talking to a bloke in my local café about the boring out of my B31 to 399cc he asked can you up the cc on a B33 500cc engine I didn't know but said I knew a man who would thanks roger

email (option): roger.beck@node6.com

Hi Roger..The B33 barrel can be bored out to take a 90mm piston...Thet will give a capacity of 570cc on a standard crank.

Fitting a modified M20 crank with B31 rod and a 90mm piston will take it up to 600cc...

My own B33 has the 90mm piston with a modified M21 crank fitted (with short B33 conrod) and that makes it 712cc...

The last above is a lot of work though, as the engine is dimensionally pretty much at the limit. I also had to fit a second crankcase breather to cope with the additional air that has to be moved on the down stroke....Ian

That's interesting Ian, what 90mm piston do you have to use.

Hi Ian...I get them made specially...Forged 90mm pistons with a modern 3 piece scraper etc. and a pin size to suit the BSA..They are a straight fit into the B33 after boring to suit...Ian

Thanks for that Ian, do you need to rebalance the crank and how much for one.

Hi Ian..No crank rebalancing required..The pistons are £175 plus shipping..Ian

Sorry to keep asking questions. Are they for the short rod engine, what sort of compression do they run at and what clearances are required on boring the cylinder?

Hi Ian (W),

Regarding the piston,
Please see if you can reply to my mail..

Noam.

email (option): noam10@gmail.com

Hi Ian...The piston is a 'flat top' type, made for long rod engines. Fitted to a standard B33 compression ratio is 7.2-1...

I bore them to .0035" clearance on the thrust faces and run them in for the full recommended period.
Bear in mind that the expansion rate of modern high silicone forged pistons is lower than for traditional cast types.

Modifications to the standard bottom end, head, cams etc. will yield further power increases if needed...

The very high performance option is to use a substantially modified B31 head...but that's another story....Ian

Thanks Ian being designed for a long rod engine means its no use for my short rod BB engine without major work. Shame. It would be ok for my M33 but as this engine is built from a B31 by a PO I don't know as yet what is inside it.