I've managed to source a new piston for my 84 jumbo from Italy, however when it's arrived the skirt is 5 mm shorter than the original. As this will affect the inlet port timing will the bike run or do I need another? The pistons are almost impossible to find.

May not run so well at low revs but will rev higher and produce more power at higher RPM.

 

You need to calculate what the new inlet opening duration will be and look at a tuners handbook such as A G Bell to get a better idea of what the exact effect is likely to be.

 

Also need to be sure the bottom edge of the exhaust port has adequate cover when piston is at at TDC

If it's a reed valve engine (I think it is) it should make no difference.

If it's a reed valve engine (I think it is) it should make no difference.

Agreed regarding inlet duration, forgot that jumbo had a reed valve, my friend had an SWM a 1981 or 82, non reed valve 320 I think and it was that I had in mind.

Comments regarding exhaust port still valid and unaffected by reed valve.

Thanks for help, the exhaust port is not closed at tdc so I guess I'm returning it and starting the hunt again.

Regards the reed valve, there seems to be a plate where this would go and I've read they removed it to soften power.

Looking forward to riding this as it's been in bits since it came from Italy