Leakdown?

[articfriends]

Heres my helpful suggestion that I have said before: After dynoing/breaking in a new motor do yousrself a favor and do a leak down test,if you results are screwed you won't be scratching your head for the next 2 years. Record these numbers for reference and at least once every year repeat the test (berst time is in fall before putting boat up). If you find a bad cyl you will have sufficient time/warning to get it fixed. You should always do a compression test first too,If your real lazy you can just leakdown test the cylinders with the lowest compression readings (as long as you have a baseline to compare it to from your ORIGINAL leakdown test),Smitty

[baronbob]

Leak-downs on aircraft specify taking same on the up-stroke near TDC. The primary reason is that the cylinder grind is tapered and tightest at the top. the reason for up-stroke relates to ring seating. the pressure against a properly seated ring is most representative. If you stroke backwards the rings may not be seated. Back to airplanes, you bring the prop up-to but not past TDC. You can do the same action with the big bar referred to earlier. BE CAREFUL Start @ 80#s and look for readings above 70#s. The idea is to have all cylinders readings close to one another, say +/- 5#s. If you have a suspected cylinder, listen for the leak in the intake, exhaust or the crankcase.

[Chris Sunkin]

Originally Posted by Budz Motorsports

A leakdown is a very accurate way to check valves and rings. We have done a few hundred leakdowns here and I have never heard of doing a leakdown in three positions! .

Had a motor down on power once. Turned out that on two cylinders the spirolocks had come out and scored the walls. The walls weren't scored up top though. Doing leakdown at TDC wouldn't show it. Doing it where the damage was done told us right where to look.

You also can't gauge bore wear very well by just doing it at TDC. The piston moves fastest at 90 degrees of crankshaft rotation and as such, more wear occurs in the center of the bores. Add in that the Chevy doesn't exactly have long rods and rod angles aren't optimum and you're definitely going to find ovalled bores on anything with hours on them. Again, leakdown at TDC won't tell you that you have several thousandths more ring gap at mid-bore.

The best part is it only takes a moment once you're already set up on the bore.

[Chris Sunkin]

Originally Posted by baronbob

Leak-downs on aircraft specify taking same on the up-stroke near TDC. The primary reason is that the cylinder grind is tapered and tightest at the top. the reason for up-stroke relates to ring seating. .

I know nothing of aircraft or how they do their bores... in modern automotive applications the bores are very finely honed and the rings all come pre-lapped and virtually always moly coated. There is virtually no "wear in" anymore. Many years back, you used to put a fairly rough cross-hatch on the bores and then "break in" the plain iron rings. If you look at the bores on a modern engine after a significant amount of hours of operation, you'll still see the hone cross-hatch very clearly.