Servicing my own gear

[redacted]

The MK25 is pretty consistent up to around 3000 (< 5 PSI) and then it starts to rise at higher pressures due to increased friction and extrusion in the HP o-ring. Probably the best way to address this is with a 90 duro polyurethane o-ring, well lubricated, in the HP piston spot. If you are looking at one, I would suggest going with a MK20 and using the MK25 composite piston. That way you don't have the added complexity of the external IP adjustment, but you still get the MK25 flow rate and performance. It doesn't really matter......

The earlier SP balanced piston 1sts have a greater IP swing; my MK10s go at least 10 PSI from 3000-500. It doesn't bother me in the least, but for HP tanks, you'd be better off with either a MK15 or 20, which have the bushings for addressing extrusion at higher tank pressures. I think the higher duro PU o-ring is very helpful.

The Mk20/25 piston does have a interesting design feature to help compensate for the almost balanced performance of a BP. The last couple mm of the piston has a slight flare that results in an additional closing force proportional to supply pressure. I believe I have heard of this being referred to as an "overbalanced" design. You can detect it with a caliper.

 

[bl6394]

And this discussion has raised another question. How do you identify 1st stage models. I was looking at my wife's and the only thing that identifies it as a Mk25 is the knob for tightening it on the tank. Do you just have to become familiar with each model and what they look like?

So ... by visual inspection you can identify it as a member of the Mk 25 family - but when was it manufactured? Is it a Mk 25 or Mk 25 EVO. Is it a Mk 2, or a Mk 2 Plus, Mk 2 EVO....

If I'm not sure - I would use the serial number to confirm with the manufacturer the date of mfg and exact model. The Mk 2 plus uses a different service kit than the Mk 2 Evo for instance... (Some of the o'rings are a different diameter.)

 

[bl6394]

The Mk20/25 piston does have a interesting design feature to help compensate for the almost balanced performance of a BP. The last couple mm of the piston has a slight flare that results in an additional closing force proportional to supply pressure. I believe I have heard of this being referred to as an "overbalanced" design. You can detect it with a caliper.

Interesting you mention this. Rene did sketch out the flare in the piston shaft when discussing this. I never put a caliper on it to verify.

 

[redacted]

And this discussion has raised another question. How do you identify 1st stage models. I was looking at my wife's and the only thing that identifies it as a Mk25 is the knob for tightening it on the tank. Do you just have to become familiar with each model and what they look like?

I'm sure SP can tell by the serial number. Since parts are all Interchangable, appearance is not a reliable indicator. But, other than identifying what piston is inside and whether it has the universal retainer, it really does not matter much.

The external adjuster and the vanes on the ambient chamber are good indicators of a model that started life as a Mk25, but they could also be Mk20 upgrades.

 

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Interesting you mention this. Rene did sketch out the flare in the piston shaft when discussing this. I never put a caliper on it to verify.

Did Rene, by any chance, mention the o-ring inside the composite piston that will eventually fail from age? If so, did he mention that it can be rebuilt?

Older cutaway drawings did show that o-ring, but it seems to have just disappeared on newer drawings.

 

[bl6394]

Did Rene, by any chance, mention the o-ring inside the composite piston that will eventually fail from age? If so, did he mention that it can be rebuilt?

Older cutaway drawings did show that o-ring, but it seems to have just disappeared on newer drawings.

The piston in the Mk 25 regulator we serviced (recent model Mk 25 EVO) was not a composite piston - as such the composite piston was not discussed.

Edit. Actually - I'll amend that. My recollection (subject to all of the imperfections of a 46 year old) is that the Mk 25 EVO had a metal head and shaft. The equipment tutorial shows a plastic head and metal shaft. But no discussion of an internal o'ring subject to failure or replacement was mentioned. Was this in the interface between the plastic head and metal shaft?

 

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What is it now? Solid SS?

 

[bl6394]

What is it now? Solid SS?

Edited above. By composite - the Mk25 piston may have had a plastic head and SS shaft. The tutorial video shows one. And my memory may be incorrect. (I do a lot more Mk2 and Mk11/17 rebuilds than Mk 25)

No mention was made of an internal o'ring. (I assume that would have been between the plastic head and SS shaft.) No further disassembly of the piston was discussed.

Looking at the attached cutaway - (http://www.scubapro.com/media/90858/mk25_ass_01.jpg) all three of the o'rings on the piston are accessible / serviceabe (the two on the perimeter of the head - and one at the interface between SS and plastic.) I'm unaware of any other internal o'rings.

 

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The piston is actually two metal pieces shown as the dark green shaft and the yellow piece that screws in to it capturing the brown plastic head. The black round bit where the 3 pieces join is the sealing o-ring.

 

[bl6394]

The piston is actually two metal pieces shown as the dark green shaft and the yellow piece that screws in to it capturing the brown plastic head. The black round bit where the 3 pieces join is the sealing o-ring.

Gottcha! I understand your point.

Restating - the green in the picture is the SS piston shaft. The brown is the plastic piston head. The yellow is a SS shoulder that screws on to the shaft and captures the internal o'ring while securing the piston head to the shaft.

No - we never disassembled / removed the shoulder from the piston assembly. It is not discussed as part of annual service on a Mk25.