Can anyone explain the MK25 Evo Piston? (with pictures)

[NowTeelus]

Hello everybody,

recently, I got a bunch of parts of an MK25 Evo from my dive buddy and I'm wondering about the new piston design. Compared to the pistons of the MK20 (at least those I've seen), the metal part of this piston seems to be made of two seperate pieces which had been screwed together: There's a little "window" in the transition between the stem and the "head" which seems to show (at least this is my interpretation) the screw threads that were covered with lock-tite. Also, there's a little O-Ring and some kind of (also my interpretation) a leverage point for a wrench (the flattened part of the "circle").

Can anyone explain A) these features and B) their impact on the cleaning process (e.g.: Has the O-Ring to be changed? Goes the piston into the ultrasonic cleaner in one piece?)?
Thank you very much!

 

[lexvil]

While I’m not 100% positive I don’t believe this is a part you can take apart, it’s the MK 25 composite piston which is a SP method of cold proofing along with coatings etc.
The o ring under the hat is a replaceable part that should be in the service kit. The MK 25 is a good regulator but is does, over time, suffer some scoring from grit in the ambient chamber although it would probably take 25 years to fail.

I prefer the Atomic method, fill the ambient chamber with lube and cover the holes with a seal, keeps all of the bad stuff out.

 

[NowTeelus]

Thanks a lot!

As far as I can see, the O-Ring is not in the service kit. Accordingly to the schematic of the MK25 AF from "Frogkick" (not MK25 Evo!), this could be the O-Ring 01050360, which is not part of the service kit 10.750.045.

For the filling of the ambient chamber: Vance Harlow wrote in his Book ("Scuba Regulator Maintenance and Repair") on page 205 about the MK10+, MK20 and MK25: "Scubapro now allows the use of grease in these regs, but recommends that it not be left in year round". I don't know, if this information is correct or at least still correct (for the MK25 Evo).
A couple of years ago, I owned an Atomic reg that was filled with Cristolube and "sealed" with such a protecting (rubber/silicone) ring. Sometimes after a dive, tiny amounts of Cristolube and salt water leaked out of the ambient chamber. So, maybe it is not completely without problems to have this solutions since tiny amounts of (salt) water could stay "trapped" in the ambient chamber.

 

[lexvil]

I usually slightly overfill my Atomics to make sure stuff stays on the outside. I am more concerned with grit than cold, my cold tolerance is lower than the regulators.

 

[rsingler]

The Mk25 composite piston was designed to facilitate better ice shedding compared with its Mk20 predecessor.
The black plastic bushing can be removed at servicing to check for corrosion underneath, but the piston shaft is not designed to be removed (hence the Loktite). The space under the black bushing is designed as an air chamber, which is why the oring is there (although it was also present with the previous version of the piston). That air space reduces cold transfer as ice accumulates around the piston shaft in low temperature conditions due to adiabatic cooling. Similarly, the white soft compressible washer around the shaft is designed to flex and crack off accumulating ice.
As a result, you should put no lube under the bushing, but just lube the orings and add a bead of lube between the main piston oring and the wiper ring above it.
The small oring under the bushing is a static ring which should not need replacement unless damaged during removal for piston ultrasonic cleaning. As you noted, it is not in the service kit.

Here is a pic from Service Bulletin #272:

 

[rsingler]

For the filling of the ambient chamber: Vance Harlow wrote in his Book ("Scuba Regulator Maintenance and Repair") on page 205 about the MK10+, MK20 and MK25: "Scubapro now allows the use of grease in these regs, but recommends that it not be left in year round". I don't know, if this information is correct or at least still correct (for the MK25 Evo).

The Mk25 Evo is NOT designed to be sealed. The blue coated spring and other "improvements" are their answer to ice accumulation, but do not address the problem of retained grit damaging the piston land where particles get stuck in the crack where the piston ring meets the ambient chamber.

Scubapro's discontinuation of SPEC boots and lube-filled environmental chambers is one of the reasons Atomic is eating into their market share. While many deride the cost and hassle of filling the environmental chamber, it markedly improves time between services, and decreases salt/sand/grit-induced damage to the piston land inside the ambient chamber. I seal ALL my Atomics, warm water and cold, to keep out the junk.

 

[Hawaii Treasure Finder]

The same white plastic washer/o-ring is in the MK20. Pain to replace as they can crack with use and as you say, not in the service kit...

 

[halocline]

The composite piston in the MK25 is not designed to be taken apart, and I heard the story of one failure of a tiny o-ring that seals the metal shaft to the plastic piston head. It results in a slow leak out the ambient chamber ports. I don't think it's the one that sits under the bushing, I think you have to screw the shaft off, breaking the loctite seal, to get to that o-ring. My friend who discovered the leak on his did this, and successfully replaced the o-ring and put it back together with new loctite, but I'm sure that SP's 'solution' is to simply sell you a new composite piston.

As rsingler explained, SP no longer supports packing the ambient chamber with grease, and the enlarged ambient chamber vent holes would make packing the MK25 messy and likely ineffective. More 'progress' on their part. I'll stick with my MK10s with spec boots and one piece pistons that don't have built-in obsolescence.

 

[rhwestfall]

I just dive diaphragm (MK-17) regs....

 

[rsingler]

I just dive diaphragm (MK-17) regs....

Six years and counting since last teardown.

IP 135 with crisp lockup and no creep. Jes' saying...