Question Scubapro MK10 - spring positioner. Do I need one?

[kdlv]

Folks, 3 lame questions from my side

Some diagrams indicate the presence of a spring positioner.
Rev C from 12/1997 indicates only the presence of a washer/shim 01.060.101. #10 on the diagram


Rev M from 02/2001 of the diagrams however has now #34 - 10.500.118 which is a spring positioner washer. The piston, bushing and sleeve (#19,#33, #22) on the diagram look exactly like mine.


Same spring positioner/washer is on the previous Rev L as well from 03/99

Supposedly Rev M is the latest and I should have both at least 1 shim and the washer together contacting the main body and under the spring.

And then... there is the confusing MK10 Plus conversion infographic.

My piston looks similar to the Plus on the graphic, but the rings are not amber. It has the TIB, but doesn't show the silicone sleeve (#19 on diagrams)! I know mine is normal (non-Plus) MK10. But this diagram doesn't show the silicone sleeve at all, wondering why...?

* Now... my MK10 is in pristine condition and it has been serviced recently. It appears to be a late model. But... there is no spring positioner, just a shim under the spring and the tallest HP seat is installed. I couldn't test the IP at low tank pressure, to see if it drops significantly. Should I source one? And do I need a shim under it? Probably not...

* Is there a need to lubricate the piston stem under #33 cap?

* What is the right position of 19? the groove is on the outside, or the flat surface is on the outside? I assume the groove is on the outside like on the photo and it was like that when I took the piston out. #19 is soft and it is very easy to turn it inside out. What's the purpose of it?

{and then I found this topic: When did MK10 get spring alignment washer (10-500-118) and why?}

 

[halocline]

Your second diagram is of a MK10+ which has a different piston and seat from the MK10. You might already know that. That plastic centering washer is only for the MK10+ piston. The silicone sleeves you can use or discard, it doesn’t really matter. They were part of SP’s ongoing attempts at keeping ice from forming on the piston in very cold water. You don’t need to have an IP shim on both sides of the spring; sometimes they help to break up an annoying resonance that can occur with the MK10 and cause a sort of harmonica sound on inhalation at certain tank pressures.

When you say “my piston looks like the MK10+” do you mean it has a rounded edge instead of a sharp knife edge on the end of the shaft? If so, you have a MK10+, if not, regular MK10. It’s very important to be clear on this in terms of rebuilding, of course. And the MK10+ has a radically different seat and seat retainer. It’s basically a MK20 trapped in a MK10 body.

The amber o-rings were polyurethane that SP used for a few generations of their rebuild kits. They switched back to EPDM on later kits. The presence of these amber o-rings does not indicate either MK10 or MK10+, they can appear in any SP piston reg that was rebuilt during the time SP was using them. The problem I have had with polyurethane o-rings is their shelf life is not great. EPDM works really well, the only reason not to use those is if you are planning higher than 40% O2 use, in which case viton is the standard, but there are other characteristics of viton that are not great.

 

[kdlv]

When you say “my piston looks like the MK10+” do you mean it has a rounded edge instead of a sharp knife edge on the end of the shaft? If so, you have a MK10+, if not, regular MK10.

It's a sharp edge piston, definitely MK10 (non Plus). The seat is also definitely MK10 (non Plus) I have seen older versions of MK10 however with a different shape of the thicker part of the stem under the gray cap. Without the little groove where the cap "clicks" onto. That's what i meant.

Ah, the silicone sleeve preventing ice... that makes sense. Thanks.

Strange thing with the spring positioner is that the diagrams (i.e. rev M) indicate this part is needed for both + and non + versions, but I now understand it is optional.

Thanks @halocline!

 

[halocline]

Strange thing with the spring positioner is that the diagrams (i.e. rev M) indicate this part is needed for both + and non + versions, but I now understand it is optional.

Thanks @halocline!

Someone else might know something I don’t, but I’ve worked on probably 20-25 MK10s over the years and I’ve never seen one with that centering washer. Only on the single MK10+ I ever worked on, which was a conversion from MK10 to MK10+.