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The BSA Twin Carb Mystery


Cylinder Head History


The Official Record

To start any piece of research, look at existing records.  What do the BSA parts lists tell us?

In 1948 BSA produced a sports version of the A7, the Star Twin, with twin carburettors and a 7.5:1 compression motor giving it 31 bhp. However, this was an all iron engine superseded in 1954 by the A7 which had an alloy head.

The A7 head most will be familiar with has an integral single carb manifold, is made of alloy and is listed as part no. (67-1121) and was used for all A7 models from 1956 to 1962 when BSA stopped producing the A7.  A photo and illustration from the original BSA parts list are shown below.

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

 

1956 – 1962 A7 Shooting Star Cylinder Head (67-1121) - the real thing

     
   

 

Eagle-eyed readers will have spotted the gap between 1954 and 1956.  What kind of head did BSA produce then?  The alloy head produced between 1954 and 1956 (67-1101) is shown below.

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1954 – 1956 A7 Shooting Star Cylinder Head (67-1101) - parts list illustration.  

1954 – 1956 A7 Shooting Star Cylinder Head (67-1101) - the real thing

 

If you came across one of these at an autojumble, you might think it's a twin carb item, but it's not.  BSA made these with a detachable ‘Y’ shaped manifold, onto which you would fit a single carburettor.  The published A10 cylinder head story is rather simpler. Though the part numbers differ with different models (Gold Flash, Super Flash, Road Rocket, Super Rocket, RGS) illustrations all show the same single one-piece iron or alloy casting as was used for the later A7.

So, with disappointed resignation I decided that my own 'job-lot' A7 head was just an unusual early alloy head, nothing exotic or sporting about it.

But niggling doubts persisted. My 'job-lot' engine had certainly been tuned with much polishing, lightening and balancing in evidence, drilled and slimmed timing pinions, different breather timing and a 358 camshaft, which wasn't listed in the parts books - perhaps this engine had been raced?  In addtition, the BSA press photo clearly showed a racer, though I didn't know what since it was supplied without a caption.  Finally, my contacts in North America were talking about both A7 and  A10 heads with the bolt-on manifold design but with twin carburettors fitted instead of the 'Y' shaped manifold.  Why would BSA have bothered with the needless expense of a bolt-on manifold?

The Unofficial Record. 

All the illustrations and part numbers shown above can be found in the literature available – Haynes, Bacon,  BSA parts lists and owners manuals. What were my sources in North America telling me?

The US and Canadian twin carb heads. The people I had been corresponding with about these heads had told me they had A10 heads as well as  A7. These cylinder heads are externally similar to the alloy A7 head produced between 1954-56, But with a different part no. (67-1106) and separate bolt-on inlet manifolds (67-1330R and 67-1331L). These manifolds are separate alloy castings. On the head end there is a1/2-' thick flange shaped like the standard Amal flange and drilled for studs with normal 2" centres. Behind this flange is a 1/2" thick duct and then another flange set at about 55 to the head flange to allow the carbs to be bolted vertically. Bores on the manifolds are 5/16" and bored straight through. The manifolds do not have splayed bores.

 Some also mentioned these being fitted to machines from new and mentioned the Spitfire scrambler, a model produced for the American market only between 1958 –1963.   About 300 were built, which makes it a rarer than the Rocket Gold Star.  Then, in response to an appeal for infromation in the UK BSAOC magazine, an email and photo arrived from Finland appearing to

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1957 A10 Spitfire Scrambler originally supplied by Hap Alzina, allegedyl with twin carbs fitted

 

show a Spitfire scrambler supplied allegedly supplied with twin carbs by Hap Alzina, one of BSA's two distributors in the US during the 1950's.  Clearly there was more to know than was detailed in the official texts.

The BSA Press Photo. Although the bike shown in the press photo I bought from the owners club many years ago carries no caption, US contacts reckoned this almost certainly shows one of the machines the factory prepared for the Daytona 100 and 200 mile races during the 1950’s.

Since BSA's racing history is documented, I turned once again to the official texts to investigate BSA's Racing Success.



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