Mk10 air leak ambient holes

[Candiru]

I hap previously posted about the mk10 with the ip creep a couple posts down the list. I got bored today and decided to put in my new piston along with a -1 seat and a shim. The ip is 135 after break in with no creep. I did notice after an hour that the ip was actually decreasing. I'm thinking leak at this point. I filled up tub and sure enough a small leak out of the ambient chamber. I assume that this is the 010 oring in the recess or the oring on the piston.

---------- Post added December 1st, 2013 at 12:36 PM ----------

Replaced the hp oring in the body and the oring on the piston, reassembled, still leaking. Is this symptomatic for scratching the body?

 

[herman]

It could be a bad piston head oring or a scratch somewhere on the piston or a scratch inside of the stem oring area or the incorrect HP oring (too soft or wrong one)

 

[Candiru]

I put the original piston back in with a new oring. No leak. I wonder if the new piston is ripping the oring when I insert it. I'm using a bullet by the way.

---------- Post added December 1st, 2013 at 01:34 PM ----------

The oring is 010 Duro 90

 

[redacted]

Replaced the hp oring in the body and the oring on the piston, reassembled, still leaking. Is this symptomatic for scratching the body?

I would re-install the old piston. If the leak stops, that would be a rather clear indication of a problem (damaged sealing surface) with the new piston. If the leak persists, that would suggest a damaged sealing surface on the body. If you can licate the damaged sealing surface, you may be able to buff out the damage (scratch) enough to stop the leak.

edit: Crossed posts. Inspect the old o-ring for damage. Re-install the new piston again. If no joy, go back to looking for damaged metal seating surfaces on the new piston, It appears to body is OK.

 

[Candiru]

Yes the great deal on a new piston on eBay turned to be not so great a deal. There is clearly a scratch running down the side of the "new" piston. It's not at all obvious but is visible with my magnifying glass. It's probably a micro mesh job. Uhhgh. Oh well, at least the old piston randomly stopped doing the ip creep.

 

[Diver0001]

Inspect the o-ring for size as well...

 

[halocline]

If your IP was dropping, that's not a leak at the HP o-ring. That o-ring separates HP air from ambient. The only o-rings that seal IP air on the MK10 are the piston head, turret retainer, and the big body o-ring. (And of course port plugs/hoses)

So if replacing the HP o-ring stopped the leak, your IP was dropping for some other reason. I will say that on several MK10s I've worked on, if you pressurize and leave the IP gauge on, let it sit for a while, the IP does sometimes drop a few PSI. I don't know why. But it's not a leak like you see the needle moving down until it drops enough to burp in some HP air. That sort of leak would either be in the o-rings I mentioned before or in the 2nd stage. A not-uncommon place is the tiny o-ring in the 2nd stage balance chamber on the G250 et al.

 

[Candiru]

there were bubles coming from the ambient holes that stoped with a different piston. visual scratch on piston.

---------- Post added December 1st, 2013 at 07:16 PM ----------

seems to be the most likely culprit.

 

[halocline]

there were bubles coming from the ambient holes that stoped with a different piston. visual scratch on piston.

---------- Post added December 1st, 2013 at 07:16 PM ----------

seems to be the most likely culprit.

Right, I got that. What I was saying is that the leak at the HP o-ring, whether it was caused by a scratch in the piston or a bad o-ring, doesn't have anything to do with your dropping IP.

But, regarding the piston, it's worth getting some micromesh and trying to polish the scratch out. Depending on the depth of the scratch you'll either be able to fix it or not, but it sounds like you don't have anything to lose. Micromesh starts at 1200 grit and goes up to 12000, works great wet or dry.