Need help on piston size

enalkarion

New Member
Dec 5, 2012
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Levittown, PA
So I broke my piston being careless when pulling my crank. I used a telescoping bore gauge, took the 12 measurements and came up with 66.84 average bore size. Will a 66.75mm leave to much piston/cylinder clearance? Do I have to go up to 67? Cylinder looks clean with no scoring or scratching.
 
I belive the piston manufacturer figures in the piston to cylinder wall clearnce. Meaning that a 67mm piston is sligtly under 67mm's. In your case you will probobly need a 66.75mm piston. As your bore is probobly worn out a little larger than the 66.75mm it should have been origianlly. One thing is for sure.... Your not going to fit a 67mm piston in that bore.

Also be sure to calibrate your equipment before taking any measurments.

Corect me if im wrong guys.
 
So I broke my piston being careless when pulling my crank. I used a telescoping bore gauge, took the 12 measurements and came up with 66.84 average bore size. Will a 66.75mm leave to much piston/cylinder clearance? Do I have to go up to 67? Cylinder looks clean with no scoring or scratching.

With the 67.75mm piston and your 66.84mm measurement,you'll be hitting right about .004 "thousanths (not mm) clearance. too much for a cast piston. You also have to take into account "out of round" and "bore taper" top to bottom. Don't half ass it. Too many people have done this,and wonder why things go wrong ,oa don't last. Remember that you still have to hone that cylinder.What piston are you running?

The reason I ask this is because there are slight variations among manufactures,plus not all manufacture's offer quarter milimeter (.025mm) increments. WSM and Pro X,and wiseco does. Namura does not. Something to consider. Also WSM piston skirts always tend to be .002 thousandths bigger than the other guys,which may help you out. Things to consider.
A 67mm over bore (.010 thousandths) will clean that cylinder up nicely if its in the condition you say it is. When it comes to pistons...Cast,or forged,...pick your poison. Both work well when set up properly and engine is brought to operating temperature before riding.
 
Couple things don't jive. Only need six measurements. Front to back @ top, middle, bottom. Then side to side same way. compare front to back to determine taper. Compare f/f to s/s to determine out of round. Averaging ALL the numbers tells us nothing really, other than at some point the cylinder is WAY bigger than 66.84. You have it apart, you need a new piston, might as well bore it and start out fresh.

Only once have I put a new piston in an old bore as the largest dimension was less than half the service limit.