Skip, that's very interesting, about the valves warping a little from
standing open hot. Many years ago I recall some fellas I knew with hot
Harley drag bikes had so much aggravation from the valves warping after
a hard blast down the strip with their fuel bikes, they had a practice
of jamming rags in the open pipes they used, as soon as they shut them
off. They didn't remove the rags until it was time to run the bike
again.
Ardun Doug pointed out to me a while back that when leakdown testing,
small movements of the crank to change the piston position of a given
cylinder minutely, around the TDC where you are supposed to test it,
make big differences in the leakdown rate. From nothing more than tiny
shifts in the lie of the rings on the cylinder walls and in the piston.
I've found the same thing. I believe that any running of a perfect
valve is going to create a little bit of seepage, and the rings have to
leak a little even if they are "Total Seal" type. Don't forget the
leakdown test is a static test, and doesn't entirely prove how much air
will be trapped in the chamber under running conditions. It takes
"time" for leakage to occur and there isn't much of it. I'm told the
FAA is pretty lenient about the leakdown rates that FAA-licensed
mechanics can approve on piston engines at periodic inspection
intervals. I think you have to combine leakdown testing with
compression testing to get a feel for what's going on in your cylinders,
but true, if you have 30+% or more leakdown, it's too much. It needs
some work.
Cheers Bill
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