BSA M21 Parts & Spares
The M21 is the big side-valve single BSA kept building long after the overhead-valve twins had taken over the showroom, because it did work no sports machine would do: pulling a sidecar, day after day, at low revs and without protest. A good number are still earning their keep with a chair attached.
At Obsolete Bike Parts the M21 sits alongside the rest of BSA’s M and B series, which is half the point. Many clutch, gearbox and bottom-end components are common to several of those models, and our listings quote the numbers so you can check rather than guess.
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€ 44,00 Select options This product has multiple variants. The options may be chosen on the product page -
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€ 45,00 Select options This product has multiple variants. The options may be chosen on the product page -
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M21 or M20?
The two look much alike and share most of the cycle side, the gearbox and a fair amount of hardware, but the M21 is the larger-capacity engine of the pair and the internals do not all carry across. Several items we list are marked for B31, B33, M20 and M21 together, which is a reliable sign that a part really is shared. Where a listing names one model only, take it at its word. Several listings quote two numbers for one component because the reference changed during production; either number is enough for us to identify it.
What we stock
- Bottom end — big end bearings, bushes, timing side components
- Top end — cylinder head and joint gaskets, valve gear parts
- Clutch — friction plate sets including Surflex, springs and centres
- Transmission — engine and gearbox sprockets in several tooth counts, chains
- Cycle parts — brake, control and fastener items shared across the M series
Running an outfit
A solo and a sidecar outfit wear differently, and gearing is usually the first thing an owner wants to change. Tell us what you are pulling and we will talk through sprocket sizes before you commit to a set.