'Pimp my dive !' Suggestions for modernizing my SP Mk 5 / 109 ?

[scubafanatic]

I recently acquired a couple of MK 5/109 reg sets, Originally I really only wanted the 109 2nd stages, the MK5 1-st stages were just part of the deals.

One set is a NIB (minus the actual original box) where the 1st stage has 1 HP / 4 LP ports, and no PSI rating is stamped on the yoke 1st-stage. (as this set is new/old stock, it has NOT been refreshed with an annual service or anything, so it's NOT dive ready until I take it in for inspection/service.) No SPEC cap installed.

The other set is a later version, with 2 HP / 5 LP ports, yoke is stamped @ 3000 PSI. It's not NIB, but it looks new, as was fully serviced by the seller. It appears to have the SPEC cap installed.

(both sets have the 'old' metal banana shaped yoke wing nut)

For the time being, I'm not doing anything with the 'older' version set, as it only has 1 HP port, and a yoke of unknown PSI 'rating'. I AM planning to get the newer version set fully up and running. I'm looking for suggested upgrades to bring either/both these regs up to their full potential, but have never dove/owned SP regs until now. I've strated reading SB about these 1st and 2nd stages, and here's what suggested 'upgrades' I've seen/or thought about, so far:

1) upgrade the 109's to the 'balanced' 156 version
2) make sure the 1st-stage swivel nut is Stainless Steel, instead of the OEM brass version (available at Vintagedoublehose.com @ $ 15 each)
3) upgrade the yoke to something beyond 3000 PSI, assuming such an upgrade part even exists ? (would anyone have a problem diving the 3000 PSI yoke with a HP steel tank filled to 3500 PSI ?) Would the 1st-stage internals even be able/rated to handle a 3500 PSI fill ?

MK5 1st-stage service kits run $ 15 @ VDH
other parts I saw: OEM Scubapro 2nd stage orifice Part # 11500-109 Fits all 109's and G250's $ 6 each

....additionally, I'll admit I'm a little concerned about the extra maintainance of diving an unsealed piston 1st-stage (all my other egs are either diaphragms or sealed piston Atomics).

I'm debating sending the work to either my not-so-local SP dealer or maybe Bryan @ VDH ? Odds are I'd need to try Bryan as many dealers would have 'issues' installing a non-OEM upgrade like the SS 'swivel nut'.

Can anyone think of any other upgrades / pitfalls I might want to consider on this project ?

Thanks, Karl

 

[Wookie]

Yes, upgrade to a Mk 10B/G250B. Best set of regs SP ever made, in my not so humble opinion.

 

[scubafanatic]

Yes, upgrade to a Mk 10B/G250B. Best set of regs SP ever made, in my not so humble opinion.

I kinda like the classic look of the 109/156s though, with the metal/adjustable 2nd stage, nobody makes diver adjustable, metal 2nd-stages anymore (other than the SP A700, not a fan of that styling though).

 

[Nemrod]

I believe the stainless nut was a service bulletin from Scubapro.

The yoke, if it is the one here, it is probably able to handle 10,000 psi, well, not really but it is stout:

Unless you are ice diving or never rinse your regulators I do not think you should have an issue with the non sealed first stage, it is not like a zillion have seen service in sea water for the last forty plus years.

The balanced kit conversion from vdh is nice. Make sure you have the softest, thinest exhaust valve you can find, it is a 25mm, kind of small by modern standards, check the diaphragms and make sure they are soft, I think the newest version is the clear one used in the G250?

N

 

[redacted]

it sounds like your NIB Mk5 has the oldest 3/8th inch yoke screw which was designed for LP tank use. I have heard of folks using it on a 3000 psi tank with not problem. If it did fail, it would bend the yoke and leak - not a catastrophic failure. Your newer Mk5 would be a heavy yoke if the 3000 is up on the screw (9/16 thread) end of the yoke. I use that yoke regularly on an HP (3442 psi ) tank. There is an intermediate yoke (7/16 thread that is marked 3000 at the base. Scubapro recommends that also be used only on LP tanks, but I use it on 3000 psi tank with no problem.

I prefer the Mk5 over the Mk10 because the IP is more stable as a function of tank pressure. The Mk10 uses the smallest diameter piston in the scubapro line which means HP o-ring friction has a greater impact. The Mk5 has the same HP o-ring configuration but uses a larger piston (twice the surface area of the Mk10) which means that friction has less effect. The modern SP balanced pistons(Mk20/25) use the same size piston as the Mk5 which tells me SP learned an engineering lesson.

Don't get me wrong, the Mk10 is still a good regulator. But I replaced most of mine with Mk5s.

