Polini EVO 47.6
- Thread starter dmann
- Start date
BB Roxx
Minarelli Maniac
dmann,
When the pistons are manufactured, they are never exactly the same tolerances, so typically when you purchase a cylinder for the first time it shoud be an "A" piston, or a "B" piston preferably. As the cylinder wears, the inside diameter will increase slightly. When you rebuild it, you can do only piston ring changes as long as you still have a good tight clearance between the piston ring edge and the cylinder wall. When the wear on the cylinder walls becomes too great and a simple ring change will not be enough to have the right fit (i.e. the piston is too small in diameter) to be a proper distance from the wall of the cylinder, you would then increase to the next greater piston size, for example A up to a B, or B up to a C to maintain the close fit between the two.
When it becomes necessary for the piston to have a larger diameter, it is also time to hone the cylinder wall to the correct inside diameter that is recommended for the next greater size piston. You will need to bore it up to size, which might not be all that much and then check it with a caliper for fit with the new piston. The manufacturer should have a chart available either online or direct from them to know the exact size the cylinder should measure inside for the various piston sizes.
I think that most of them follow the same basic size ratings.
So, before you buy the new piston, take apart yours, and see what letter it is first, then measure with a caliper the inside diameter of the bore and see if it still falls in the range of your current piston. If so, simply hone the bore just slightly to clean up the surface and get new piston rings (and a new piston if necessary) of the same size rating. If it is nearing the next greater range, then hone it up to the lower end of the next higher range, and get the next upsized piston and rings for a good fit. Good Luck with the work man. ~Roxx
For a Minarelli Evo H20, like mine,
http://www.polini.com/catalogo/en/risultato.php?op=po&id=69023
Last edited:
