I have been led to believe that the only difference between the M20 & the M21 is the crank, the M21 one being heavier and having a longer stroke.Therefore could i produce 100cc more grunt for my WM20 just by changing to an M21 crank and rod, or is there more to it than that? Has anyone done it? Or maybe it isn't worth the agro?.

email (option): michaelggilbert@btinternet.com

I have an M21 crank and piston fitted to an M20 motor.
The only difference is obviously the crank with the longer stroke, and a piston with a gudgeon pin that sits further up the piston, which reduces the crown height.
I'm not sure about the dimensions, you would have to research this.
I recall the distance between the centre line of the gudgeon pin and the top of the piston at about 30mm.
Sorry can't tell you much more than this, as I assembled it years ago.
The motor does seem to have a bit more pull, but doesn't like to rev.
Keith

email (option): keithatkinson@hotmail.com

Hi Mike,

The main difference is in the flywheel, which has the bigend pin closer to center,
Giving a longer stroke, and the piston is same diameter, but the pin sits higher.

Barrel is the same, with a slightly larger input port, and a bigger carb.

The conrod is the same.

Noam.

email (option): noam10@gmail.com

Most strange is the main jet in the carburator it is smaller than the M20 for the M21 is it 160 and for the M20 is it 170 its for the 276 carb.Gr Theo

email (option): oldbikes@kpnmail.nl

Theo, do you think maybe because longer stroke pulls more air fuel mixture & burn is more efficient with the slightly higher compression ratio? Its my best guess.

Does anyone know if 66-635 is M20 or M21 crank.It maybe post 1958 as it does'nt tally with my parts book and that only goes upto then? Thanks.

I believe the heads are different also, as I have one of each and the M21 is aluminum. Of course this may mean nothing performance wise.

And it is also possible I was misinformed about what I have.

Bob

email (option): baatfam@sbcglobal.net

Neither started out with alloy head, that came later & although noisier was often used as replacement on early machines.easiest way to tell between m20 & m21 crank is the stroke, either 94mm or 112mm.

Hi Mike,

Yep,
I meant that the crankpin would be further from centre on the 600 for a longer stroke.

Mixed the 2..

Part No:
On the technical section it says regarding the part No.-
"At some point after 1949 production of flywheels was concentrated on a single pattern of steel forging that was used for four different models…The M20, M21, M33 and B33. These carried the same part number for both the drive side and timing side flywheels.. 66-635. and were machined to provide the appropriate stroke etc. for the different engines."

Also see:

http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1127000&cmd=show

email (option): noam10@gmail.com

Don't forget to change to the larger engine sprocket for the M21 conversion. Worth noting as well that the alloy head requires different head bolts and a different plug grade if you change to that type...Ian

email (option): ian@wright52.plus.com

Do those alloy heads also use a longer reach plug, i know some alloy heads had an insert to continue using 1/2" reach plug but i think M21 was alloy thread & long reach? ?

For 10 cents of mig wire, just fill your head up to this shape and you'll fly. If you don't like it just grind it off again!

Ian Wright

Don't forget to change to the larger engine sprocket for the M21 conversion. Worth noting as well that the alloy head requires different head bolts and a different plug grade if you change to that type...Ian

email (option): baatfam@sbcglobal.net

email (option): noam10@gmail.com

email (option): noam10@gmail.com

Hi Noam/Bob..The head bolts for the alloy head are all the length of the long bolts in the iron head. Correct plug for the alloy head is a long reach Champion N5C or equivalent...
Check alloy heads for flatness at the gasket face before use and torque down to 28-30 ft.lbs. using only a very light machine oil on the bolts for lubrication...
A plug insert if not already fitted is a 'must' on the alloy heads and it is worth considering a helicoil in the timing plug bolt hole as well if you intend to use it. Also fit an annealed copper washer under the head of this bolt. Full thickness 'machine washers' under the head bolts are also worth fitting to avoid collapse of the washers when the bolts are tightened.
I can't say I agree with Douglas's 'blanket statement' about cylinder head modifications..Care must be taken not to reduce gas flow when modifying heads and too high a compression ratio will result in 'pinking' under load..The Harley Davidson had both valves tilted towards the axis of the cylinder, had a more compact combustion chamber than the BSA and a 'pop up' piston layout to improve the breathing characteristics of the 'throat' where the gas enters the cylinder....along with other detail work this was the result of a long period (years)of detailed bench and dyno testing to 'fine tune' the layout for that application. They even put perspex 'windows into the cylinder heads and used coloured dyes to study airflow patterns within the head on the flow bench..
Whilst a study of the KR Harleys is helpful in providing a basic guide to the principles and some ideas for modifying an M20, the design IMHO is not directly transferable to the M20 engine...for example, the M20 valves are a different shape and size and much further from the cylinder, the combustion chamber is wider and the standard porting arrangements are completely different...plus the Harley had vastly different cam designs, the engine revved to a much higher level (so gas speeds were completely different)..and later versions had a different spark plug location. Looking at it logically there is no way the head design can be regarded as directly compatible...Ian

email (option): ian@wright52.plus.com

Proof in the pudding as they say. A pair of 4446 goldy cams this week ,will inform if any improvement. A pic of the hack for experiments!

Proof is in the pudding! Here is the hack. Will try a pair of4446 goldy cams this week!

Here's my nearly-finished alloy head for a 720cc WM20 engine. Waiting for the 90mm pop-up piston to cut the squish band. It will have a 2inch intake valve.

Jeff

email (option): jjbandoo@aol.com

Douglas, do you mean 2446 Scrambles Cams ?
Did they make a 4446 ?
Can't help mentioning after seeing sparkplug thread inserts mentioned on here that its good idea to steer clear of stainless with its poor heat conductivity, i use alloy inserts on alloy heads that are plain from the factory.

It is maybe strange bud i have make this stroke shorter it was 94 and now is the stroke 88 i did it only for that it is faster on the top of RPM It is not the same for a side valve. A frend of my put a Jaguar piston in the M20 barrel and has olso an M21 crank it go's like hell even with a side car on it Gr Theo

email (option): oldbikes@kpnmail.nl

Nice looking bike Theo..What year is it? I bet it sounds sweet as well with that silencer? ...Ian

email (option): ian@wright52.plus.com

It is a mixture from all my old stuff Bud i call it an M22 special 1938 if you looking in my shed i can build another one some newer i have an B31 bore to B44 piston with a B33 head the M22 has now touring GS cams bud when it good running it come with racing cams bud i think he wont go stationair Oh the exhoust i hope its no probleme. Gr Theo

email (option): oldbikes@kpnmail.nl