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BSA B40 GB trials gearing info

beardy2000
in Pre-65 Bikes

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Continuing on this theme, I have a B40 GB motor with 18/52 primary gearing and 14/64 final drive sprockets. Standard sleeve gear/ layshaft is 22/14, NEB do a conversion which is 21/13. I suppose the gears must be of a larger profile otherwise I would have thought they should add up to the same total. If I went from 22/14 to 21/13 does anyone know how to calculate what difference I would have to make to the final drive sprockets to retain the same overall gearing for the bottom three ratios? The aim is to keep 1st, 2nd and 3rd the same and end up with a higher top.

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