Proper leakdown test procedure

[Honus]

A search of the threads on leakdown tests reveals a split between those who do leakdown tests with the piston at TDC and those who do it at BDC.

For those who do the test at TDC, how do you ensure that the motor doesn't spin? I am particularly worried about spinning the motor backwards if I don't get the piston precisely at TDC.

For those who do the test at BDC, how do you ensure that both valves are fully closed during the test?

By the way, the factory service manual says to do it at TDC and to make sure the engine doesn't spin. Unfortunately, it does not say how to keep the engine from spinning.

[dmorrison]

Ive done a leak down test on my sons 65 Mustang.
1. A leak down test at bottom dead center???? Makes no sence. The valves are open. so do it a TDC.

With the piston at TDC both valves should be closed. Hold the crankshaft steady with a breaker bar and a socket on the front bolt. You not putting a lot of pressure on the piston. 100PSI may be on the guage but the hole that releases it is very small to it is quite easy to hold. Apply the pressure and read the guages. IN the 289CUin V8 a 80% second guage reading was considered OK. Same should hold true for a diesel.
Listen for an air leak around the valves and the valve cover oil filler opening.
for intake valves listen to the intake manifold. With the air
e filter off. For the exaust you will listen around the turbo. for the rings listen and "feel" for air around the oil fill opening.
If the second guage shows 80% of the first guage then they are probably ok.
a final diesel consideration is to look for any leak around the injectors or glowplugs, depending on where your connecting the leak down guage.

Dave

[Honus]

Thank you for the reply.

TDC makes sense to me. One of the recommendations I read in favor of doing the test at BDC said to remove the rocker arms, which would ensure that the valves stay shut. That seems like a lot a trouble, though.

Someone on this board, I think it was leathermang, recommended doing the test at BDC because 100 psi in the cylinder puts an enormous amount of force on the piston. For a 617 motor, 100 psi works out to about 1000 pounds on the piston. On the other hand, if you get the piston reasonably close to TDC, that force will result in a small amount of torque. If you get it within, say, 10 degress of TDC, the torque from 100 psi will be in the neighborhood of 65 or 70 ft-lb, which seems managable to me.