Hi, this is my first post. I have a late AA M21 with alternator. I would like to try to extract a bit more power, perhaps up to about 23hp as was the B33. I have 2 M20 engines in pieces and have also bought a new aluminium cylinder head. Can I get a larger piston to suit? What cams? What valves should I use? How can I best modify the head for compression and flow? Can it be twin plugged when fired by a magneto? I know I have a few questions but I would like to gather some information first.
Thanks in advance. Dave. Australia.
email (option): merumbles@bigpond.com
You say more power for an M21 but you also say you have 2 M20 engines. M21 and M20 have different flywheels and stroke so you gain 100cc just by having M21 flywheels. M21 has a shorter piston. You can use an early B33 piston in an M21 which gives a 3mm bigger bore or more depending on oversize piston. M20 and M21 have STD bore of 82mm and B33 is 85mm.
I am intending to use the M21 crank. I am thinking I could start work using one of the M20 barrels. I have looked at Yamaha xt500 pistons. They are available in sizes from 87 mm to 91mm I think. The wiseco ones are similar in weight and dimensions except for the gudgeon pin which is 20mm. I am not sure what to do about that.
email (option): merumbles@bigpond.com
David it has been discussed on here several times. If you have a wartime type barrel with a liner in it, you can get away with an 85mm piston. I have a new early B33 piston for my bike which already has an M21 crank, so have done some checking. If you fit a bigger piston (Ian Wright has supplied several specially commissioned 90mm pistons over the years). You must either use a post war barrel which has no liner, or get a bigger liner specially made. NB. The post war barrels were a better casting and didn't require a liner.
I haven't got a small end bush to hand to determine if you can take another .475mm from it for your Yamaha gudgeon pin? Ron
email (option): ronpier@talk21.com
I've recently fitted an oversize liner to a 'linered' M20 barrel to take a 90mm piston..So it's possible but the liner can't be too thick..
Westwood Liners stock a vast number of cylinder liners and if you give them the overall dimensional requirements they can usually come up with something that can be easily modified for application..Altering the length and the 'top hat' dimensions(if used) is straightforward so try to select a liner that has the O.D and I.D that you need and surplus material elsewhere if necessary....The top hat keeps the liner in place but if you are going to a 90mm piston and you have the engine apart the crankcase mouth can be bored out to 'seat' the liner at that end and the cylinder head/gasket will stop it going the other way....In that case you can use a simple parallel liner...
Regarding pin size, 20mm is .7874" in proper measurements...The BSA pins are .750" diameter so you only have to remove .0374" (nominally) to fit the 20mm pin...There is plenty of material in the BSA small end bush to do that...
When planning your engine you have to decide whether you are going to get more power through building a 'big banger' with, primarily, a boosted capacity or a more revvy engine...A slow revver won't benefit much from wild cams and larger ports as it will have insufficient gas speed...Conversely, a high revver with an M21 crank might have piston speed issues due to the long stroke and the majority of M21 flywheels are cast iron, not steel where as the reverse is the case with M20s...High revs and cast iron flywheels aren't the best combination....
Saying all that it's all down to what you intend to try and do with the engine and its final application..That's what would lead you towards choosing the right specification for it....A key thing to remember is that all the elements of the engine work together in a complimentary way (cams/ports/compression ratios etc. etc.) so it's about a balanced package more than individual changes.....Ian
email (option): ian@wright52.plus.com
Thanks for the replies. I will be using the bike on the road, mostly riding with a group of British bikes that are faster. I think I will go for the 90mm bore. I am thinking standard cams with maybe 2mm extra lift. A bit larger valves, particularly the inlet. Radius the valve side of the barrel. Machine the head down as much as practical. Would that sound sensible?
Ian, I see on another post that you have had some pistons made. Would any of these be available? It would simplify things.
email (option): merumbles@bigpond.com