Scubapro MK 20 Mysteries

[Zung]

I just opened up my Mk20 #2 and found the spring that secures the bushings has 4 turns in the coil, and it's 10mm long!

This reg was serviced by a LDS about a year ago, I haven't messed with it so far. And it's a yoke type, so there's no confusion with the DIN filter retainer spring.

I looked into the Technical Service Reference & Repair Guide, and all of the drawings show the bushing spring has 4 turns, and the longer DIN spring has 5 to 6 turns, depending on the view angle.

Is it possible that the 10mm one really is for the bushings?

 

[redacted]

I just opened up my Mk20 #2 and found the spring that secures the bushings has 4 turns in the coil, and it's 10mm long!

This reg was serviced by a LDS about a year ago, I haven't messed with it so far. And it's a yoke type, so there's no confusion with the DIN filter retainer spring.

I looked into the Technical Service Reference & Repair Guide, and all of the drawings show the bushing spring has 4 turns, and the longer DIN spring has 5 to 6 turns, depending on the view angle.

Is it possible that the 10mm one really is for the bushings?

I just pulled my Mk20 DIN filter retainer apart and the spring is about 14 mm long. Mine is a 300 bar din. A 200 bar DIN fitting has a different part number for that spring and other din components

 

[Zung]

Thanks awap, you've saved my day.

Actually, my 14mm spring comes from a 232 bar DIN fitting. Between the 232 bar and the 300 bar, the knobs, the knob retainers and the filter retainers are different, but the springs are the same.

Btw, I found an error in the Rev. L schematic: the seat retainer is spec'ed to 30nm/266in.-lbs. I almost went all the way, then thought it couldn't be right. The revision H says15nm/133in.-lbs: that sounds more reasonable.

 

[Zung]

Just finished rebuilding the reg. with an original SP kit, cleaned it real well, polish the part of the body where the crown of the piston rides with the local equivalent of Brasso, real smooth, replaced all o-rings because they're all in the kit anyway, lubed "copiously" all the piston o-rings, leaved the original shims in place, 1 black, thick, spring end, and 1 white, regular, retainer end, and pressurize.

Initially, the IP is too high, removed the white shim, now down to 142 PSI, still a bit high, and most importantly, it took about a minute to get there; no real step, the IP just took its own sweet time to swing up to where it belonged. The piston is the lastest & greatest composite.

So I take it apart again to remove the black shim that's at the bottom of the spring; while I'm there and just for the heck of it, I put a brand new piston to see what's up, with the same 2 shims. And here's what:

- IP down to 129 PSI YeeeeeHaaaaa!
- More importantly, revovery is INSTANTANUOUS!
- No creep whatsoever over a 10' period, no leak either

So another question: do pistons wear out? I look at the old one with a loupe: no nick, no nothing. But the measurements are there: minus 13 PSI, and minus 60' to recovery. Am I supposed to budget in 40 bucks worth of piston every so often?

Edit
I'm thinking about mattboy's trick with Micromesh, except I can't find them around here. Maybe a little polish of the tip with Brasso or it's local equivalent?

 

[BlueWaterDiving]

How may dives are on that piston? I do change them for time to time. I change a lot mon the instructor's regs because they have a lot of dive's on them. If the reg is in warranty it would be covered. So this is another reason to keep your regs in warranty.

 

[redacted]

How may dives are on that piston? I do change them for time to time. I change a lot mon the instructor's regs because they have a lot of dive's on them. If the reg is in warranty it would be covered. So this is another reason to keep your regs in warranty.

What is the failure mode(s) you are seeing that are causing the change? I had one composite piston develop a leak through the ambient chamber. Turned out to be the O-ring that seals the composite sandwich. IP creep suggests a bad seat ot a bad piston failing to make the seal. Seat was good so I would take another look at the piston. Not just the rather dull "knife edge" but the area on the outside where it penetrated into the seat. Perhaps another good cleaning or even a light polishing would do the trick.

 

[Zung]

I bought this one second hand, so the warranty is out. I've got another one I bought new that has a few hundreds dive in it, works perfect, instant recovery. Both are in very good conditions externally, barely any scratches; so, unless the previous owner is VERY careful, this one shouldn't have more than maybe 3-400 dives. Does this come close to your definition of "a lot"?

 

[BlueWaterDiving]

Unfortunately I do not have to go threw all that. With one if it dose not work I put in a new one. And they do wear out. I do use a eraser on the top to clean it but thats about it. I have had them wear a gove where the stem o-ring gose. But that makes them whistle. ScubaPro dose allow a 5 pound creep. The ip can be 135psi + or - 10 psi. The GUE boys like there Ip 115 psi. And I do not know way.

 

[Zung]

Anyway, this one is fixed. I'll look at still another one tomorrow and report back. Thanks.

 

[Zung]

BlueWaterDiving: btw, great collection of clear G250's! I love them. I've got one, but as soon as my #2 son saw it, he liberated it. So I have to live with a plain black one myself:depressed: