Here's how Cummins does it on the 6BT's.

Measuring Blow by, The Real Test For Internal Engine Condition - Seaboard Marine

Quite similar to the plastic bag method but just a little more "professional".

The old school method was to remove the dipstick, place a tightly fitting hose over the tube, drop the end of the hose into a bucket of water and observe the bubbles. The amount and frequency of the bubbles would give you a pretty good indication of the amount of blow-by.

Small, steady, slow bubbles meant good
Erratic, uneven bubbles meant at least one weak cylinder
Frothing like an Alka-seltzer meant you have effectively created a 2 stroke from a 4 stroke design.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
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