This is where it gets complicated.....
Early plunger bikes have barrels and heads with narrow or thin fins. The number of fins stays the same, its the profile that changed. On the introduction of the S/A Range BSA increased the fin area to improve cooling. Wide fin heads and barrels were fitted to later (post '53 or thereabouts) Plunger bikes on both A7 and A10 Models. Gaskets and head bolt spacings are the same for early and late motors.
A10 Plunger head thin fin casting number is 67-251, found on the underside of the inlet manifold.
A10 Wide Fin Plunger and S/A Iron Head Casting is 67-1066, usually on the underside of one of the lower fins. Both thin fin and wide fin heads have a combustion chamber diameter of 70mm. If it measures 66mm, it's A7. Barrel fins match the head profile.
Other important points. Rocket and Shooting Star have their own expensive Alloy Heads. A7 barrels have a narrower bore, 7 fins and no cut outs to the lower spigots. Head gaskets differ to reflect smaller bore on A7 models.
Swarfy.
