Scubapro MK20 Maintenance Issue/Question

[rhwestfall]

I think everything should stay in place but it might not seal well on the used HP seat.

Fully rebuilt 14 months ago, and only 1 dive on it.....

 

[redacted]

question: I've got a MK-20 with the brass piston, and also have a composite piston sitting here to update it. I don't have any of the MK-20 tools, but with the flat edge (not knife edge) can I just pull it and replace the composite? No insertion or alignment tools.... (I don't have the bushing/piston tool at this time). Does the seat and retainer hold the bushing in place if it's all assembled?

Inserting the piston without holding the bushing/o-ring sandwich in place may result in one of those components being dislodged. If that happens, you should feel a little extra resistance as the piston passes through. As long as you don't force it, there should be no damage. Worth a try but may not work unless you kludge something to hold stuff in place. Perhaps a correct size straw along with the bullet tool would work.

 

[guruboy]

Inserting the piston without holding the bushing/o-ring sandwich in place may result in one of those components being dislodged. If that happens, you should feel a little extra resistance as the piston passes through. As long as you don't force it, there should be no damage. Worth a try but may not work unless you kludge something to hold stuff in place. Perhaps a correct size straw along with the bullet tool would work.

He plans to leave the HP seat side assembled and closed.

 

[rhwestfall]

He plans to leave the HP seat side assembled and closed.

correct (was hoping it had an advantage)

 

[redacted]

He plans to leave the HP seat side assembled and closed.

correct (was hoping it had an advantage)

I would expect the force from the spring between the bushing and the seat retainer to just make the potential problem of dislodged bits worse. But again, as long as you don't use much force, it should not damage anything.

 

[Tbar]

correct (was hoping it had an advantage)

I have done this to replace a piston to fix creep. There is a risk that you will nick the HP Piston o-ring. I am pretty sure this caused a failure on one of my regs but a few dives after the piston change. When I pulled the reg apart the o-ring was completely split.

 

[halocline]

In the MK20 repair guide from several years ago it does say that an alternate method of assembly is to put the bushings/o-ring in place first, try to seat them with some sort of rod (maybe a chopstick or the handle of an o-ring pick) then install the spring/seat retainer, then push the piston through with some lube on the piston edge. I have done it that way but now I have the tool and it's much better.

The surface of the edge just looked a little rough under magnification. I basically put a piece of 3600 grit micromesh on a granite countertop, then placed the piston edge down on the micromesh and spun it around with my fingers. Then I relieved the outside corner a bit, and tried to polish the inside bevel by wrapping some micromesh around a pencil tip to make a cone. I think couv has some fancy abrasive cones that he uses on the inside bevel. That probably works better, especially if you have a sizable nick in the piston.

If I were doing this regularly, I'd make a little clamping jig for the piston, and glue some micromesh to a really flat surface, so that I could precisely re-surface the edge. I don't know if it would be a real improvement in terms of lock up but it would make it faster and easier.