N231BN

That is interesting, does it have a reference line attached to the aneroid?

Rich, All "pop-off" valves are absolute pressure controllers, it makes no sense to design them any other way. However, I seriously doubt they intended it to be the primary regulator in a turbo system. It would be a very inefficient way to accomplish that as you are wasting a lot the energy used to spin the turbo. I obviously don't own one of those systems so I could be wrong on that but it doesn't sound right to me. Respectfully,

Did you call on any of those Ad's on barnstormers?

https://www.barnstormers.com/category-20608-Mooney--M20K.html Couple of choices here.

They are basically cavity drains and serve no purpose normally. Uses a regular crush gasket.

14 CFR 43 Appendix D paragraph (d)(3): Internal engine - for cylinder compression and for metal particles or foreign matter on screens and sump drain plugs. If there is weak cylinder compression, for improper internal condition and improper internal tolerances.

Now that I think about it, I was looking for a power setting that was WOT so I had the RPM down around 2250. Pure experimentation, trying to see if the fuel balance changed while WOT. I also don't think I have tried higher altitudes(15k+) since getting my Merlyn overhauled.

What RPM are you running?

Continental or continental, your choice. I've been getting overhauled steel cylinders when needed and have had good luck.

The density altitude today at FL210 was 23,760. I think Merlyn advertises critical altitude of 19k but seems like most get 20-21. I haven't had my 231 that high but I'm surprised you could get 30" while LOP. I'm usually full throttle at 15k while cruising LOP. What RPM were you running?

The "pop-off" valve pressure is absolute, it wouldn't be very useful if it wasn't. In addition to the other things you mentioned check the alternate air valve and of course check for exhaust leaks.

Another vote for Jet-Glo. Sherwin Williams makes some good primers too.

Bringing this post back, the number is there.

VLOC Preview

Do you have the yellow tags for all the parts? Realistically, at this point time SMOH is meaningless since the engine is over 12 years old. Whether it was an actual overhaul or just a repair and reassembly is moot. There is no requirement for a mechanic to log TSMOH. Sent from my Pixel 4a using Tapatalk