The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: Miker on 29.03. 2026 10:00
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Hi, I have a 1951 a7 star twin and a 1957 a7. They came in pieces and I am unsure which pistons go with which engine. I have one set of domed pistons and 1 set of flat top pistons. Any idea which would go with which engine. Also would the crank be the same for both years. Thanks for your time.
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Have you had a look on the Draganfly website? Click on this link > https://draganfly.co.uk/product-category/bsa/sparesparts-for-bsa-a710-b3133-c101112-m202133/engine-gaskets-gearbox/pistons-rings-and-liners/ and it will take you to the pistons for the A7 and when you click on the model it will show you a photograph. HTH.
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If the pistons are genuine BSA there will probably be a part number stamped on the crown.
If the pistons are Hepolite you will find a cast in part number on the inside edge.
Below is a page from the old Hepolite catalogue showing part numbers/compression/year which may assist identifying what you have.
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Draganfly do have a piston chart, (somewhere!)....search in the information section. It references BSA part numbers and Hepolite equivalents. Remember BSA part number is for a complete piston, not just the bare casting which is the number on the piston crown. Close but not quite the same.
Flat top pistons are unlikely to be A7. Both the Long stroke 62mm bore and later Short stroke engine 66mm bore pistons have valve cut out pockets and raised crowns.
If the flat tops are 70mm....they're A10.
Crank is the same for all Shortstroke A7 variants. Looks like both bikes are single carb.
Swarfy.