Scubapro piston: is there a life after death?

[Zung]

I've had a few cases of IP creep, some bad and some mild.

A pretty bad case was reported here: it's a MK10 Plus (the "Plus" uses a different piston, seat and seat retainer), with a brass piston; I call it MK10Plus #1. After a full service and settling in, the IP crept to 170 PSI, with no shim. After much wrestling and with the help of all the gurus in general and akscubaduck1 in particular, I managed to downgrade it to straight MK10, and now it's working fine.

I also reported a milder case here, with a MK20 I call MK20 #2. After a full rebuild, IP was creeping to 140 PSI. I replaced the piston, et voilà, IP is now 129 PSI AND NO CREEP AT ALL!

So now I'm left with 1 MK10 Plus and 1 MK20 piston, and the thoughts of trying out mattboy's magic. Basically, the magic consists of:

• Inspecting the edge with a jeweler's loupe, 15x or more magnification prefered.
• Lap sanding the edge and the body with a magical ingredient called Micro-Mesh, with increasingly finer grit.

awap has also reported success with "cleaning" the tip: he actually means removing the imperfections with abrasive.

Micro-Mesh is all over eBay, and the price is sensible: I've got 1 sheet of each grit for about $12 shipped. Just do a search with and without the dash.

Case 1: the guinea pig

So I have this MK10 Plus piston made of brass I don't care about, and I have another MK10 Plus body I call MK10Plus #3 that has a stainless steel piston and that guy creeps to 152 PSI. I'll try the #1 piston in the #3 body, so there's a flaw in the methodology because we're not comparing things that are directly comparable, and I'm too lazy to fit the #1 piston as is just to check.

Anyway, the first 2 pics are "before". I took them with a 10x loupe held over the lens of my cell phone; it works better than my digital camera. There's a little thing in -01 that looks like a crack but it's just a lint.

Lap sanding is actually easier that I thought, the Micro-Mesh bites easily into the brass. I started with the 1500; it's a bit too coarse. I went through 1800, 2400, 3200, 3600, 4000, 6000, to 8000 and stopped there, checking often with the loupe. The whole operation took no more than maybe 15 min.

The next 2 pics are "after". The difference is huge.

So what gives? I don't know for sure yet, the thing is still settling; what I have so far:

• Pressure on: 9.5 bar /138 PSI
• 10 min. creep to 10.2 bar/148 PSI
• 1 hour later: after a (light) purge, swing to 8.5 bar/123 PSI, level off at 9.8 bar/142 PSI
• Purge again another hour later: 9.6 bar/139 PSI

After a couple of hour, it appears to settle for 139 PSI, and that's without the piston centering washer, but with 1 regular shim in its place. I'll leave it over night and report back.

Anyway, I'm VERY HAPPY with the mattmagic! Thank you mattboy!

 

[halocline]

It looks like you did a great job, and excellent photos.

 

[Texfrazer]

Congrats Zung!

It looks like you're developing a nice touch. The difference in the pistons is amazing.

Now to see if the local LDS will collect any problem pistons they remove and donate them for the cause.....

 

[Couv]

Well done!

 

[herman]

Nice job, it's always great to see old gear returned to service.
I am curious to know how much effect the refacing has had on the balancing of the reg. Have you done a check to see how much the IP varies as tank pressure changes? From the photos it appears that the mating surface of the piston has been increased a little which would have some impact on the balancing of the reg. I am sure it's not enough to be an issue. Considering all the balancing discussions we have had on the board I tend to get curious about such things.

 

[Rick Inman]

Thanks for the post. My buddy & I were just overhauling our Atomics and discovered a bad piston in one of his regs (IP creeping up to 175psi), so of course he ordered a new one. Never considered re-working the existing one - just for fun, of course...

 

[Zung]

Nice job, it's always great to see old gear returned to service.
I am curious to know how much effect the refacing has had on the balancing of the reg. Have you done a check to see how much the IP varies as tank pressure changes? From the photos it appears that the mating surface of the piston has been increased a little which would have some impact on the balancing of the reg. I am sure it's not enough to be an issue. Considering all the balancing discussions we have had on the board I tend to get curious about such things.

Good point. I think the effect is negligible, because the balancing depends on the external diameter vs internal diameter ratio of the piston stem [(A2 - A1) x P], and this ratio is not modified. Actually, the balanced pistons have a flare (MK20 composite) or a larger diameter at the stem (MK10 Plus) that's visible. My next project will be my MK20 #2 and I'll do a low air test to prove or to rebute this point.

Another point worth noting is the lockup is potentially mushier, because the area of contact is larger.

Btw, the IP is finally 136 PSI.

 

[halocline]

You might need to take it for a dive, or at least cycle it a few hundred times, to really get it to settle in, but if I'm understanding you correctly, the IP creep is gone. Is that right? I predict that the same thing will happen with the MK20 piston.

What you've done should not affect the balancing of the reg, except if you've resurfaced the piston shaft where the HP o-ring goes through, you might decrease friction there, which should keep the IP from rising at very high tank pressures.

If I'm thinking about it correctly, it's only horizontal surface area subjected to HP air that would have a downstream force on the piston and cause IP to drop with supply pressure. If the bottom of the piston edge is flat and it's fitting into a conical seat, that should be close to zero. The majority of the horizontal area would be inside the piston, only subjected to IP.

I have a feeling that the primary reason IP is a little higher at high supply pressures on a MK5/10 has to do with slight HP o-ring extrusion and the resulting friction. Just a guess...

 

[Zung]

Well, the creep is not "gone", it's reduced to some OK-ish value: after a purge, the IP returns quickly to 8.5 bar / 123 PSI, then slowly to 9.4 bar / 136 PSI, over a couple of minutes, so the creep is about 13 PSI.

Maybe this is specific to the brass piston: I have a MK10 Plus with a stainless steel piston that has no creep at all. But then again, this one is only a guinea pig, the "real things" are yet to come.

I'm waiting for the lake to warm up to 70°F before I start my diving season; yes I'm a WWS (warm water sissy). Until then, I have a question for the gurus: if I leave the air on for a long time, like a week, would that be equivalent to cycling the reg a hundred times? In other words, the difference between actively/dynamically plowing the seat and statically "shaping" it?

 

[halocline]

I notice that my MK10s are more stable after the first dive. You can try leaving it pressurized for several hours, tapping the purge a few dozen times, leave it pressurized, repeat, etc...