John Simic:
I’ve read a few posts about valve leakage with normal compression including “waxing & waning”, associated particularly (why) with Continental. I’m assuming valve ‘seat’ leakage. Guessing it has to do with a worn guide, or stem? How does this occur? I don’t understand how compression can be ok and yet have leakage, that can also wax & wane, no less.
Technical Editor:
Continentals do not have sodium-filled valve stems. As a result, the exhaust valve heads tend to run hotter than on Lycs, and more heat transfer has to take place between head and seat, as opposed to going out through the valve stem as it does in Lycs. There are pros and cons to both approaches, but the short story is that, in most cases, the Continentals have a shorter exhaust valve life span, particularly on the large-displacement engines. Since the IO346 uses the same cylinders and compression as the IO520, it basically experiences the same stresses (just as does the Lyc IO360/540 derivative).
There is often some lead build-up around the valve face, as well as carbon. At times small granules get “hammered” and flattened into the pitting that results from heat erosion on the valve face and seat. These granules are pretty hard, and they can hold the valve off its seat just enough to cause audible slight leakage on a compression test; even though the leakage is so slight that good compression numbers are achieved. As the valve rotates and hits the seat during further service, the deposits wax and wane. As they do, compressions and leakage rises and falls. This is a larger problem on the exhaust versus the intake, as the exhaust tends to have much more erosion on the face and seat where granules can get trapped; but it can happen on the intake valves as well.
One of the temporary patch jobs sometimes done on Continentals, to stave off premature cylinder work (“topping the engine”), involves freeing the valve and removing the spring. You can apply valve grinding compound to the valve edge through a plug hole, then use a drill to spin the valve (using a home-made adapter) while pulling it into the valve seat. This is crude but it will work surprisingly well, and can easily raise compressions by 20-30 “points”. Any compound that you don’t manage to get off of the seat area gets melted and carried out the exhaust during ops, so it doesn’t contaminate the cylinder area.
The two main drawbacks to this are that it is a royal PITA to do it, and it usually only holds up for 100 hours or so, then has to be repeated. If done on a timely basis, it can prevent valve burning. Once the edge of the valve burns (you can see the discoloration on the edge of a burned valve by using a borescope, or while attempting the above procedure), you cannot recover it with this lapping process.
This is another example of the vast differences between our air-cooled aircraft engines and modern car engines. Our engines are so “loose” that valve lapping is done to help “nest” the valve head in the seat; even on new cylinders. No auto engine factory has lapped valves for thirty years or more, due to precision machining tolerances and in-service temperature control. Lapping in a modern car engine is considered a bad thing, as the slight circumferential ridges it creates are a detriment to good sealing, heat transfer, and valve life. New valves and seats in new car engines look almost polished, they are so smooth; and they fit together perfectly.