I have a problem with the spark plug stripping out of the head on this engine. After starting it one day I noticed one plug was loose. I retightened it and after the engine was restarted it blew the plug out of the head.
After I installed a head from an RCS140(this engine uses the same head) and finally got a good run out of the engine I found both spark plugs were loose. Since I had tightened them firmly and they were clamped in the RFI boot with a hose clamp they could only come loose by deformation of the threads. It seems logical that sooner or later both of the heads will lose the plugs.
I did not have this trouble with my RCS 140 with about 100 flights. The head that was on the 140 is now on the twin. I wonder if the mass of the RFI boot is causing the trouble.
Putting a helicoil in this head has proved to be a problem. C. Lee does not do this size. The local auto parts store has helicoil kits for 10mm but not the correct thread pitch. I have ordered a tap and helicoils for the correct thread pitch but it will take awhile to get it.
Has anyone else had this problem with this engine? If so what is the solution?


1/29/2004 8:13:16 PM

Isn't this the same CM6 plug that is used on the 3W and DA motors? Doesn't it use a copper gasket on the plug? The only thing that I can think of that would cause it to come loose is if the head is overheating. The aluminum will expand a lot more than the steel spark plug and it'll get loose. I have never had a plug come loose on a 3W. I would talk to RCS and find out what they think is going on.

Yes it is the CM-6 plug. I used the gasket that came with the plug. It does not appear to be copper.
It could be an overheat problem, but the head is hanging out about 1/2in in the breeze. Perhaps I an not getting enough air to the cylinder.
I never had this problem with the 140 and it had about the same exposure.

You are right, it's a regular crush washer on the spark plug, not copper. Maybe you could use some cowl baffles to cool the cylinders?