Scubapro MK15 serivce

[halocline]

The other white bushing should come out and get replaced, or at least very carefully cleaned and inspected. The HP o-ring sits against that bushing and that’s where all the pressure goes. I have several of them, but since I’m in the process of moving across the country, it will be at least a month until I can get to them. The brown bushing and metal spacer are ok to reuse, they don’t do much other than keep the piston alignment correct, I guess.

There are some of these that have no shims, but look carefully in the ambient chamber and on the underside of the piston head. The shims are very thin and easy to miss.

 

[rnln]

Thanks Halocline, thanks Couv, hi everyone,

I finally took my MK15 apart, cleaned all parts well, and found that there are pitting/scratching on the cap/ambient-chamber (#20) and piston base (#15). I run my finger nail through it slowly and I do feel it. Do you think these can be reuse or trash?

Thanks

 

[Couv]

Those surfaces can be restored with micro mesh. All DIY guys and gals ought to have a micro mesh kit. https://www.amazon.com/gp/product/B000H6HIK2/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1

 

[fmerkel]

I've not tried micromesh on that, nor have I had to deal with the piston pitting. Got worse scoring on a MK20. Took some fine grain wet and dry to it on the end of my finger, then used a dremel and polishing compound on it. Plating is long gone. Doesn't leak at all.
The MK20 does have the double piston O-ring. That may make a difference. I don't think the air will migrate under the piston O-ring, but that's conjecture that you would need to test.
I'd try cleaning it up, and doing a full pressurized dunk test after assembly. You'll be able to see if it's leaking.

The MK20 I have has a tendency to nibble the piston HP O-ring. I know it needs replacement when it starts to do a slow bubble out of the ambient chamber., usually 40-50 dives. Put in a polyurethane O-ring last overhaul and that seems to be holding up better. Did the same replacement with the MK15 but that was because of 'honking', not leaking. The regular SP O-ring was nearly new. Honkiing went away. Not sure if it was the poly O-ring, better lube, or just luck.

 

[Couv]

The MK20 I have has a tendency to nibble the piston HP O-ring.

Put in a polyurethane O-ring last overhaul

The nibbling is due to extrusion of the hp o-ring. If you've tried a hard Viton seal (duro 90) and it still extruded, then yes, the PU seal was the right way to go. PU is known for it's resistance to extrusion.

 

[fmerkel]

The nibbling is due to extrusion of the hp o-ring. If you've tried a hard Viton seal (duro 90) and it still extruded, then yes, the PU seal was the right way to go. PU is known for it's resistance to extrusion.

I had a number of problems. One was not getting the gray support rings in correctly. Guess I had a 50:50 chance of getting it right. The manual is not very useful for that, especially the one without the shoulder. Got that straightened out with help from this forum. Was using the stock 90 duro SP O-ring.

Made me appreciate the MK5 & 10 system HP O-ring system more.

 

[rnln]

Thanks guys,
By eye balling, the piston is SS but the cap looks like brass(?) (looking at the color at the scratch mark). If I polish them, should I have to use thicker o-ring), assuming if I can make it perfectly flat/round?

 

[Couv]

Thanks guys,
By eye balling, the piston is SS but the cap looks like brass(?) (looking at the color at the scratch mark). If I polish them, should I have to use thicker o-ring), assuming if I can make it perfectly flat/round?

Correct on the SS and brass.

Use the same size o-rings. You're just going to lightly abrade the sealing surfaces-if they go out of round you've spent way too much time, effort, and sandpaper on them. Polishing both surfaces should not take you more than 5 minutes including clean up.

 

[fmerkel]

Don't be surprised if you see more brass in the cap with cleaning it up. Plating is not thick. Mine has no trace of plating in the piston wear area, pure brass showing. I use the stock O-ring and it's fine.

 

[rsingler]

Don't forget the advantage of mirror polishing the piston shaft in the area where it meets the HP oring.
An old wives tale is that a satin finish will retain traces of lube longer than a mirror polish. Even if that's true, the microabrasion of a satin finish on the HP oring will markedly decrease its lifespan.
And do you really think the lube is well retained after a thousand back-and-forth strokes? That's two 60 min dives at 8 breaths/min.

For me, it's a pinch of a tiny square of 6,000, then 8,000 then 12,000 grit Micromesh over 3 min before inserting the piston.