MK10 squeals like a pig

[Six2Life]

I have a pair of MK10's that I just finished rebuilding. One is working great with a solid lock up at 135psi and is quiet. The other squeals like a stuck pig. Both have almost identical parts in them. All orings are EDPM 70's except for Poly 90's for the piston stem. The quiet one has a new SP "+" seat and no shims. The squealer has a "-1" seat with 2 shims. The squealer also doesn't lock up all that great with about 5-10psi of creep over 5 min or so. So far I have tried flipping the spring and heavily lubing the HP oring without any success. I also tried adding shims. The only variable I can think of that I haven't changed is the piston. Is it possible that the piston is causing the noise? I know it's possible that it is causing the IP creep.

Any ideas?

 

[apertyx]

Mine does it all the time when there isn't enough grease packed in during service ... seems to want much MORE than what the specs say. Once the techs pack it in it's usually fine. Been doing it for 5 years now, and never a problem breathing - just whistles away when it's dry in there.

 

[redacted]

You might try changing the location of the 2 washers. Assuming you have both between the spring and the body, try one on each end of the spring. I do have a couple Mk10 pistons if that does not help. I would be a bit more concerned about the creep with a new seat. Take a good look at the knife edge surfaces. I have had some success in cleaning up the knife edge and reducing creep. But I don't believe that is directly related to the noise.

 

[halocline]

I think he said the honking one had no washers. I would try switching the seat to the middle height (-1) and add a washer (shim) or two and see if that helps. Sometimes the plastic acts as a dampener between the spring and piston. I also would be a bit concerned about the creep. 10PSI is ALOT of creep.

Oops, I see you already tried adding shims. Didn't that bring the IP pretty high? I usually leave my MK10s around 125, definitely not higher than 130. They do seem to creep less that way, and there's plenty of flow.

Awap is, as usual, probably right about the piston edge. What I would do is check the edge with a jeweler's loupe, I bet you'll find some scratches and/or nicks right on the edge that you can't even see with the naked eye. You can polish those out with micromesh wrapped around a tapered stick of some sort, but you really need to be careful. Then I'd take a look at the shaft where the HP o-ring goes, and if there's a glaze, knock that off with micromesh as well. It seems to help with keeping lube on the piston shaft.

Or you can take the easy way and get a new piston, that will probably cure the creep and maybe even the honking.

 

[drbill]

I had a Mk10 that squealed like crazy about 6 years ago, but it turned out there were two cracks in the body. Since you serviced yours yourself, I'm sure that isn't the problem since you would have noticed them.

 

[Six2Life]

On the quiet MK10 I cleaned it, replaced the orings, lubed it, and reassembled it leaving the old seat in the retainer. Testing it with the old seat had it squealing worse than the one that I'm now having problems with. I then swapped the old flat seat with a new grey + seat and the noise was completely gone and it locks up great at 135. 135 is the middle of the spec so I figured it would be fine if it allowed me to use up a + seat. I can't help but wonder though what it was about the new seat that cured the noise. Was it the cone shape of the seat? I doubt the material would have an effect. Maybe the different seat height put the piston in a position where it does not resonate?

I tried swapping around a few used seats in the noisy MK10 to see if it would have an effect on the squeal but that didn't help. The shims are one on top and one on bottom. Although I think I should fix the creep problem before I worry about the noise. It looks like the culprit has to be the knife edge since the seat is new. I looked at it through a magnifier and nothing obvious jumped out at me. I'll give it a closer look and see if I missed anything.

 

[Zung]

You need at least a 15x loupe and good lighting and good angle to see the problems with the knife edge.

From my experience, scratches at the edge will cause your reg to creep, but I don't have anyone that "squeals", even though I've heard some would "whistle" with the flat seat.

 

[halocline]

The flat seats are known noise-makers, in fact I suspect that's a big part of the reason they were phased out. The conical seat has completely different air flow characteristics. I don't know exactly what those are (you'd need some sort of fancy software to graphically show it) but the basic idea is that air flows into the piston-really fast moving- through the tiny gap between the seat and piston edge. The conical seat puts quite a bit more space directly under the center of the piston, allowing the air to flow more easily once it gets around the edge; I'm guessing it slows down because it's got more space. This means less turbulence right at the piston edge, so (I guess) less potential for the piston to start resonating.

 

[Couv]

If you have a bit of time to play around with your regs, swap out the only pistons between the two regulators. If the problem follows the piston you have "convicted" the part. You might also get lucky and solve the issue. You may end up having to replace the hp seat too as they may engrave in different spots.

To resurface the knife edge see: Matt Magic thread: (refinishing the piston stem)
http://www.scubaboard.com/forums/re...6-scubapro-piston-there-life-after-death.html

Then, heed this warning: If you do the following, you will most certainly die! So when your life support equipment fails and you drown, don't come swimming to me for help....you've been warned.

See this thread http://www.scubaboard.com/forums/repairing-your-own-gear/398154-stuck-mk10-piston-cap-scubapro.html starting at post 35 for my insane idea of treating the piston stem by abrading it.

 

[Six2Life]

Well, the micromesh arrived from jolly ole England and I proceded to use it to turn my MK10 into a MK2. It looks like I flattened the knife edge of the piston too much which messed up the balancing of the reg. Now a 1000psi difference in tank pressure will change IP by about 8psi. Over the full range of tank pressure it will probably be close to 20psi. I did try fixing it by using some fine lapping compound with a tapered steel rod on the inside piston surface. It seems like it is helping but I don't think I have the patience to work it as long as it would need to get back to an acceptable condition. So I guess it's piston replacement time.

One thing I did notice though is that polishing the piston shaft with finer grades of the micromesh seemed to help the squealing a lot. After the polishing it was still there but much less noisy than before. It was quiet enough that I probably wouldn't have worried about it if the reg was otherwise in good shape.