The twin carburettor kits

In the mid to late 1950s, the USA was BSA's biggest export customer.  The importance of this market to BSA meant that they produced some models for the US only like the Super Flash or made bikes available in the US before the UK like the Road Rocket. They also made variations specifically for the US like the flat-track and Catalina Gold Stars or some finishes that were only available on US models.

Many of these differences are not listed in BSA parts books and appear only in BSA's service sheets or as notes sent to dealers and distributors. It is not a subject that is covered comprehensively in books about BSA history or restoration. This keeps restorers and website forums very busy!

To make things even more interesting, US dealers could source or manufacture accessories in the US and list them in their own catalogues as 'BSA approved', Sometimes these would be given part numbers using the same BSA 2 digit model prefix (e.g. 67- for twins) as the model to which they would be fitted.  The size of the US market gave dealers 'clout' their UK counterparts did not have and helped remove any objections BSA might have had about control of spare parts issue.

Usually, the accessories were very definitely aimed at the US market - tooled leather saddlebags, whitewall tyre paint, etc.  But in 1958 ??? listed a range of speed equipment - 'Speed Parts for increased performance from your BSA motorcycle' -  that included high compression pistons, racing camshafts, oversize valves, etc.  What were also listed were twin carb conversion kits for 500cc and  650cc pre-unit twins comprising special cylinder heads, inlet manifolds and matched twin carburettors.

Although sold as special parts the the cylinder heads  were in fact the old detachable manifold parts fitted between 1954-55 to Shooting Stars and Road Rockets.  The separate right and left manifolds were new, as were the twin control cables. Though TT carbs are shown in the illustration on the right these were not listed as items in the catalogue.

These parts were sold by dealers right up until 1964, a year after the unit twins had appeared and the old pre-unit twins had been discontinued.

Both heads cost $109 in 1958, cost $112.60 in 1960 and by 1964 were available as a full kit comprising the head, manifolds, twin throttle & cables, nuts, washers, studs and twin monoblocs, one with cut-off float chambernow complete with matched monoblocs and cables - cost $158.05.

1959 catalogue cover

Comparing the part numbers in the speed accessories catalogue with those in BSA spares list is interesting.

The original 54-55 Shooting Star head was listed as part no. 67-1101.  The speed accessories part is also listed as part no. 67-1105.

The original 54-55 Road Rocket head was listed as part no 67-1102. But the speed accessories part is listed part no. Although the 54-55 design of heads were used the original part numbers were not.

Are the parts speed accessory heads different from the original 54-55 heads?  Were the speed accessory heads simply surplus 54-55 stock being sold under the guise of performance accessories?

I would guess that the Shooting Star heads were surplus stock but that the road rocket heads were either re-cast since these were only available in the US with less likelihood of surplus stock on the shelves..

There is also the possibility that the speed accessory 650 heads were actually modified Shooting Star heads. I have held examples of both side by side and the inlet port dimensions are the same diameter.

Also, the casting number on the 650 head looked like it had been added or over stamped on top of the original number.

I don't know what the anwers are and I suspect the proof, if it could be found would reside in the heads or filing cabinets of old BSA dealers.  But this is less interesting than the reason why US dealers decided to sell twin carburettor kits in the first place.

54 –55 Shooting Star Head - 67-1101

54-55 Road Rocket Head