BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: Mike Farmer on 28 November, 2017, 18:45:19

Title: M20/M21 block
Post by: Mike Farmer on 28 November, 2017, 18:45:19
 :)

Just a thought. Do the M20 and M21 have identical cylinders. The reason that I'm asking is because my book says that the carb bore is different, that of course makes sense, and I am wondering about whether there is a difference in the inlet orifice.

Mike 8) 8)
Title: Re: M20/M21 block
Post by: TTJOHN on 29 November, 2017, 14:35:04
Not entirely sure but I do believe that they are one of the same, the only difference is the piston & piston stroke, I think they were both fitted with the same carb, 276 if I remember correctly
Title: Re: M20/M21 block
Post by: Mike Farmer on 29 November, 2017, 19:45:53
 :)

Thanks for that. My book says for the Monobloc- M20 376/21   1"
                                                                       M21 376/23   1..1/16"
However as far as I can tell the receiving orifice on the block appears to be only the one size 1.1/8".
However its really not important as its only for my interest. But I would be interested if anyone has a definitive answer.

Thanks agen

Mike 8) 8)

 
Title: Re: M20/M21 block
Post by: Pete C on 30 November, 2017, 13:09:50
I think they are different but only in the size of the inlet port. As you noted the carbs are slightly different sizes. A definitive answer can certainly be got from the kindly folks on the WM20 site forum at:

http://www.wdbsa.nl/

Cheers
Pete
Title: Re: M20/M21 block
Post by: JulianS on 30 November, 2017, 14:12:20
Parts book shows the same part number for both.
Title: Re: M20/M21 block
Post by: Pete C on 01 December, 2017, 05:54:24
same in my parts book..I stand corrected  :-X

I have an M21 on which I think someone has previously opened the inlet out a little to suit the carb as it is more than 1" diameter.

cheers
Title: Re: M20/M21 block
Post by: Mike Farmer on 01 December, 2017, 08:30:38
Hi

Thanks for all your replies.

As some of you may recall I 'accidently' converted mine about 12 months ago so I'm scratching around a bit.

My inlet port is 1 1/8th which is larger than either so no problem.

We'll see what happens some when down the line "if and when"

Mike 8) 8)