Am trying to decide what size piston to purchase for a rebore. What is the standard size?
Thanks
The size will be determined by the rebore specialist .
Dictated by wear and ovality of your cylinder , they will determine how much to rebore to true it up.
They will inevitably supply the piston also as the piston to bore clearances need to be allowed for in the calculations
Your manual will have all dimensions
Standard C15 bore is 67mm.
In most cases of normal wear of a standard bore a +20 (1/2mm) oversize piston and barrel rebore to suit will be sufficient to restore the barrel. After that +40, then +60 pistons are usually available.
At some point in the past Hepolite also produced +10 and +30 pistons.
The engineer reboring your barrel may or may not be able to supply the oversize piston, depends what suppliers he uses.
I'd do the following -
Take your barrel to the person to rebore it.
Have them measure the worst worn area (near the top of the barrel, front to back) and make an assessment as to whether they reckon it will clean up at the next oversize or whether it will need two oversizes to clean up.
Buy the piston size they recommend, including if necessary a liner to take it back to standard size if the barrel won't clean up at +60 thou or less, although Triumph 5TA low comp pistons can be used to give sizes +80 and over.
Great. Thanks