[gboutch]

I'm trying to do a leak down test on my motor. I have a hydraulic roller cam. How do I put the piston at bottom dead center? What rocker movement am I looking for? Once it's on bottom do I have to loosen the rocker arms up to proceed with the test? Greg

[oldbogie]

Originally Posted by gboutch

I'm trying to do a leak down test on my motor. I have a hydraulic roller cam. How do I put the piston at bottom dead center? What rocker movement am I looking for? Once it's on bottom do I have to loosen the rocker arms up to proceed with the test? Greg

Are you referring to a cylinder leakdown test or a hydraulic lifter leak down test?

I'm guessing you mean a cylinder leakdown test. This is a case where you need both valves closed, that will be on compression near TDC when both valves are (hopefully) tightly closed. The crankshaft must be jammed to insure the pressure doesn't push the piston down the bore. You don't need to do anything with lifter adjustment. That would only happen if you use the bottom dead center position for the piston and unwind the rockers to get the valves closed, that's one giant adventure as everything in the valve train has to be readjusted.

This isn't an easy test under any circustances it will be time consuming. The TDC is faster but has its obvious risks from crank rotation if it gets away from you. The BDC process is even way longer to do but is safer to body parts vehicles and yours.

Bogie

[gboutch]

What is normal cylinder leakage, I just tested all of mine using BDC and valves closed? Sounded like air was going past rings.

[cobalt327]

Originally Posted by gboutch

What is normal cylinder leakage, I just tested all of mine using BDC and valves closed? Sounded like air was going past rings.

Leakdown can vary quite a bit between an engine that's cold or if it's been brought up to temperature before the test was done, and by the piston being at BDC (where the wear is less as opposed to at TDC). Oil being squirted into the cylinders (like sometimes done during a compression test) will temporarily decrease leakdown past the rings.

But generally speaking, 10% is the max I'd want to see- that's at the edge of needing some serious work. Like a compression test, you'd want the readings to be close to each other, if one were to be outside the others could be an indication of a more serious problem w/that cylinder.

[oldbogie]

Originally Posted by gboutch

What is normal cylinder leakage, I just tested all of mine using BDC and valves closed? Sounded like air was going past rings.

A competition engine should be 1-2 percent

A street engine in good condition should be 10-15%, less is always better.

A street engine in worn but servicable condition 15-20%

A street engine showing 20% or more is reaching the overhaul point.

The difference range between the low and high leak cylinders shouldn't exceed 4-5%

Leaking past the rings is expected and normal if the engine has gapped rings as threre will always be some leakage through the ring gap even if nowhere else.

Testing at BDC gives the best reading you'll ever see as at TDC there is usually more bore wear from where the operating pressures are the greatest and the lubrication the least.

Leakage past the exhaust valves needs to be addressed as when operating that leakage is not compressed air but flames hot enough to erode the seat material which means this situation only gets worse.

Older model engines with low silicon cast pistons will leak more than newer engines with hypereutectic amounts of silicon because the hypers have a tighter clearance which keeps the rings better aligned to the cylinder wall. This can also be said for forged pistons where in general forgings have more clearance than cast so the rings don't address the wall quite as well and within forgings the 2618 alloy needs wider clearances than the 4032 so one would expect the 2618 to show more leakage again because of ring alignment to the cylinder wall. All the for-going assumes a lot of other things have been done correctly in terms of optimizing skirt to wall clearances for the piston materail type and having the correcct wall finish for the type of ring material i.e. straight cast, chrome plated, moly filled or plasma sprayed. So this is a gross generality of what to expect from a pretty new engine.

Bogie