Motorcycle Repair: Is a re-bore really necessary?, stroke motorcycle, feeler gauge

Question
Mark, I recently purchased a 1973 Yamaha RT3 360 Enduro 2-stroke motorcycle with approximately 2600 total miles according to the speedometer. The guy I bought it from was the second owner and he removed the engine, cosmetically restored the bike, then reinstalled the untouched engine .
It starts and runs fine, but it seemed to me the noise coming from the cylinder head seemed excessive, almost like a knocking sound. I rode to my local Yamaha dealer and they tell me I need to have it rebored, new piston and rings due to the piston "rattling around" in there. I have removed the top of the head myself and the vacuum compression seems alive and well and the rings seem to seal like a glove. I checked for horizontal movement of the piston and I can move it ever so slightly back and forth with my fingers. It's so slight, you almost don't see it move, but I can feel it move with my fingers. I checked the clearance with a feeler gauge according to the manual spec (0.3MM - 0.5MM) and I could not even slide my guage between the piston and head. Based upon this limited info, is a re-bore necessary? What do you suggest I do?  Thanks, Mike  

Answer
Hi Mike,

To know if the cylinder needs bored, more thorough measurements must be made. Disassembly is necessary to make these measurements.

Measure cylinder taper and out-of-round. Measure ring end gap, piston diameters, and check head warpage. There's really too many measruements to list here.

Check the following websites for more detailed information. You should have all the measurements in the service manual.

http://www.type2.com/library/engineg/pcdec.htm

http://www.macdizzy.com/2stroke.htm

And, if you can find (or afford) Bruce Jennings' Two Stroke Tuners Handbook, get it....engeering formulas and more.

Respectfully,
Mark Shively