Someone emailed me last night about an alternator crankshaft for an M21, as he wasn't sure what he needed to sort out an engine sprocket, so I looked in my parts list for special AA parts, I didn't find the crankshaft as my parts book stops at 1952, but what I did notice was that the AA had a different flywheel part numbers which were 66-636 and 66-643, what was the difference? It's not for an alternator model, so what could possibly be different (materials?) I noted also that the barrel had a different part number (66-88) What was the difference with the barrel? Was it unlinered or was it different materials?

email (option): davmax@ntlworld.com

The AA models had heavier flywheels.
Don't know why, they just did.

email (option): wariron@tpg.com.au

M21 flywheels changed from cast iron (66-705/707) to steel (66-636/634) part way through the 1947 model year. BSA appears to have reversed this sometime in the 1950’s (1953-54?) and went back to the original cast iron wheels for normal production, but AA bikes retained the steel ones. I wonder whether the AA preferred steel flywheels (I can’t think why they should) or whether there was a contractual agreement which prevented BSA from changing the AA machine specification.

Similarly, BSA also changed from lined to unlined barrels in the same period but the AA stuck with the lined version.

It sounds to me like BSA went through some form of ‘value engineering’ exercise in order to cut manufacturing costs, but either the AA were having none of it or BSA was otherwise prevented from altering AA bikes.

ChrisJ

WD M20s and all OHV models also always had steel flywheels. Later B33s, M20s and M21s all used the same (steel) flywheel set machined differently to provide the appropriate crank throw for each model...Numbers for steel wheels start with a six..iron wheels a seven..I have a set of the alternator type AA flywheels but have never checked the numbers. They were only used in the last 3 years of production....Ian

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