Bax,

I need your help on this one! S-90K
I'm pretty sure that I've detected air coming from around the glow-plug.

When I hold my ear to the top of the cylinder and turn the prop, I can
hear a small amount of air coming out around the plug. But how could
this be? I checked the threads, and they are not cross-threaded. I've
only taken the plug out twice. Once to replace a bad stock plug with an
OS-8, and then again last night to inspect the threads. Could there be
some sort of compatibility problem between the OS-8 plug, its washer,
and the S-90K? Seems unlikely.

How big of a problem is this? There is only a wisper of air coming out.
With a rapid compression and ignition, I can't imagine that this would
result in much pressure loss, but then again, maybe I can.

Eagerly awaiting your diagnosis,
WH

Whenever a model engine, especially a ringed one, is turned over slowly, you'll hear the hiss of pressure passing by the piston and ring. That's normal. To check to see if you have an actual leak around the glowplug, just spray a bit of soapy water or window cleaner around the plug and its seat. Be generous. Now turn the engine over. If a leak exists, you'll see the liquid bubble at the point of the leak. You can also do this at the jucture of the crankcase and cylinder head. That will tell you if you have a compression leak through the cylinder head/cylinder liner mating surfaces. If you see no bubbles, then the leak is past the piston and ring, and is normal.

If the engine actually does have a leak, then it should be sent to us for service. The cylinder head would likely have to be replaced.

[edit for typos]

Thanks Bill. That addresses my concerns.