if all the valves are closed does the piston need to be in the TDC position to get true leak down test results? Should there be a total seal even if the piston is at the BDC position? The rocker arms are off the the motor and I would like to do the test without having to rotate the crank to get TDC on each piston. thanks

 

It is best to do the test with the piston at or at least near TDC; since leakage there will have the most effect on how the engine runs. It is also possible for there to be damage to the cyl wall (vertical scoring in particular) at the top, but not at the bottom.

If the rockers are off, you should be able to do 2 cyls at each TDC position: 1 & 6, 8 & 5, 4 & 7, and 3 & 2.

 

thanks guy for the info.

 

If the piston is not close to TDC, the air pressure will push the piston down.

 

even if it is down as long as the valves are closed will the test results be correct?

 

The test results will be correct as long as the valves are closed.

 

when you apply pressure to the cylinder if its not all the way up or down.

 

The test results will be "correct" if the piston is down in the bore, as far as that goes; the only thing is, what you'll be measuring, is irrelevant. What matters is the condition of the bore up at the top, where the piston is when it's doing the work. And you can only measure that with the piston up at the top.

In fact, you could even use the difference between the results with it at the top and with it at the bottom, to assess the overall condition of the bore, and of the rest of the cylinder's seal. For instance, if you got great leakdown with it at the bottom but terrible at the top, you could conclude that the important part of the bore (at the top) is wiped out, but the valves seal well. Or if they were bad both at the top and the bottom, the valves or rings are probably leaking. Etc.

But if you want one single go/no-go type of indication, do it right.... with the piston at the top.