 

[scubafanatic]

it sounds like your NIB Mk5 has the oldest 3/8th inch yoke screw which was designed for LP tank use. I have heard of folks using it on a 3000 psi tank with not problem. If it did fail, it would bend the yoke and leak - not a catastrophic failure. Your newer Mk5 would be a heavy yoke if the 3000 is up on the screw (9/16 thread) end of the yoke. I use that yoke regularly on an HP (3442 psi ) tank. There is an intermediate yoke (7/16 thread that is marked 3000 at the base. Scubapro recommends that also be used only on LP tanks, but I use it on 3000 psi tank with no problem.

I prefer the Mk5 over the Mk10 because the IP is more stable as a function of tank pressure. The Mk10 uses the smallest diameter piston in the scubapro line which means HP o-ring friction has a greater impact. The Mk5 has the same HP o-ring configuration but uses a larger piston (twice the surface area of the Mk10) which means that friction has less effect. The modern SP balanced pistons(Mk20/25) use the same size piston as the Mk5 which tells me SP learned an engineering lesson.

Don't get me wrong, the Mk10 is still a good regulator. But I replaced most of mine with Mk5s.

Based on your info, it appears my later generation MK 5 does have the 'heavy' yoke since the '3000 psi' stamp is on the end of the yoke next to the metal banana-shaped yoke screw. My older generation MK 5 will likely remain in reserve as a parts-donor/reserve reg, as it only has 1 HP port, and I don't know how much luck I'll have locating a spare 3000 psi yoke either.

...while browsing the SB archives, I came across some controversy, debating the 'safety' of the old style piston vs the newer style piston in the MK 5, and that neither piston is available from Scubapro anymore regardless, so if the piston gets damaged, the MK 5 owner is screwed! I have no idea which piston is inside my MK 5 (I saw photos of the differences here on SB, so I can distinguish old version from new version, but I haven't opened the unit.) If I had to place a bet, I'd bet it has the old version.

(this safety debate is in addition to the suggestion of upgrading the MK 5 swivel bolt from brass to stainless steel, which as far as I can tell, is only available in the aftermarket @ VDH.)

Also, I'm confused as to if Scubapro allows it's dealers to work on a MK 5 anymore ? Some parts are still available from SP, while others are not, so SP official support falls into a gray area, does anyone know the official SP position on this ?

---------- Post added January 12th, 2014 at 10:43 PM ----------

I believe the stainless nut was a service bulletin from Scubapro.

The yoke, if it is the one here, it is probably able to handle 10,000 psi, well, not really but it is stout:

Unless you are ice diving or never rinse your regulators I do not think you should have an issue with the non sealed first stage, it is not like a zillion have seen service in sea water for the last forty plus years.

The balanced kit conversion from vdh is nice. Make sure you have the softest, thinest exhaust valve you can find, it is a 25mm, kind of small by modern standards, check the diaphragms and make sure they are soft, I think the newest version is the clear one used in the G250?

N

I've got the SPEC cap on my later version (with the tiny holes), I'm hoping this isn't a corrosion magnet !

 

[John C. Ratliff]

A couple of points:

--It is very difficult to damage a piston in one of these regulators. Because the piston is held off the seat by the spring, there is no pressure on the seat/piston interface except when it is on a regulator and pressurized.

--Given what you've said about the yolk, I don't think you'll have a problem with the yolk.

SeaRat

 

[scubafanatic]

A couple of points:

--It is very difficult to damage a piston in one of these regulators. Because the piston is held off the seat by the spring, there is no pressure on the seat/piston interface except when it is on a regulator and pressurized.

--Given what you've said about the yolk, I don't think you'll have a problem with the yolk.

SeaRat

Hi John,

Per previous posters, the likelihood of piston damage seems to be based on the skill of the service tech, somehow it sounds trickier to service than the average piston. I did manage to buy a pair of MK 5 NIB pistons with related service kit on EBay last night, as spares, as they're out of production, and I was all paranoid about having to scrap the 1st-stage if the piston were ever damaged. The seller had 2 of those kits left for sale as of last night, if anyone else is looking for spares. (the pistons appear to be the older version though)

 

[Zung]

The MK5 is actually easier to service than the MK10 because the "tunnel" where the HP O-ring lives is longer, so the O-ring stays put while you try to coax it into its groove. Unless you go for herman's magic tool that makes servicing both a no brainer.

Of course, that's totally unrelated to the piston. If you don't drop it, you're good for a long long time!

 

[coldwaterlloyd]

I just got this baby back from rslinger , complete upgrade / overhaul , great reg