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Bsa m20 wheelbase

Hi
Can anyone confirm whether the wheelbase dimension’s of the m20 differed between solo & sidecar contracts
I have looked but cannot find any details on this

I know they ran 18” wheels as opposed to 19” but wondered if the rear frame section differed in length to the std solo frame

The frame part numbers are different to reflect the sidecar lugs etc - war time & post war being different too - to each other
Br
Jo’b

email (option): jonnyob1@googlemail.com

Re: Bsa m20 wheelbase

John it's never occurred to me that the rear frames might be different.....Why would they be? If they were different, I'm sure other items would also have to be different too?? The mudguard to stays would probably be off?

As you say the sidecar outfits were fitted with WM3 x 18" rims, but 400 tyres were used which gives about the same radius but with a bigger foot print.

Maybe Rob Miller can confirm as I think he has both. Ron

email (option): ronpier@talk21.com

Re: Bsa m20 wheelbase

I don't think there is a difference unless you count the shorter fork legs, these are from a WW2 RAF data book and it doesn't mention the 400-18s on the outfit, I think that was only on certain contracts?

Rob

198

208

email (option): robmiller11(a)yahoo.co.uk

Re: Bsa m20 wheelbase

Hi
I used a pre/early war - single sided side car lug frame & during the dry build a good whole back - I never fitted the inner primary cover - what could go wrong !!

This week the discovery of a school boy error was that in fact I had blasted and painted an inner primary cover from what I can make out is perhaps a C10
The aperture for the crankcase is smaller & is the only difference - the outer cover fitting nicely
So
I overcame that as I have inner covers here which I positioned only to find now that the upper chain guard when in place will not align - this was an nos item too
Another primary & another upper chain guard - used - same -

Hole in the front of the guard when the stud protrudes - leaves the rear fitting bracket approx 1/2” to the rear of the frame stud it is supposed to fix to

I have a replica version here - I think it is anyway - fits like a glove
Baffled here in the tropics of Ireland
Br
Jo’b

email (option): jonnyob1@googlemail.com

Re: Bsa m20 wheelbase

Rob, I agree, not all the outfits had 18" wheels. But that is all that is listed in the RAF illustrated parts list for 509 bikes for contracts C10655 and C14052. The 18" sidecar wheel is listed in a different section.

John there are lots of BSA primary case types and some very similar to each other....You are not alone with that conundrum. Ron

Scan11-2

email (option): ronpier@talk21.com

Re: Bsa m20 wheelbase

John,

I can check if the length is different, if you want to know for sure. I have the following rearframes:

1. post war with 1 side car lug,
2. Pre war/1940'ish Dutch Army, 1 side car lug and attachment for the short 'jiffy stand'
3. 1939 KM20 rear frame, as fitted to my KM20

I would be surprised if there is a large difference in length betweenj those frames.

Regards,
Bastiaan

email (option): wdmotorcycles@gmail.com

Re: Bsa m20 wheelbase

"1. post war with 1 side car lug,"

? Bastiaan I thought the post war frames have two lugs?

Ron

email (option): ronpier@talk21.com

Re: Bsa m20 wheelbase

John, I think you have run into the same issue I had some years back. There is a very small difference between the rear primary for an M20 rigid and an M20 sprung heel.
Your issue may be different but I will run through my issue as it might help someone else.

This an original WM20 rigid inner primary - note the distance between the end of the rear chain guard fitting and the square.



This is what bought at a swap meet and was very pleased with the purchase as all the screw holes were perfect, no holes worn from loose chain etc. Again, note the distance to the edge of the square. :angry:



It gets worse - Not long after that I came across a NOS inner in OD, complete with sliding plate and thought I won the lottery and paid quite handsomely for it. Who would have suspected it wasn't for a rigid?



Time passed and when I tried to fit the used one I ran into the exact issue of the rear chan guard missing the mounting stud by 1/2"-3/4".

This is from the 1952 parts list.



Check the part number stamped on the inside of your inner chainguard if it is still visible - my original WM20 has 66-760? and the two that have the shorter chain guard mounting point are 65-7587.

This wouldn't be a difficult thing to modify but I haven't brought myself to hacking up a NOS part (yet).

Ray

email (option): rays54@hotmail.com

Re: Bsa m20 wheelbase

Thanks all

Yes it seems that my issue is not a wheel base length issue but an inner chain case
No matter how many times you handle these parts / sometimes you can learn something new or observe a new feature
Those pictures speak 1000 words ray / I’m off to the lock up to check what I have here
Many thanks

Br
Jo’b

email (option): jonnyob1@googlemail.com

Re: Bsa m20 wheelbase

Hi ray
Found another 2 inner covers here
66-7602
65-7578
65-754 - the one I chose incorrectly

7602 seems compatible with m20
7578 rigid b31 it seems
754 is unknown and not seen in any docs I have

Br
Job

email (option): jonnyob1@googlemail.com

Re: Bsa m20 wheelbase

The B series with a 6 spring clutch have a thinner badket & thus a shallower outer
I would have thought the inners were the same
The B series outer will bolt up to an M series inner but the clutch cover will rub on the outer when the clutch is in.

Re: Bsa m20 wheelbase

Hi Trevor
It’s the inner cover that I had the issue with - all my outers are correct for single spring clutch

After ray posted photos here on the forum & I checked my cases here - it was clear to see the difference and is in fact the positioning of the top bracket for the upper chain guard
If you study his photos you will see this
They can be modified easily I believe with the undoing of the spot welds & re positioning it
But another school day
Br
Jo’b

email (option): Jonnyob1

Re: Bsa m20 wheelbase

Was there a longer top chain guard later in or after the war?

Rob

email (option): robmiller11(a)yahoo.co.uk

Re: Bsa m20 wheelbase

Just to confuse things more, my 65-7578 has the longer chain cover. ???

email (option): pes.sales@btconnect.com

Re: Bsa m20 wheelbase

Hi John

Pre '39 inner case is listed as 66-7504 for all M models, could it be the same as 66-754.
I just had same problem but other way around, being sure that later case fits to the earlier engine.

edit: I did misread the prefix, 65 is something to do with B group models.

Kalle

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