Scubapro MK 20 Mysteries

[halocline]

I've just assembled the HP side, trusting the spring to hold it all in place, then carefully pushed the piston through. It worked, but if I was going to do a lot of them I'd want the tool.

Using a bullet with a piece of tubing that's the right size is a great idea.

 

[Zung]

Actually, I have both the bullet and the plastic insertion tools from Scubatools. I'm just being cautious because of the contradictory instructions from SP. I like the EB #267 way of fitting the swivel/cap/piston on the LP end with the 1st half of the bushing from the HP end, then the o-ring and the 2nd half of the bushing; I guess this is the safest. I'll try that out & report back. But first I have to order the Maintenance Kit.

 

[DA Aquamaster]

When you install the first bushing, be sure that the 4 little divots in it face away from the o-ring.

I prefer to stack the bushings and o-ring on the tool, then lubreicate them on the tool and install them all at once.

 

[Zung]

Thanks DA.
It's good to have a unanimous decision by all the guru's, and so all at once is what it's gonna be!

 

[Zung]

Another Q: does anybody have experience re-using the plastic bushings mentioned above, #01-700-105 & #01-060-607? SP calls them "Rings" and they keep the HP o-ring in place.

When I try the current ones on the piston stem, the one nearer the crown is tight, the one nearer the seat is loose, with maybe a bit less than half a mm (.02 inch) of play.

If that works, I have everything else to service this reg.

 

[DA Aquamaster]

The idea behind the bushings was to:

1. Reduce the incidence of pinching of the high pressure o-ring, and

2. Provide a means to renew the tolerances in this area.

With the Mk 5 and Mk 10, there is potential for the surfaces around the HP o-ring to either be on the loose end of the tolerance range and/or to increase due to wear in service. If the tolerances are too loose, the HP o-ring can extrude into the gap between the piston stem and the first stage body and that can result in cresent shaped pieces getting cut out of the o-ring, which quickly leads to leaks.

SP addressed this to some extent with harder 90 duro o-rings, but for a Mk 10 that has issue at higher pressures, the only real fix was to replace the first stage body.

With the Mk 15, 20 and 25 SP wanted a reg that could handle 300 bar / 4530 psi service pressures. To accomplish this SP moved to using replaceable bushings. The positive aspect of this is that it is now cheap and simple to renew the original tolerances and any wear between the piston stem and body is a non issue, as any wear will occur on the softer but replaceable plastic bushings. The downside is that you now have to replace the bushings on a regular basis.

How long they last is not something I really know. They last at least a year of even heavy diving so the number of dives is probably at least 150-200 and could be more, but that's hard to tell as they get replaced annually in most cases.

On any given day you can find a new Mk 25 annual service parts kit on e-bay and it would be cheap insurance to just get one.

 

[Zung]

Thanks DA, this truly is the voice of wisdom.
Over here in Europe, the kits can be bought fairly openly from the LDS, except mine has just run out. It looks like this is gonna be a skiing weekend

 

[Zung]

There're 2 little springs inside the MK 20 DIN version: one for the HP o-ring/bushings, 01-020-700, and one for the DIN air filter, 01-020-179, and I forgot to write down which is which! The smaller one is about 10 mm (0.4 in.) long, and the taller one is about 14.5 mm (0.57 in); both have a diameter of 9 mm (0.35 in).

Help please!

 

[DA Aquamaster]

The DIN retainer spring is the 10mm long spring and the one that secures the bushings is the 14.5 mm long spring.

You've just discovered the reason why you should lay the parts out on the bench in order like a schematic when you take the reg apart and at a minimum keep them sorted by sub assembly - at least until you are 100% confident what each part is what the exact relationship and order of assembly is for all the parts. It's by far the most effective way to learn that particular technique.

 

[Zung]

I stand corrected.
On my 1st attempt at a MK 10, i did as you said, and I took a picture of the whole thing. This is my second and I assumed prematurely I've reached the status of "old hand" and could afford some sloppyness
Thanks.