Piston size question

[One1]

Is there a tolerable allowance between pistons if the PSI of them is within the spec of the allowable difference? For example, if I have a 78 or 78.5mm and I have a chance to buy a replacement jug and piston that is 79mm (1mm over), should that work to run that way? With a 78mm and a 79mm, each having their own jug that is proper size. I know every little bit counts, but is it close enough if you have no option? I really need my ski running within 2 weeks or I'll completely miss the summer with it, and my stock jug is fine on the back, the front has to be replaced after it hooked the exhaust.

 

[mwat65]

I personally wouldn’t do it. Someone may tell you different. You need to find out why one piston and jug went bad on you or you will just have the same problem again.

 

[Dr Honda]

As above... in the Rotax world... it's frowned upon. But in the rest of the boating industry... it's common to have one jug reworked, and different sized pistons in the engine.

But as above... you must know why the jug failed in the first place. If it was a plugged carb, and it's fixed... OK. BUT, if it was a leaky crank seal... you will just kill it again.

 

[mikidymac]

Also with two different pistons and sizes the weight will be different which is not good for the balance of the engine.