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]

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

Oh that specific mk20 needs to be retired — or as per SP new ambient cap, new piston, and new main spring; it served for 25 year daily diving at a shop in Dahab
It was a futile exercise in how far restoration efforts can reanimate it (it had sentimental value to its owner)

 

[Nemrod]

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

As does the second stage balance chamber and poppet stem with dual O-rings.

 

[USdiver1]

Last year, I rebuilt a MK20, the old piston was chromed brass, except for the tip, that was only brass. Was it made that way by SP, or had the chrome worn off over the years? I don't recall exactly but I think it was made around 1997/98.

 

[CG43]

If the piston has two O-rings, there are considerations to fill the gap between the O-rings with grease.

1.) The first O-ring keeps dirt out.

2.) Without enough grease, the pressure between the two O-rings remains at about 1 bar even when diving deep. This means that the inner O-ring (towards the IP side) must seal against IP plus ambient pressure.
In the case of a single O-ring, this would not be the case :
Similarly, if the gap is almost fully filled with grease, the ambient pressure is transmitted through the deformation of the O-rings and the grease. In any case, there will be enough grease for good lubrication.

Even dynamic O-rings can get old under low stress (pressure) .
I have three simple (non-compensated) piston first stages, the oldest being at least 56 years old (8 bar IP).
So far, it has not been necessary to replace even one piston O-ring of the three stages.
One cannot expect such a service life from a dynamic O-ring that has to withstand 200 bar sealing.
I do not mean to say anything against maintenance here, but there are more or less maintenance-intensive stages, and of course the surfaces that the O-ring seals against must be in good condition through adequate maintenance .

 

[Still Kicking]

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.

Rene said it was to increase the likelihood that the wall of the body will remain coated with a layer of grease between service intervals. IDK if that's a reasonable expectation but that's what he said.

 

[Pearlman]

A little OT, But now that we are discussing this topic - I followed that video to the T when servicing mine, but now I just remember seeing something that I forget - what is the purpose of the air access flat shown in the video?

 

[Mobulai]

As does the second stage balance chamber and poppet stem with dual O-rings.

True, and I’m not debating the part whether we should or not slober all that lube when we have wiper oring configurations; but the impossibility of recommending the excessive lubing and creating a reservoir with only a single oring (like the old poppet)

Last year, I rebuilt a MK20, the old piston was chromed brass, except for the tip, that was only brass. Was it made that way by SP, or had the chrome worn off over the years? I don't recall exactly but I think it was made around 1997/98.

Like this one

These were the flared tip pistons before the composite ones (that were recommended as a replacement)

That’s how it looks, no chrome loss

 

[halocline]

Last year, I rebuilt a MK20, the old piston was chromed brass, except for the tip, that was only brass. Was it made that way by SP, or had the chrome worn off over the years? I don't recall exactly but I think it was made around 1997/98.

That’s the middle generation MK20 piston. The rest of the piston is stainless steel, none of the pistons are chromed brass. That brass-tip piston was terrible and should be replaced at servicing. The only thing available new is the composite piston (plastic head) which locks up great and generally performs the best of all three generations of MK20/25 pistons. But….there is an internal o-ring which is not officially user-replaceable and eventually will fail, causing the piston to leak. I know someone who did replace it successfully (it seals the plastic head to the steel shaft) but the approved SP method is to replace the whole piston at, I’m guessing, $80-100.

 

[USdiver1]

Composite piston used in the rebuild. I also upgraded. My MK20 is now a MK25.

 

[Duglum]

If you have the brass piston you can still get a new replacement composite piston for free from Scubapro. Same with the knob retainer if you still have the old one from the recall before 2006.