How do I find Bottom Dead center?

[chris91LX]

I got my leak down tester and the instructions say to warm up the car, remove the spark plugs, set the piston at Bottom Dead Center, loosen the rockers to make sure both valves are closed and hook up the gauge. Do I guesstimate BDC and let the air pressure shove the piston the rest of the way down or what? I thought I was just supposed to set the piston at TDC for this test, ain't it funny how nothing is ever as easy as it is supposed to be? Thanks for any info.

[88workcar]

Chris. set #1 at TDC. Mark the damper. Stick a screwdriver in the spark plug hole. Feel the top of the piston. Get some one to slowly roll the engine with a breaking bar and socket on the crankshaft bolt. When the piston gets back to TDC again, mark the damper again. Now go back to the center of the two marks. Hope you get it

[PKRWUD]

Written by 88workcar:
Chris. set #1 at TDC. Mark the damper. Stick a screwdriver in the spark plug hole. Feel the top of the piston. Get some one to slowly roll the engine with a breaking bar and socket on the crankshaft bolt. When the piston gets back to TDC again, mark the damper again. Now go back to the center of the two marks. Hope you get it

Actually, that won't work. Each pistons TDC mark on the balancer will remain the same. If you were to try the above suggestion, you will find that the damper has rotated 360 degrees, and the mark will already be there. The easiest way to determine piston position is to mark the outer pulley edge at the same spot as the TDC mark on the balancer, then, with a ruler from underneath the car, make a straight line through the center of the pulley, starting at the mark you just made, and mark the pulley at the other side (opposite from the TDC mark). Now, make two more marks that are 90 degrees apart from the two marks you have. You will end up with 4 equally spaced (90 degrees apart) marks on your pulley. Don't put extra marks on your balancer, as that may confuse things later. The only mark on your balancer should be the TDC mark for the #1 cylinder.

In order to find TDC for any cylinder in your engine now, start at the TDC mark on the balancer (check the rotor to verify it's on the compression stroke), and for each mark that comes up on the pulley, it is TDC for the next cylinder in the firing order. The mark at 180 degrees from a cylinders TDC mark is that cylinders BDC mark.

Keep in mind that your cylinder leakage will be much higher with the piston at BDC than at TDC.

Hope that helps.

Take care,
-Chris