ZB33 8004

[Lantech31]

Hi everyone, I'm new to this forum so please forgive me if I'm not following protocol or the rules.

I have a BSA 500 M33 - I think.

The engine number is ZB33 8004.

I have searched everywhere and cannot find this combination of letters and numbers.

It was first registered in 1959 and is built into a plunger frame that I think is for a sidecar.

Could anyone help me with the year of manufacture and what engine manuals I should be following.

Much appreciated.

Tim.

[JulianS]

ZB33 prefixed engines used between 1949 and 1952. 

Yours from autumn 1950 and would have originally been in a plunger or rigid frame with number prefix ZB31.

[Lantech31]

Thanks for your reply.

The chassis number is BM20s11925.

It's in a plunger sidecar frame.

As I look more into the bike more doesn't make sense. The wiring colors are all wrong for any documentation I have found, e.g. the wires for the rear light are yellow. It does appear that the wiring is original.

Do you know if BSA used a different range of numbers for bikes built for sidecars ?

I haven't been able to find a zb range of 8000, only 7000 and 11000.

Thanks your your assistance.

[Lantech31]

I think, with your input, I'm much closer to the truth.

I think I have a 51 ZB33 motor, 8004 would make it the 1003'rd produced that year.

It is in a 54 m33 plunger frame and could have been the 5th for the year.

Awesome.

I thought I bought a 59 bike and ended up with a 51/54. Cool.

Thanks.

[JulianS]

Your bike frame is from a late 1958 BSA M21 600cc. This makes it a 1959 season bike hense your 1959 date.

Only the M21 was using the BM20S prefix frame at this time.

As said your engine is from autumn 1950 which makes it from a 1951 season bike.

[Lantech31]

I went off this list ( found googling ).

[JulianS]

You need to check later for frame 11925.

See link to BSAOC;

https://www.bsaownersclub.co.uk/number_display.php?theyear=5

And Glass's check book below.

[Lantech31]

 

I finally GET IT.

Thank you for your patience as your info seeped slowly into my brain.

I see the reference to 11001 and now realize mine is 924 frames later.

That also helps explain the first registered in 1959 situation.

One last question ( as my first question is well and truly done ) - do you think they had an old motor lying around at BSA or has someone cobbled together an old motor and a less old frame ? Maybe someone ordered a frame only ?

NO - the bike was original in 1959 but the motor is a 1951 replacement.

Tim.

[berniej]

I suspect possibly a change by a previous owner from side-valve to OHV. I have a 1952 B31 frame with a 1949 M21 motor in it.

The bottom ends of the side valve and OHV pre-unit B/M 350/500/600 singles were very similar and in my case I only had to change the engine plates (from memory).

Cheers,
Bernie

[idie]

My 1949 M33 started life as an M20. Somewhere in the past it was re-registered as an M33 probably when they sold the original reg number.