MK 25 Piston Preparation

[GLiebmann]

I was hoping to get some clarification on how to properly prepare a MK25 Piston for reassembly. I have been though several other manufacture courses and the general consensus when replacing the main piston Oring is to lightly lubricate it only.

I have not taken an official SP service seminar yet, but I have the service manual and I have spoken with a couple of people that have taken the SP service seminars and I am receiving conflicting information. Some say that light lubrication with the appropriate lube is all that is necessary, on the other hand, I have spoken to some techs, and seen some youtube videos where they are filling both Main Oring channels with lube along with slightly coating the Oring itself.

The Service Manual states to "Grease the o rings of the piston by filling the cavity between the 2 o rings". When they mention cavity, are they referring to the groove the o rings sits in, or the space/gap on the piston between the two o rings? Seems like this is the only piston design that suggests this aggressive of a lubrication. I was hoping to get thoughts from more experienced technicians out there.

 

[Mobulai]

It has 2 orings for a reason — one is to wipe the grit that drags and scratches at the walls of the ambient chamber, (intentionally or not) between their peaks there’s a middle (uncarved) channel that acts as a gland to carry some lube
Regs that have seen some serious time and some wear can benefit from this to extend the service interval (to match a new reg)

Most of the regs I serviced are old mk20s (same piston) (some have been doing daily salt water dives for 25 years); and I try to restore them and undo the wear as much as possible

On a new mk25 I’d want to just dab the orings with a tiny dot of lube and move on as O should do on other dynamic orings, but it started becoming more of a habbit to also fill the gland with extra lube on that piston head design

Honestly it’s not that good of a policy if I want to identify any LP leaks from scratches in the piston land, as the excessive lube will cover it up for quite a while; and you can’t pin it after servicing

Some might even suspect it would provide more stickiness that might drag the grit even more and trap it causing further damage like this tiny red grain of sand has crept all the way to the IP side oring

My recommendation is to take it on case by case basis:
if you already did a pre disassembly test and on immersion saw no leaks from the ambient chamber that suggests a scratch in the ambient chamber wall (comes from below not above) —> only the minimal lube works (but don’t underlube it; it’s still an exposed dynamic oring)

If there’s a leak, you either replace the ambient cap (as per SP recommendation); or resurfaced it with micromesh; use minimal lube, retest; if seal is good un assemble the cap and add some extra lube

You pick your own flavor, and maybe ask for us at the seminar what the instructors thoughts are

 

[Still Kicking]

In SP class they told us to load those orings up as they act as a reservoir of grease to coat the wall of the body. I remember that advice distinctly.

 

[happy-diver]

Yeah they're in with the grease manufacturers, distributors, whoever makes the money people, the cream

Do the orings roll or slide the wiper does

 

[Nemrod]

I was hoping to get some clarification on how to properly prepare a MK25 Piston for reassembly.

The Service Manual states to "Grease the o rings of the piston by filling the cavity between the 2 o rings". When they mention cavity, are they referring to the groove the o rings sits in, or the space/gap on the piston between the two o rings? Seems like this is the only piston design that suggests this aggressive of a lubrication. I was hoping to get thoughts from more experienced technicians out there.

The "cavity" is the space formed between the O-rings and their respective grooves. Excess lubricant on the ambient side of the piston will only attract grit, sand, whatever.

 

[Duglum]

I'm doing it like they show in that video. Zero issues, all my Mk20 and 25 work great.
I'm not sure if that video is outdated or not, but it is an official (leaked?) Scubapro Video.

 

[GLiebmann]

I'm doing it like they show in that video. Zero issues, all my Mk20 and 25 work great.
I'm not sure if that video is outdated or not, but it is an official (leaked?) Scubapro Video.

Yeah, this is the video I saw also. Looks very thorough, just seemed a little excessive especially when you compare the recommendations from other Piston First Stage manufactures. For Example, Atomic and Hollis just recommend a light film on the main piston Oring and thats it. Even the MK2 seems to recommend Slightly Lubricating the Oring on the piston. I have always been taught less is more but evidently not in the case of the MK25.
I appreciate all the responses from everybody!

 

[Mobulai]

For Example, Atomic and Hollis just recommend a light film on the main piston Oring and thats it. Even the MK2

They all have one thing in common different from the mk20/25 piston head: only one oring
They have no way to create such a reservoir

Btw the piston stem oring/bushing technically has a similar lube reservoir between the 2bushings

 

[USdiver1]

there’s a middle (uncarved) channel that acts as a gland to carry some lube

it started becoming more of a habbit to also fill the gland with extra lube on that piston head

Some might even suspect it would provide more stickiness

only the minimal lube works (but don’t underlube it)

You pick your own flavor, and maybe ask for us at the seminar

Hmmm, great write-up, but the subliminal theme running through this MK 20 /25 rebuild would suggest you seek professional help. Nudge, nudge,

 

[halocline]

Yeah, this is the video I saw also. Looks very thorough, just seemed a little excessive especially when you compare the recommendations from other Piston First Stage manufactures. For Example, Atomic and Hollis just recommend a light film on the main piston Oring and thats it. Even the MK2 seems to recommend Slightly Lubricating the Oring on the piston. I have always been taught less is more but evidently not in the case of the MK25.
I appreciate all the responses from everybody!

I believer the general practice of applying extra lubricant to the piston o-ring refers to the HP piston o-ring, not the piston head o-ring(s). The reason for loading up the HP o-ring this to help prevent the harmonic whistling that can occur in those regs when air moving across the piston edge starts a vibration, kind of like a flute, and then the spring starts vibrating and it’s annoying. The excess lube tends to dampen the piston shaft vibrating.

There’s no reason to excessively lube the piston head o-ring on any of the SP balanced pistons. It’s a small pressure gradient.