of course you do).

I've attached a photo. Thanks.

 

[redacted]

I've never seen it before (all my Mk2 stuff is older) but it looks like a thermal insulating bushing like they put on the Mk20/25 piston to reduce freeze ups. It probably also takes the place of at least 1 IP adjusting shim. I'd leave it off but save it unless you are diving cold stuff. This is one way scubapro implements upgrades.

Wait till you see all the extra parts you get in a Mk20/25 kit.