Hi, I replaced the engine on my M20 with a b31 then shortly after made that into a 400cc with advice off this site but I spoke with a guy the other day who had fitting a m21 crank and made his own barrel using a liner with shaped aluminum and spacers to build the barrel up on his b33.
I never asked enough questions but will a m21 crankshaft fit without modification and do you know of a piston that would be suitable for the b31 standard barrel to give more cc
I already have a m21 crankshaft and was wondering if the piston would be the only cost with rebore, it seems almost wrong not to do it 
Thanks steve nick
email (option): herlocksolmes@hotmail.co.uk
To fit an M21 crank you will need to use a 'short' B33 conrod from a post 54 B33 to accommodate the stroke increase and to maintain overall engine height...
That will also call for a lengthened cylinder liner and bored out crankcase mouth as the piston goes much lower at BDC...
Dependent on what power characteristics you require you might also need to lighten the flywheels.. The finished assembly will also require rebalancing...
Again dependent on what you want from the engine you may need to do some headwork, change the cams and the inlet valve. Also changes to the carb specifications...
A far easier option would be to fit M20 flywheels with a standard B31 conrod and the piston you already have (Triumph?)...
That would still require work but less of it...Final capacity (if you are using a 76mm Triumph piston) would be 426cc...
Regarding a larger piston into the B31 barrel I think there are some Japanese ones that will do the job but don't have that information myself...
My advice to you would be to consider what power characteristics you want from the engine and that will dictate its specification more closely...And of course, think about how much work you want to do and how much money you want to spend..
I run a very oversize B33 with an M21 crank and other mods. (above), but the engine is taken to its limits dimensionally to achieve the capacity (720cc) and frankly, I think the B33 based 600cc unit, which is also bored and stroked to achieve that size is a far better and more versatile engine...Ian
email (option): ian@wright52.plus.com
I forgot to mention...You also have to modify the timing side main shaft and machine up a spacer to fit SV flywheels into the OHV engines...Ian
email (option): ian@wright52.plus.com
Maybe i would be much easier to look for a B33 engine all together.
And fit it with gold star cams !
Without a doubt the B33 is a better base from which to start...The original engine from my bike is still at 500cc...The larger ones have all been 'new' engines...
B33 parts are harder to find and more expensive than B31 parts however and that may be a factor in some cases...and as I said in my original post its about what you want...
I've run these B Series engines for about 40 years and have tried most combinations of parts and specifications...Up until fairly recently it still didn't cost to much too experiment but now it's getting pricey if you want to use B33/Goldie parts...Ian
email (option): ian@wright52.plus.com
Thank you for your reply. I was hoping for something more simple 
I do under stand the amount of work involved now.
The bike is scary at 60mph never mind any faster.
I was just thinking about something to do this winter and more cc,s
looked interesting.
email (option): herlocksolmes@hotmail.co.uk
Hi Simon...No it's not possible, as the M21 has an extra main bearing on the timing side of the engine and consequently the tappets are further from the crankcase centreline than in the OHV engines...
Also the tappets (cam followers) are vertical in the crankcases in SV engines but are tilted from the vertical in OHV units to align the pushrods with the rocker arms...So, even if the top end would fit there would still be pushrod alignment problems...Ian
email (option): ian@wright52.plus.com
Thanx Ian!
Not exactly a bolt-on fix then huh!
I have seen pictures of all sorts of extravagant OHV-modifications such as fitting Harley shovel head top ends and more. Maybe I'll attempt something like that one day. If I should happen to run out of other troubles...
On the other hand it looks like a blueprint for disaster having the valves hanging right above the pistons like that doesn't it? 
Much safer to keep them to the side I would say!