Cylinder Head History

The focus of this web site is on the alloy cylinder heads produced for the pre-unit A7 and A10  twins made between 1954 and 1963. 

The earlier Herbert Perkins designed 500 twin and later Hopwood designed semi-unit 650 are outside the scope of enquiry.

Alloy cylinder heads first appeared in 1954 as part of a package of improvements for 'A' series twins that included a separate gearbox and swinging arm suspension. 

Although small numbers of all-alloy engines had been made for privateers and alloy heads fitted to bikes prepared for the Daytona beach races, 1954 was the first year alloy heads were available to the public.

During the lifetime of the pre-unit twins, BSA used two different designs of alloy head - one with a detachable manifold, the other with a cast-in manifold.

Detachable Manifold head

Between 1954 and 1956 heads were fitted with a detachable 'Y' shaped manifold. The design rationale may not have been obvious to the buying public, but BSA had good reason make them this way.

BSA had prepared bikes for the Daytona beach races in the US since the beginning of the 1950's but for 1954 they made a greater effort then ever before with ightweight frames, new engines and twin carburettors. 

Bikes raced at Daytona had to comply with  AMA class 'C' regulations - essentially production based bikes.   If BSA wanted to use twin carburettor heads on the Daytona bikes, the same heads also had to be available to the public.

Although BSA could have produced just the minimum number required for homologation they decided instead to fit them to bikes sold to the public.

They planned to capitalise on the success of the Daytona twins by producing a super sports 500 twin that would replace the Gold Star. This bike might be produced and it might have twin carburettors - by making a detachable manifold head BSA kept their options open.

Of course the public could remove the manifold and fit twin carbs if they wanted, but without the same preparation the Daytona bikes received, they were unlikely to get Daytona performance.

This design of head was also fitted to the 650 Road Rockets of the same period. Cycle magazine of September 1954 tested a Road Rocket and said:  

"As I mentioned before, the cylinder head is cast of aluminum alloy incorporating a removable intake manifold.  This was done with the idea of easily installing a set of twin carburetors.  Angle of the intake manifold is such as to allow easy fuel flow to the valve chamber.  These improvements in the BSA 40-incher are not secrets, but are incorporated with the idea in mind to put a "hot" machine within the reach of any sports rider who is in the market for a vertical twin."

Since Road Rockets export -only models until 1955,  few of these heads were seen in the UK.  

1954 – 1956 A7 Shooting Star Cylinder
Head Part no. 67-1101

Cast-in manifold head

Cylinder head design was changed on both 500cc and 650cc models in 1956.  The separate inlet manifold was dropped and a one-piece cast-in manifold design adopted.

BSA had been very successful at Daytona with twins  getting positions 1, 2, 5 and 6 in 1954 and just missing with positions  2, 3, 4 and 5 in 1955.  This was just what Bert Hopwood had hoped for and should have justified his  'Gold Star Twin' for the public but it was not to be. 

Hopwwod had even grander plans for new BSA models that would need more investment than the directors were prepared to give him. Frustrated, Hopwwood moved to Norton in 1955 to pursue his dreams of sports twins elsewhere.

The impetus to produce a 500cc super-sports twin left with Hopwood and support for the Daytona event waned.  In any case, Gold Stars were still selling and winning  - did BSA really need 2 sports bikes in the 500cc class anyway?   If there was no longer any need for a twin carburettor head then a one-piece casting could be used that was cheaper and simpler to manufacture.

So in 1956 both 500cc and 650cc models were fitted with a cast-in manifold that was used until pre-unit twin production ceased in 1963.


 

1956 – 1962 A7 Shooting Star Cylinder
Head (67-1121) - parts list illustration.