I know that when you do a leak down test the cylinder being tested should be at TDC of compression stroke. I know one way is to feel for pressure by placing your thumb over the spark plug hole and feeling for pressure while someone else kicks the engine over. My question is is it important for it to be at exactly TDC or close to it?? The only purpose is so that both valves are closed and all the presser being introduced into the cylinder will be contained provided the rings and valves are all good, correct??

 

yep all you're really after is to have both valves closed

 

Another reason is that as a cylinder wears, there is commonly more wear at the lower end of the stroke (taper), so rings fit tightest (with less gap) at TDC - where maximum compression pressure is achieved. A cylinder with its piston at TDC will also present a smaller reservoir to the tester. The percentage of leakage may be cushioned slightly by the excess aircapacity of a cylinder at BDC, possibly resulting in an inaccurate test.

And if the piston is not at TDC when the air pressure is applied, it will be driven downward by the force of the air. As long as you are within ±5° or TDC, the test should be O.K.