Briggs 402437-0666-01.
I have had this engine re-bored and piston .020 over. Valve job was done. Compression has been weak since rebuild. It was only run briefly. I checked compression and got numbers in the low to mid 50lb and thought the rebuild was done poorly.

After much investigation and testing: does this engine have an intake valve compression release built into the cam lobe? I have a briggs manual for this engine...not a word.

I checked the valve action with head off and find that the intake valve stays open a few thousanths until very near TDC. This seems to tell me the compression release cam lobe "system" is at work. (no spring, arms, etc. on camshaft.).

How does this not effect compression/power at operating speeds?

I put a bolt in the PTO end of crankshaft and spun the engine backwards a few turns with a variable speed electric drill with a socket on the end. the compression read 120lbs. (cold) in both cylinders. In normal operating direction, checking compression with engine starter, I only got around 55lbs. Does this confirm an "intact compression chamber", and that I should put the engine back in the tractor and cut grass?

any briggs bulletins or explanations of this system? any list of which engines do or do not carry this compression feature?

Many thanks for help!