Measure from the centre of the main shaft to the centre of the crankpin and times by 2 this will give you the stroke.

Centre to centre for an M21 is 56mm, so a stroke of 112m....The M20 is 47mm/94mm and the B33 44mm/88mm...
The 20 and 21 use the same con rod which is 8.250" between centres and the B33 has a rod that is 7.375" between centres for the 'long rod' type and 6.875" for the 'short rod' type...Ian

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

thanks for the great advice. unfortunately I'm surfing through ebay listings so have to eye it. To me it seems the nut on the end of the shaft would not fould the oil slinger even if it were round. Most of the others have the nut positioned closer to the oil flinger.

So while I'm here, hoping the lords of speed (Ian et al) might help me a little further. If I add an M20/1 con rod and an M21 piston thats all I need to build an M21? Or is there an alternative that is more desirable? I live on an island which has peaks of 14,000 ft so I encounter the odd hill.

email (option): cas.vanderwoude@gmail.com

Cas it's exactly as you say. Once you have the M21 flywheels, you use the M20/21 conrod and the M21 piston. And if you still had the B33 piston that you sent me....you could have 635cc.:yum:

If you're viewing stuff on ebay, it would be better to message the seller for the measurement. Even just from the top of the mainshaft to the center of the big end, to make it easier for them, would confirm it. Ron

email (option): ronpier@talk21.com

Hi Cas, it is a 600 for sure, pin is placed very much to the outside.

On a 500 the machined flats leave much more room for a hole further out of centre, on yours not so must be M21.




BR Michiel

You'll need an unlinered barrel (if you don't have one) or will have to fit an oversize liner to your linered barrel if you want to use a B33 piston....Ian

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

here's an M21 as comparison

email (option): ronpier@talk21.com

Thanks for correcting that Ron...I didn't recall the previous discussion and I really don't like misinforming people....I've previously used liners with a 1.5mm wall thickness myself so I know 2mm is OK...

For anyone who may be contemplating an even bigger engine I recently fitted an oversize liner to a linered barrel big enough to fit a 90mm piston, so it is possible (just)......Ian

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

thank you all, especially Michiel for confirming. I now know what to look for.


The stock piston should be enough. I had an M21 previously and the extra 2 horses was enough to be able to cruise at 60mph which is ample around here. Not planning on racing. Trevor might have some advice on carbs...

email (option): cas.vanderwoude@gmail.com

The M21 runs on a 1 1/16"276 carb. (M20 is 1") Other models are available of course. Ron

email (option): ronpier@talk21.com

Cas
You seem to live in an interesting place. Do you mind telling us where it is?

Sorry to be off topic.

He lives on that "Five Oh" Island where the girls wear a grass skirt and a coconut bra!:relaxed: Ron

email (option): ronpier@talk21.com

Thanks Ron.

The five-oh island would be Oahu which is where the tourists go to see the grass huts we all live in and the telephone the professor made with 2 coconuts and a vine. I live on Hawaii Island which is a couple hundred miles to the east; more agricultural, less touristy. It is about 60 miles across and 14,000 ft high, and in true American style, you can drive to the summit. Basically it's a 60 mile wide volcano (still active).

Only about 150,000 people here and one BSA sidevalve rider. You're all welcome to come visit and I'm sure I can find you something to ride. A bit different from where I was born which is just a few miles from Bastiaan.

If I need parts I can order them online on Ebay or from some of the excellent British parts stores, but with used parts, I need to know what to look for before buying. Sometimes I get it wrong and that is the price I willingly pay to live in paradise.

Recently I refurbished an M21 which originally came from Adelaide (God knows how) and sold it to someone on Maui. Riding that, I was surprised at how more "eager" it seemed and definitely happier at 55-65 MPH. Figured my next one would have to be with an M21 crank. Its amazing how many old Brit bikes there are rotting away in people's barns, and they somehow seem to find their way into my stash of project bikes.

email (option): cas.vanderwoude@gmail.com

Cas please continue with the geography lesson. Hawaii became the (last) 50th state of America in 1959. Therefore does that mean the whole island group? Is the TV show Hawaii Five Oh based on the island Oahu? I've seen snippets of the show but never really watched it. Of course Pearl Harbour was also at Oahu, but again always referred to as Hawaii. :thinking_face: Ron

email (option): ronpier@talk21.com

In the 50's the importer for the Hawaii Islands was Shuberts Cyclery Stores, corner of Archer and South Kings Streets, Honolulu, Hawaii...Ian

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

I could only find 'Archer Lane' off South Kings Street on Google maps and it looks like a hotel has been built on the corner....Ian

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

Correction, it's a tower block of flats...Ian

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

Cas
Thanks for the update.
Living outside Adelaide, I'm an ex neighbour of the M21 you did up. I still call my bike an M20 (1941) but it's now 600 & I agree that for road riding nowadays these are a better bet.
Thanks for the invitation to visit. If I ever come I'll bring you a kangaroo. They can be kept quite easily in grass huts.

hi Ron,
I never thought I'd be giving a geography lesson on a motorcycle forum....

The group of 8 islands is the state of Hawaii, but the main island is Oahu where the capitol Honolulu is situated. Both the old and new versions of Hawaii five oh were recorded there as was Jurasic Park and Gilligan's island (for those of us old enough to remember). Hawaii island is normally called the Big Island which helps to help reduce confusion. I came here about 12 years ago for a short contract job and saw absolutely no reason to leave.

There are a handful of old brit bike geezers scattered throughout the state but very few on the Big Island, and even fewer Beeza riders.

email (option): cas.vanderwoude@gmail.com

Thanks Cas, I'm fully enlightened now. They had to rearrange the "Star Spangled Banner" because of the extra two post war states:- Hawaii and Alaska :relaxed: Ron

email (option): ronpier@talk21.com