Who knows old Big Twins

The goal is a well done engine under a Lark IV cowl. The question is what should the compression be? I now have a '61 & two '62s, all 40 hp. All of those cylinders read between 89 and 93 PSI checked with two gauges. Apparently OMC never let compression #s out, but somebody knew back in the day. I've read here and there anything between 90 and 130 with 100ish being acceptable. I need a # to shoot for before I start tearing down 3 to make one. On a 4 stroke if you know the compression ratio you can find the proper compression with a bit of math. But it seems jonnyrude didn't let that info out either. Anybody know? Gonna drive me to Merc, how can you build a proper engine w/o the basic info!!!

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No numbers were ever listed.
Your numbers sound pretty good. The general rule on these is that they need to be within 10% of each other.

Johnny said:

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https://www.aomci.org/ Dedicated to the Preservation and Restoration of Antique Outboards



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Click to expand...

Yep, checked the archives and posted the question last week. Some good sound advice but if anybody knows the #s they're not fessing up. FREE BEER for anyone that can give me the answer! Otherwise I'm gonna have to strip one down, check / mic the bores and re-ring just to see what it can give.

Pappy has posted some very sound advice over the years
about reviving old motors. Flush with OMC Tuner, then
run a tank or two of 24:1 mix. (time and patience - time and patience).
Some of these older motors have never been opened up. So there
is no telling what the past owners have or have not done.
Always approach each project as if the previous owners knew
zero about maintenance and upkeep.
When you say you want a "well done" motor - exactly what is it
that you are expecting ?
Just refurbish it to the best of your abilities - then after running it for
awhile and you are still not satisfied with the performance, then,
tear it down completely and rebuild it from scratch yourself.
90-100 compression is decent for a 56 year old motor. (IMO)

For me a "well done engine" is sound on the inside and tidy on the outside. I "expect" the best I can do with what I have to work with. Pappy, I took your advice seriously. Removed the side plates, carbon but what shows looks OK. I just had to remove the head. Its true & waterways look fine. Bores look great, surprised there's no ring grove up top, is that normal? No tool marks or paint chips on the bolts or screws on the block or head to indicate its been apart. I won't go any deeper unless you recommend crank seals or ?????