looks like i have a standard barrel and piston on wd engine, as i can see the half semi honing circles on the sides, i was looking to fit hi comp piston but i suppose you cant get one for a standard barrel size, so being the standard compression is 7:1, would i be able to fit a standard 8:75 , its not hi comp but its more than 7:1, would it make much difference?
I would honestly doubt that you would still see the honing marks if this was the original piston and cylinder - more likely it has been re-bored in the not too distant past. Either way, you should be able to fit the higher compression piston from an SS90. In terms of power, the latter provided ~5 bhp more power but that was also with a larger inlet valve and sportier camshaft. Thus, with just the higher compression, you will probably be gaining ~2.5 bhp at most which is still ~12.5% more power and might be worth having??
i have seen ss90 pistons but they are 20 oversize, cant seem to find standard new, and £200,
so a larger inlet valve would that need a new bigger valve seat cutting in?
It probably isn't worth anyone stocking a STD piston - normally you only change a piston after a rebore! The only way of being sure what size piston you've got is to measure the bore which I assume you haven't done yet?
You would probably only get a benefit from a larger inlet valve if you fitted the sportier camshaft and the larger 389 carb (1 1/8" choke compared with std 1 1/16" choke).
no not yet measured the bore, will do this week end, seems like a lot of work finding the parts, maybe leave as it is unless the parts fall my way, but thanks for your knowledge and advice.
It would be a lot of effort with probably marginal benefits. The biggest performance problem with the B40WD is its standard gear box and ultra low 1-3 gears - it is far from the pleasant trying to ride one on the road! If the gearbox is still the original, I would definitely think of changing it to a standard B40 box before trying to improve the performance in 4th gear!