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Piston with marks



When I had the cylinder head off, some scratches or marks became visible as a discolored area on the right hand piston. Does anyone have an idea what this could be due to? Se picture.





Whatever caused it, I would be checking the strobe timing on both cylinders and the mixture for weakness. It could be just corrosion from sitting with  valves open.


Has it even been run? Maybe I'm unlucky or plain incompetent but I've never seen pistons as clean as that after they've done enough miles to give cause for taking the head off.  If the discolouration was buried under deposits which you have already removed, it surely can't be problematical?


The discolouration here is not the issue, have another look at the pistons and the proximity of the 4 bolts.

Personally, if has not run I would strip and rebuild checking as you build back up. 


Is it a 750 converted to through-bolts ?


Rgm 920 kit with sleeved barrel which reduces the holes for the through bolts, turn down the bolt heads and/or grind the sleeve in the hole.

The pistons are cleaned from carbon and the discolored area was then unveiled, so corrosion is good guess, I think.

It is a 920 kit bought from Fair Spares/Les Emry in 1982.


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