B31 Piston

[martinh10]

I have a B31 that has a good +".020 bore with a not so good piston. My attempts at finding a NOS BSA/Hepolite/Heplex +".020 piston have come to nothing so I have purchased a JP piston. Apart from a ring gap issue which I hope to resolve soon my main concern is the weight of the JP item. My existing Heplex piston weighs 217g but the JP is 50g heavier at 267g.
BSA Service Sheet 305 states that when replacing a big end you should ensure that the difference between old and new does not exceed 1-1/2 ounces (42.5g) or the balance may be adversely affected. The dynamics of a complete crank assembly are quite complex but there must be some correlation between big end weight and piston weight.
I would like to know if anyone else has used these pistons and if they have had any issues. Maybe in an old chugger like a B31 it's not a problem.
I have looked at machining the inside of the piston and there is scope for reducing some weight but not I think 50g which equates to almost 18cc of aluminum.
If anyone has a quality +".020 or +".040 piston they wish to dispose of I would be very interested to hear from you.

[Pete Gill]

What weight of piston was in originally when it was new?

[martinh10]

Good question but I don't know.

[Calum]

That seems quite a difference to me, but it's not something I have ever looked at on a bike engine before to be honest.

Out of interest have you measured the weight of the con rod, gudgeon pin etc too? Obviously the big end of the con rod is a rotating mass, whereas the little end of the con rod is a reciprocating mass but it might give you an idea of the change in reciprocating mass as a percentage. You can weigh only the little end of the con rod by holding it horizontally by the big end with the little end on the scales/spring balance or whatever you are using. I'm just thinking the rod is quite a lot heavier than the piston so overall the weight might not change all that much. I think balancing the crank is covered in the service sheets from memory?

[martinh10]

Hi Calum, You raise a good point. The conrod small end with pin weighs 250g so plus original 217g piston = 467g. So +50g is a 10.7% increase which may not be significant.
I believe these Australian JP pistons have been quite widely used so I would appreciate any feedback from anyone who has used them.