SP MK-3 - First Stage Piston Seat

[rhwestfall]

Okay, got a MK-3 from a friend as a "throw-in" with another item. Its the first generation with the threaded yoke stem integral with the stage body, and the yoke is trapped with a nut rather than the current way they have their stages now. It is leaking when pressurized, and when I took it apart, its completely dry (no lube). It has no evidence of any issues with the piston. I picked up a VDH service kit, and want to do a re-build. Nothing complex to these, but a "stupid question": What is the best way to remove the old seat from inside the piston stem?

[vintagefrank]

Just take some compressed air and sent it on the site of the Piston head through the Piston. It will shoot out the seat.

[rhwestfall]

thanks!

[rhwestfall]

135 psi and it won't budge. all the air escapes out the two ports in the stem....

got it! Fine pointed straight pick from the back side......

[Bryan]

Compressed air usually works great but there is a removal tool consisting of a metal rod centered in a delrin base. Basically you put the rod up through the center of the piston and give the base a tap to dislodge the hp seat. I'm sure you can wing it with what you have around the shop.

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[rhwestfall]

Yep, there was a simple work around!

Next question (yep, I'm a PITA):

There was a washer behind the disc filter. The rebuild kit doesn't have that one.... suggestions as to specs for one?

[Bryan]

I'd re-use it. It's a spacer only and most other Scubapro 1st stages use an O ring in that place. I can send you a metal spacer washer if yours is not reusable

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[rhwestfall]

nope, I'm ok with what I have. I just put a little lube on it to "re-season" it, and reinstalled it under the filter.

New issue - have you ever run into a piston from a MK-3 (early model) that needs a smaller diameter seat than the ones you sell in the re-build kit? The one I just got from you is too wide.

FWIW - I re-installed the original, but the IP was creeping to 150+. It starts at 130, purge it, and it was slowly creeping.... Pulled it, and flipped the seat, and it appears to be locking at 135 psi (no shims).

Cycled it 20 times, and is stops at 130, and just ever so slightly goes to 135 and locks. Think I got it with just the piston head o-ring and the stem o-ring being replaced.

Any ideas/options on the seat?

I just noticed there is a "Vintage ScubaPro Regulators" forum - maybe this post should be moved there....

[Bryan]

First I've heard of an issue with the seat size. I sell lots of them and service quite a few Mk2's vintage and modern with them during the year.

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[rhwestfall]

its not a MK-2, its a MK-3.... Just curious if you were aware of the difference or had access to the apparently smaller seat?

The piston o-ring and the stem o-ring from the kit work.

[Bryan]

I read the title of your first post clearly. 102-4 is the part number for the Mk2 and Mk3 seats according to the 1976 Scubapro schematics. Best of luck with your regulator.

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[luis]

I serviced a lot of Mk-2 and Mk-3 in the early 70's and the only difference was the piston head size. The big diameter O-ring was different, but everything else including the seat was always the same.

I was going to ask you if you could post some pictures (preferably with a dial caliper), but if your regulator is working now, you do not want to open it. I would say, that you have an odd Mk-3.

[Herman]

Or possibly a wayward seat from some other reg that somehow got in the mix??

[antique diver]

Bob, I have a small container with various sizes of unmarked and unknown but similar seats, and could be a winner in there.

I know that you probably don't want to remove the seat now that it is holding pressure, but if you ever do, please take accurate measurements of diameter and thickness with good dial or digital calipers. Also good close-up photo. I'll check for a match.

[rhwestfall]

Everyone, thanks for the input.

I've ordered a digital caliper (about time I have one handy), and thanks to Amazon Prime it will be here in a few days. At that time, I will confirm the seat that was in the re-build package, and, if I feel ambitious, tear the stage down again to get a measurement of the seat. This reg was a way for me to gain some confidence in rebuilding, so nothing lost if I tinker some with it.

As this is a really early model (2 lp, weak yoke, hp thread is same as lp, integral yoke stem), I'm wondering if Greg Barlow might have some insight as to the possibility of a piston change in the early model to a next generation that is now standard....

I'll update as I find out more.