Get yourself a Triumph T140 piston and bore the B31 barrel to suit. The pin diameter is the same and also the distance from the pin centre to top of piston. This will take a B31 up to 399cc.
It would be advisable to fit a 2mm thick aluminium spacer under the barrel to lower the compression slightly and ensure valve to piston crown clearance.
After that you could fit a larger main jet and alter the ignition to 1/2inch (12.5mm) BTDC. I just slowly turned the distributor around a bit at a time until the engine felt lively.
Then, if you want, gear the bike up by fitting an 18tooth engine sprocket and fit Goldstar Scrambles Cams if you want to make the bike really goooooooo.
Oh...one thing...18 tooth engine sprockets are not available for Alternator Model B31's and I've found that the gearing is too high for normal road use if you go up to the B33's 19 tooth sprocket. I had an 18 tooth made for mine.
With the exception of fitting the scrambles cams I did this conversion on my own B31 almost 10 years ago and it's proved itself to be ultra-reliable.
Apparently you can also use a B44 piston which takes the capacity up to 425cc.
The ignition timing is the position that the engine is in when the spark is produced. The normal setting for the GB31 is when the piston is 7/16 inch before top dead centre (on the compression stroke with both valves closed) and the automatic advance and retard mechanism held in the fully advanced position.
I was told that the B44 piston will fit your engine. The same as the T140 piston but I can't advise on any other details because I don't have any personal experience of using this one.
yes i found in eBay there are two different top shape of piston with same part number..i don't weather it is a typo error or else.. anyway thanks again to inform me about the crown shape & the number.. keep in touch.. vinotkumar
Interesting thread. I just pulled cylinder and head off my new BSA acquisition,(XB31 motor in a ZM20 plunger frame)and find that I will be fitting a new piston. Current piston is +.40 and worn out. Ring gap ranges from .024 at bottom to .050 at top of cylinder. I have not yet started looking for a B31 piston (I assume they are available?) but this T140/B44 thread is intriguing. I generally prefer low compression, softly tuned engines, they stay together longer and I find them pleasant to ride.
I tend to agree with you regarding the lower compressions but I wanted a higher cruising speed in order to keep up with or stay slightly ahead of the wagons on long distance trips and this piston isn't available in low compression format. The conversion gave me the extra power to enable me to gear the bike up a tooth on the engine sprocket which in turn gave me the additional cruising speed required.
I did this conversion in 1999 and the engine has been totally reliable over 1000's of miles since then.