TxHawk

iTrader: (2)

I have recently learned of a new way to perform a leak down. I have always used a leak down meter to test the leaking of the cylinder. Now i hear that the professional's take the valve cover off and tap on the
valve untill the leak down measure is exceptable. This seems like cheating if you have to help the valve seat to get your reading. What is the proper method of performing a leak down on a high performance engine.

My other question is what is an exceptable reading on the test? 20-30% is the most i would run without looking further, where others say 90% leak is normal.

Thanks for any of your input.

TxHawk

iTrader: (2)

FormulaZ- i have always understood it the same as you. I grew concerned when i heard that some professional engine builders (not stating any names) practiced this method. I posted to to find out more on the subject.

performancemary

All my cylinders came in over 20% so I am doing a rebuild on mt HP 500'S . However I called Mercury to get some idea as to what they found acceptable. The tech said they pass motors with up to 30% leak !!

All the motor builders I talked to said 6-8% is what you should have on a new motor or quality rebuild. Makes me wonder what is going on at Mercury.

jpclear

Well, that valve slams to the seat lots harder under normal operating conditions than it does when you roll it over by hand so I guess I can see what the "tapping" thing is all about. BUT, I gotta believe that if everything is REALLY right, You shouldn't need to "tap". I'll bet the builder "taps" if it's HIS build and he needs to show you that all's O.K. and doesn't when it's someone else's build and he wants to sell another job. --- Jer

Vinny P

I have always believed in leak down tests for proper testing of internal engine condition. I have never heard of anyone tapping on a valve while doing this test. If the test is performed properly, the piston should be on T.D.C. Tapping on a valve may crash the valve into the top of the piston. Besides, if the valve isn't seated after running the motor, what will tapping it once or twice do??

TxHawk

iTrader: (2)

I agree with all of you. I can see a builder covering his rear by making a few tricks for a good leak down. Im glad to hear that no one has heard of this, just sends this builder one more notch down my list.

edb07

I have heard of it and done it. A used engine, especial one that is cold and left sitting for a while, will have some valves in the open position. These valves and even some that are closed will have an accumulation of carbon and sometimes rust build up and it will also accumulate on the seats. The running of the engine seats the valve and this is why your leak down test is usually performed on a warm engine.

When you do a leak down on an engine that has sat and is cold, you remove the valve cover and bounce the valve down several times to simulate the running and re-seating of the valve (use rubber hammer). The pounding displaces the carbon and rust build up. If that is the source of the leakage, you will see a dramatic decrease in the leakage reading.

If you do it warm it is not necessary, but sometimes engines need to be checked post removal and there are no options available but to bounce the valve to get a truer reading. It does not make a worn out engine read within specs, it does tell you if your leakage is due to carbon or rust build up. You also must listen closely to see if leakage is detected in the intake manifold or oil pan (valves or rings).

Also, on a new engine with properly seated valves it would not be necessary, but remember that the rings would not be seated.

Ed B

TxHawk

iTrader: (2)

Thankyou, that is very helpfull.

Bob280Silencer

edbo7,Right on!My race car lays up during the winter.It will show as much as 30%,a few taps,2%!BOB