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.

 

[halocline]

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.

I didn’t know that, I’ve never heard of a recall on the brass tipped piston, and I’ve seen plenty of them in Mk20s. I would guess that if you bought a MK20 new and had kept up with the warranty, and you brought it in for service with IP creep, they might give you a new piston. But otherwise this is certainly the first I’ve heard of free replacements for any brass tipped MK20 piston. It would be nice!

 

[Duglum]

It's true. I've already rebuilt and upgraded three Mk20 with the brass piston to Mk25s with brand new composite pistons and knob retainers. All of them for free. At least my dealer had no problems getting the parts from SP. I believe usually you or the dealer have to return the old/defective parts to them.

There is a Scubapro document from april 2002 called "Maintenance procedure for Mk20" that states:

3. Piston preparation:
There are 4 generations of pistons classified chronologically:
a) Stainless steel piston with one o ring and one anti extrusion ring.
b) Chrome plated brass piston with one o ring and one anti extrusion ring.
c) Stainless steel piston with one o ring and two anti extrusion rings.
d) Composite piston with 2 o rings and no anti extrusion ring.
All these pistons are equipped with a T.I.S ( Thermal Insulating System) bushing and it is
recommended to grease the piston before inserting the bushing to prevent the stagnation of water
between the parts. The last bushing generation is equipped with "blades" to cut through ice crystals
that could be formed on the spring during extreme cold diving conditions. If the regulator is going to be
used in such conditions it is advisable to upgrade the 1 st stage with the new bushing.
Of all the 4 generation of pistons, only the chrome plated piston should be replaced by the
latest configuration ( piston d.) ( composite piston P/N 10.600.155 )
Pistons a) and c) do not need replacement.

 

[rsingler]

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?

That flat speeds the seal of the main piston head oring. By allowing IP behind the oring, the oring displaces more quickly against the sealing surface in the cap.