79mm pistons???

[BigBoreSwede]

Hello there!

After getting rid of all the carbon on my 750 pistons I can see this numbers at the pistonhead.

This pistons mesures 79mm and I´m confused since the 750 pistons you buy from Mikes or Heiden Tuning or whatever seems to mesure 80mm.

Any inputs??

Thanks/BigBoreSwede

 

[yamaman]

G'day Swede, mark on piston says 79.95. As with every time you rebore a motor, buy pistons first, take barrells & pistons to machinest. Machinest will measure pistons and rebore to suit (leaving adequate piston to bore clearance).

 

[BigBoreSwede]

Ohhhh....sorry! A little unclear.

These piston come of my engine that´s I´m partly re-building right now so everything is already set. I´m only wondering why all usual "aftermarket" pistons mesures 80mm and mine are 79mm.

I can´t find any "XS web-shop" that sells 79mm pistons.......so what are these piston from?? Any ideas?
Maybe the 80mm piston infact mesures 79mm and the barrels ID is 80mm......I don´t have a clue

/BigBoreSwede

 

[grizld1]

Your piston is .05 mm. under 80 mm. because the piston is always sized smaller than the bore to allow for clearance, so a nominal 80 mm. piston will usually be very slightly less. With a cast piston in the XS650 motor you need about .002" clearance (just about the difference between your piston and a true 80 mm. bore) between piston skirt and cylinder wall, but with a forged piston such as yours you need at least .003" because forged pistons expand more. Have a machine shop mike out your pistons and cylinders.