The M21 initially had a bore and stroke of 85mm x 105mm for a swept volume of 595cc.
In 1938 it was brought into line with the M20 with a bore of 82mm and a 112mm stroke giving 591cc.

What will happen if flywheel with 112mm stroke is fitted with 85mm piston ( equal 635cc)??

Will it work?
With a standard 85mm piston?

If not, Why?

I am asking because I have both mentioned in stock. Only the piston is missing and I am afraid to use a lot of money to complete the engine , if it by some reason not will be a proper solution..

Regards
Per

email (option): p.thomsen@frogeye.dk

I think that is quite a standard modification. Using M21 flywheels and a B33 piston. In fact Ian Wright has developed a way of getting up to 720cc with a much bigger piston (with the right type of barrel) Ron

email (option): ronpier@talk21.com

Hi Per,

The 112mm stroke is M21. the M20 has 94mm of stroke.

If you use the B33 500 piston, you would need the one of the "long conrod" model.

Noam.

email (option): noam10@gmail.com

Thank you guys.
Nice to get some answers for a BSA newbie as me.

M20: ø82 x 94 = 496cc
Early M21: ø85 x 105 = 595cc
Late M21: ø82 x 112 = 591cc
My parts: ø85 x 112 = 635cc (both reconditioned and ready to use).

Tatu, who have built a Monster M20 > 800cc (see other post in this forum) sent me below mentioned answer...

Now I have some info to work with.
See you
Per



“It depends on the compression height of the pistons. I don't know how it is in the older M21's but the later M20 and M21 share the same barrel and same con rod, so the stroke difference is compensated only by the piston height.
Now IF the barrels are of same height and the con rod length is the same in these two engines, that means there should be approximately 3,5mm height difference between the pistons so that the older style 85mm piston is higher.
This means that by using the 85mm piston with the 112mm flywheels you'll get approx. 3,5mm pop-up, so you would have to make room for the piston in the cylinder head. This would improve the breathing of the engine as well.
Another thing is, if your barrel has a liner or not. If it's without liner, then you just need to have it bored to 85mm. The original liner may not have material enough to bore it to 85mm.
So, with these conditions I'd say it will work.

My bike has "home made" 5" (127mm) flywheels, the piston is from Harley Sportster, diameter is 3.53" (89,6mm). I used cylinder liner from Fordson Dexta tractor. This is kind of extreme (and very expensive) way to go...”

email (option): p.thomsen@frogeye.dk

Hi again Per. Yes some of us have been fascinated by Tatu's 800cc M20. But of course he has had to make some very nice but expensive bespoke parts.
Up to 635cc is achievable with standard BSA parts or even more with a correct bigger piston and an early non linered barrel. Ron

email (option): ronpier@talk21.com

The conversion to 635cc Ron mentions, using a 'long rod' B33 piston, will require an 'unlinered' later cylinder or a larger liner will have to be fitted to the 'linered' M20 original...

Note the 85mm piston used is the post war 'long rod' B33 piston...

If you plan to use the pre war 85mm piston from the early M21 you will have to check the gudgeon pin centre to crown height to confirm that this dimension is within useable parameters...

I ran the 635cc version for about 8 years (on standard M21 carb settings and ignition timing) and it performed quite well in that format...Ian

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

For all you SV speedsters

Just got AMA (speed record) regs - it seems that according to the Americans a 700cc SV is actually a 490cc OHV ..........

Ok, not really, but SV are allowed an extra 33% capacity for each engine class.
i.e. A 500 SV can run in the 350 OHV class, a 700 SV in the 500 OHV class, etc.

Now if I can find a 5H 500 SV top-end for my 3HW bottom end (I bet they share the same crankcases?) ... I could for fill two dreams at once...

The 1940 Triumph 5SW side valve I rebuilt didn't have the same crankcase as the 350 OHV engine...Ian

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

Oh well, I'll forget that idea ..

Per:
The earlier cylinder doesn't fit the later crankcases. The later crank won't fit the earlier cases either, as an extra timing side bearing was added on the later models. I have the same parts:-). Use a later cylinder and bore it. A Harley EVO piston can be fitted. Small end can be honed to fit, and you'll need to put about 3 mm's of material under the cylinder (compression plate). This will get you close to 700 cc's. We are a couple of Danes doing it at the moment, nice to see another one:-)