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Now that I’m doing further work on the engine and still waiting for some tools and parts, I started with a leak down test today to check the rings of the pistons.
But…
- A lot of videos say to use 100 psi. Is 100 psi not too much for the ‘old’ engine?
- When I put #1 on TDC with the piston stop (plug) tool while looking at the valves and then use of the leak tester with some pressure (60-70 psi), the engine wants to turn… I have too hold the crank in place with a braker bar and the crank socket. Is this normal? It really feels like a lot of force.
Nothing wrong at all with anything you're describing.
The engine will try to rotate when the air pressure is applied.
The 100psi isn't too much when you consider how much a cylinder's volume is compressed by the piston for the compression stroke when running -- 9:1 CR = ~130psi.
100psi is an easy and convenient number to use to calculate the % loss. With 100psi input, 80psi on the "output" gauge is a 20% loss, 40psi is a 60% loss, etc. If you use other input psi, you've got to do a little calculating to find the percentage loss.
!00 psi is OK; But anything significantly less may not reveal leakage.
Perhaps piston is Not Exactly at TDC; perhaps it's slightly over center ?
FWIW, most OE sbc pistons (and many aftermarket replacement-grade too) have a slightly (about 0.030" to 0.060") offset wristpin.
This offset designed to help minimize "piston-slap" noise and stress to skirts.
This offset results in a slight difference to where and how the piston "dwells" when at / near true TDC.
This may require You to position piston just a tiny bit before Or after "apparent" TDC in order for it to stabilize in bore, when filling combustion chamber to 100 psi.
You may find further insight into & explanation of these matters if/when you study Pistons' and Bores' "Major and Minor Thrust".
Your piston does Not HAVE to be EXACTLY at TDC when performing leakdown tests; But it does HAVE to be stable in bore.
*Lastly, do be careful ! 100 psi can slam a piston down and twist a wrench with enough force to seriously injure you.
Leak down test on a 350
