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Engine?????????

Hi everyone,

I bought, unseen, a BSA M21 1955, the frame number is fine but the engine confuses me, the number should be around BM21 1601 but it's not. I will let the photos speak for them selves.







My gut feeling is the engine could be a WM20 (brass plate), but I don't have clue, the nearest thing I have to WD is a little flying flea and before anyone asks I know the wiring needs to be sorted out before riding it.

Non- matching doesn't bother me I just want a big single.

Thanks in advance for any replies.

Kind regards Colin

Re: Engine?????????

Indeed, a WD plate indicating that it went through Southern Command workshops in 1954...It may mean though that you have a 500cc as they wouldn't have been rebuilding M21 engines at that time.

The engine number has no prefix, suggesting that it was stamped on new casings to replace the original WM20 8123 engine. Are there any other stampings anywhere on the engine ?

Re: Engine?????????

The bike may be a 1955 M21 but the engine is a M20 of much earlier vintage.

Re: Engine?????????

Thanks for that Rik, I will get the bike out of the van over the weekend and check the engine over for any other clues.

Any idea how earlier Bruce

Regards Colin

Re: Engine?????????

My guess is that it is a 1947 or earlier engine due to the engine breather still being on the drive side crack case. 1948 and later it moved to the timing side, I think?

Again, that is a guess. Other people know much more than me about the WD engines. It is definitely not a 1955 engine.

The frame is 1951 or later since it has a plunger rear. It also looks like it may have the 8" front brake which I think only came on the AA model M21's.

Re: Engine?????????

Rik
Indeed, a WD plate indicating that it went through Southern Command workshops in 1954...It may mean though that you have a 500cc as they wouldn't have been rebuilding M21 engines at that time.

The engine number has no prefix, suggesting that it was stamped on new casings to replace the original WM20 8123 engine. Are there any other stampings anywhere on the engine ?
That is interesting and explains perhaps the engine in my restored WD M.20 which also carries a small brass plate and the number 7807 I think with no WM20 prefix. The frame is a late WM20 frame and the complete machine came out of Chilwell in 1967 and to me via Dawson Motorcycles of Nottingham.

email (option): keithchandler@clistandchandler.co.uk

Re: Engine?????????

[' It also looks like it may have the 8" front brake which I think only came on the AA model M21's...']

It is the 8" brake which was also fitted to the M33...If you're not worried about it being standard I'd consider a B31 or B33 engine instead of the M20 motor...Both have more power to pull the rather heavy plunger frame and both are a lot more economical....B31 engines particularly are relatively easy to build up from parts....Ian

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

Re: Engine?????????

Sorry for not replying before but I had my 2nd Covid jab and as per the 1st one I had a bad reaction but it's small price to pay, back to the M21, it is a 1955 machine but had it's engine replaced at some point with this 1939 WM20. I have had the head off and it all looks fine so I am going to leave this engine in unless I find any nasty's when it's stripped, I wanted a big single but got a slightly smaller one, not to worry, the 8" front brake was fitted to all M21 from 55 on so I have been told, I will have to find a chain guard, tool box, case for the cut out and a horn.

Kind regards Colin

Re: Engine?????????

I have stripped the engine down and started to clean the years of gunge and have found some numbers could any kind and learned member tell me what they mean.





Many thanks Colin

Re: Engine?????????

I'm guessing Ministry of Supply with a B workshop number and a date code. It looks like the original engine number was under the plate. The two numbers repeated on the lugs are the matching case numbers.....Don't know anything else. Ron

email (option): ronpier@talk21.com

Re: Engine?????????

For all practical purposes the numbers are meaningless
If you look closely at your bike you will find them everywhere
If you look at the British Pathe BSA factory tour video you will see the QC women stamp parts as they test them with the go-no go jigs
And this all most are.
Engines were batched as were the castings so the foundry may have made 10,000 crankcase castings
The ones that went strait into the machine shop got no marks but the ones that went into storeage got a date so latter when they got machined they would know which pour they were from.
When caes got machined, they got stamped so latter on when assembled the halves matched up.
So to a point they were used to sort of blue print the engines so parts from extreme ends of the acceptable tollerances got matched to similar ones .
The post manufacture , various workshops either stamped the engine or attached a service plate .

Re: Engine?????????

If anyone is interested ......................this is how the bike turned out.

Well the wiring has been sorted, the petrol tank too, bit of black paint and polish the chrome and it doesn't look too bad now, engine may be the wrong one but it runs very sweet, I shall continue to look for a good M21 engine to replace it but am in no hurry. I know there's a lot of "not right" bits on it but it came to me to like that and this machine has had so much abuse and neglect over the years it was unbelievable.





Kind regards Colin

Re: Engine?????????

As a non-BSA chap, it looks to me like a very nice 1950s bike...A nice Kent number plate too.

Re: Engine?????????

Wow....very well done.
This bike will last forever.

Re: Engine?????????

Looks damn fine to me! Have you done all that job since your first post this april?

Best regards,

Simon

Re: Engine?????????

That is a very nice looking machine - well done!
Ray

email (option): rays54@hotmail.com

Re: Engine?????????

Thanks for the kind comments guys, I bought the bike end of April so it's taken about twelve weeks, which is about the norm, it's a hobby I share with a good friend of mine, we are both retired and I suppose we both put in about fifteen hours work a week. He is very good with tin ware repairs and a dab hand with a paint brush while I make any parts we can't get and do the electrical stuff and we share the mechanicals.


Kind regards Colin

Re: Engine?????????

I spend more than 12 weeks thinking about it!..:laughing: ....I must say you've made a lovely job of the bike in that short period of time, I really don't know how you do it....I took about two years over this, my last project...Ian

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

Re: Engine?????????

Lovely looking machine. You must be fit to ride it. My days of riding with clip-on's and rear sets are unfortunately long gone.

Re: Engine?????????

Indeed you must be fit Ian, I'm over seventy and I can only look at clip-on's and rear sets, but that bike is stunning Ian, it does look two years of work and a lot of love has gone into it, you must be very proud of that.

Kind regards Colin

Re: Engine?????????

Very nice, Ian.
Is that a John Tickle front brake ?

Re: Engine?????????

Yes, it is...Ian

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

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