Cirrus Compression

[bobden72]

Hi chaps my Cirrus is now complete and I have stripped it down to build it back with gaskets and O rings. All has gone well so far I made a jig to make the head gasket that went really well, and well worth the time. I was then reading an article on testing the valves, which involved making a blanking plate that fits on the underside of the head with a inlet for compressed air. The article says applying compressed air the pressure should hold steady. So I fitted the valve lightly oiled, pressure gauge fitted in the spark plug hole and the compressor set to 40psi and connected to the blanking plate. When I applied pressure the gauge goes up to 40psi then falls down to zero in about 5 secs leaking from the inlet and exhaust. So I applied some 600 grinding paste and ground the valves in again. When I was done all cleaned of and washed both the valve and the seat look a nice grey colour. Did the test again with the same failure.
This is my fifth engine I have built and all run but this is the first time I have done this test. What do you think should the pressure hold steady or gradually fall. As far as I can remember full size engines fall off, opinions please.

 

[Cogsy]

Just my opinion but a little leaking from brand new valves doesn't concern me much, as long as they're well made. Generally, when the engine starts to fire the combustion pressure does the final job of sealing the valves properly quite quickly and compression increases quite a bit very rapidly. Then after the engine is run for a while and the rings have also fully seated the compression goes right up to what it's supposed to be.
With such a small contact area it's very difficult to get the valves to fully seal without help from combustion pressure, although Terry Mayhugh does manage it with his builds.