Whine in a Mk10

[Tanks A Lot]

The whistle/whine is almost guaranteed to be caused by the spring and piston. I'm leaving the distinct whistle of older flow through pistons without a concave seat aside.

Fiddling with either, e.g. turning the spring end to end or rotating the piston, oftentimes cures the problem. I had stubborn ones, where this did nothing, just like in your scenario. I believe it is due to the combination of spring and piston, with special consideration towards the mass of the two. That may be why slightly polishing the spring ends did the trick.

What has done the trick without fail for me was leaving the system pressurized without any air flowing for a few hours. It's a trick I picked up from an ancient Sherwood manual and even though I was skeptic at first, it worked without a fail so far. I'm waiting for the day when this does fail me...

 

[Bloody Knuckles]

Well, @halocline , you're absolutely right: red herring.
Had two old Mk10's in the shop last week and  both whined at 3000 psi, but only one had a tiny squeak at 1500.
Traded the orange washer for 10x2 - no change
Rotated the main spring 1/2 turn. Changed seats and added shims. I started by trying to keep track of each step I took. But nothing worked. It was so frustrating by the end that I can't honestly say what did it for both regs.

It was clear that getting the IP I wanted was part of the harmonic issue. They both started over spec at 148 & 153.

I  think that flipping the spring over helped. I think that polishing the spring ends helped. And I think that settling for a slightly higher (138) IP than I had planned helped.

But it wasn't my magical o-ring.
Thanks for the skepticism, Matt. How it is I've never seen it before and now three in a row is telling me something. Maybe it's the switch from thick silicone in the old days to slippery T-71.

Well Rob, I'm glad I'm not alone in regards to your frustrations...I had the same discussion with "Halo" last spring in a DM exchange. To a tee, I experienced the same symptoms and same frustrations with two of my MK10's. I swapped springs end for end and reg to reg. Swapped pistons, replaced the seats with variable heights, installed all new O-rings many times over with varying durometers, utilized shims on one end, both ends and no ends. Micro-Meshed the hell out of all the components in contact with each other. Regardless the end result was the same, a varying degree of whistle at supply pressures above 2,000 PSI. Not sure what the magic bullet was, but eventually some combination or luck of the draw helped reduce the whistle. Problem solved? IDK? I hung e'm up and broke out the MK5's. But this much I can tell you. All I use is silicone so I don't think the "slippery T-71" is the culprit. On a side note, one of the options we discussed was SPEC utilization and Halo indicated he had good luck mitigating the whistle when packed with Tribolube. Possible fix...IDK? But alas, we come full circle back to the mythical SPEC boot dilemma and utilization on a MK10...Have you cracked that thing open yet???

 

[rsingler]

But alas, we come full circle back to the mythical SPEC boot dilemma and utilization on a MK10...Have you cracked that thing open yet???

Haha! I keep meaning to...
Too many things going on. But I will, and I'll share all the pics.