Finding TDC for leak-down test

[Diastole]

Thanks to all. So, in summary

I was also IM'd the following response

Check that the distributor rotor is pointing at plug wire for the cylinder you are checking. That will confirm TDC for the power stroke, valves closed. The piston is also at TDC between the exhaust stroke and intake stroke, valves not shut, varies with the overlap specs of the camshaft.

Cylinders #1 & #4 are in phase while cylinders #2 & #3 are in phase
(both up at same time and both down at same time)

So the dimple is TDC on #1's compression stroke and #4's exhaust stroke.
Then I turn the rear tire in 3rd gear 180 degrees to get TDC on #2's compression stroke and #3's exhaust stroke
Then I turn the rear tire in 3rd gear 180 degrees again to get TDC on #4's compression stroke and #1's exhaust stroke
Then I turn the rear tire in 3rd gear 180 degrees again to get TDC on #3's compression stroke and #2's exhaust stroke
Each time verifying the rotor is pointing to the cylinder in question. Where the rotor points, the cylinder is at its compression stroke.


Many thanks,
Diastole in the Smokey Mountains

 

[ModelA29]

I don’t know if a Model A engine will start making complete revolutions with compressed air piped into one cylinder, but when doing leak down tests on airplanes, an assistant has to hold the propeller for dear life as it tries to turn. If the piston moves off TDC it takes a lot of strength to hold the prop. If it gets too far down the stroke, it can’t be held in place.

It won't make complete revolutions. Once it's at the bottom of the stroke the air pressure holds it there. Normal engines you can pretty much check at TDC or BDC they have straight cylinders. Some aircraft engines have a tapered cylinder (choked) that gets smaller at the top causing the ring gap to narrow allowing less blowby. Some say once they come up to temp they are straight (remember they are air cooled and the head is shrink fit onto the cylinder). This is why checking at TDC on aircraft is the accepted norm. The high dollar drag racers do a leak down test between rounds to see if anything is broken/bent. If they get a low reading the engine gets a 20 minute in frame overhaul. A lot can happen to a piston in 1/4 mile......