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Gas House Gorilla said:
What is the difference between semi-unit and pre-unit construction BSA's? I have always known them as either pre-unit or unit construction engines.
I think you mean a bolt up and a pre unit, the bolt up was a seperate gear box and engine that were bolted together and were in A7 form up till 1948 and in A7 and A10 form from 1949 till 1953 ish. Then they went to the pre unit setup where the box and motor were mounted with engine plates much like triumph pre units untill they went to unit construction.
Hope this helps.
Pics of my 49 bolt up A10 can be found here http://www.chopsnbobbers.com/productssimple6.html
 
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