3. Poke the narrower end of the cable tie down the spark plug hole until it just touches the top of the piston. Bring the piston up to the top of the bore by rotating the crankshaft using a socket on the front of the crank, if there is no power. Or use the ignition key, or a remote starter, to briefly bump the motor over if there is power.
4. With the cable tie touching the piston, slide your finger down the cable tie until it is up against the spark plug hole. Carefully withdraw the cable tie from the plughole and measure from your fingertip to the end of the cable tie that was touching the piston-remember the measurement.
5. Rotate the crankshaft so the piston is at the bottom of its bore.
7. Subtract the measurement you made at step 6 from the measurement made at step 4.
8.You have just determined the stroke of the engine!
Obviously you can't use this method to accurately measure an engine's stroke, but it is close enough to tell one size motor from another. Even if you don't get the motor at exactly top dead center at the top and bottom of its travel, you will still get a good idea of the engine stroke length. Let me give you an example related to the abovementioned 302C versus 351C dilemma:
Say the measurement from the top of the spark plug hole to the piston at the top of its bore is 2 inches, and the measurement you get with the piston at the bottom of its bore is about 5.5 inches. What you have is a stroke of 3.5 inches and you are indeed looking at a 351C.
This will work for all engines (that I am aware of) that use a common block, and different stroke internals, like the Ford 429 and 460, but will not show the difference between engines that use the same stroke internals and use different bore sizes to change the engine capacity.
Cheers, mate,
Trevor Haines