need a MKV piston - shouldered one

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Have you considered just sliding a bushing over the piston stem? I'm pretty sure that's what the MK 20/25 & MK 10 plus have.

See this image I stole from DA:

44906d1208558690-upgrading-sp-mk20-adjustable-ip-stems.jpg
 
Yep, Chile is looking for the stepped piston stem version. It was an upgrade that supposedly prevents the piston from going too deep into the hp seat. If the piston travels too far, it could plug up and stop air flow or extract the seat from the seat retainer. I've never heard of a case where this has actually happened, but if the spring were to break I suppose it's a possibility.

That's why I suggest using a bushing over the stem if there is any concern. Here is a thread with pictures and a discussion.

http://www.scubaboard.com/forums/repairing-your-own-gear/303673-sp-mk5-piston.html
 
Yep, Chile is looking for the stepped piston stem version. It was an upgrade that supposedly prevents the piston from going too deep into the hp seat. If the piston travels too far, it could plug up and stop air flow or extract the seat from the seat retainer. I've never heard of a case where this has actually happened, but if the spring were to break I suppose it's a possibility.

That's why I suggest using a bushing over the stem if there is any concern. Here is a thread with pictures and a discussion.

http://www.scubaboard.com/forums/repairing-your-own-gear/303673-sp-mk5-piston.html

Thank you, Couv! But I don't believe for a second that the shoulder has anything to do with preventing the piston from impaling the seat. I could see that it would be a nice heat sink and could conceivably improve anti-freezing. But most likely it was just a change in manufacturing; maybe it was cheaper and/or helped alignment of the piston shaft.

Anyhow, I've never seen one before, and I've owned several MK5s, including several of the latest version. So I guess I would not get hung up on using one. Or, if you have one and it just has a nicked knife edge, you might try resurfacing/polishing with micromesh.
 
Agree....I think I do have a MK 5 with the newer piston, if I have time to open it up and experiment perhaps I will. But from memory, the only way I could see the shoulder performing any safety function would be in the case of a broken spring.

True enough, a beefed up stem may serve to hold alignment better. It may also dampen the dreaded vibrating/singing/buzzing piston noise.....didn't work too well in the MK 10 though.
 
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