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Engine Clinic letters Is Offline!

Dear Mr. Lee,

Sad as it was to see the demise of RCM, I’m glad you gravitated to the next best thing (lol). 

I fly Tower 75’s in 60 sized planes; a very nice combination. After a crash, the rotation was a little tight, so I sent it to a retired machinist friend in the neighbourhood. As experienced as he is, one thing puzzled him.  The tightness occurred only every other rev. After wrongly thinking it was the crank shaft, he found it was the liner. No matter which part was defective, why, in a two stroke engine would binding be only on alternate revs? With all the Tower 75’s I’ve gone through, I had a spare sleeve and it worked. I have learned though that after a repair, to break the engine in again!

Keep up the good work.

Sincerely,

C.K. 

C.K., You present a real puzzle. First off, it is very unlikely that the crash could damage the sleeve unless the engine hit on the muffler which jammed in the side of the crankcase. In this case, the crankcase would also have been damaged. My guess would be that the crash jammed the crankshaft to the rear "preloading" the bearings. Disassembly cured the pre-load. This would also explain the 'every-other' revolution bind. The inner race of the bearing turns at a different rate in relation to balls and the outer race, i.e., the inner race, makes one revolution, but the balls and retainer only travel half as far. The actual reduction depending on the bearing size, ball size, etc. The front bearing is often damaged in a crash due to taking the crash impact. You might want to recheck for roughness as there could be flat spots on the balls, etc.

Clarence Lee