The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: Dean on 21.09. 2015 21:30
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I am in the process of changing my small journal crank for a large journal one.
I was surprised to find that the big end shells for large journal rods are narrower, by about 1.5mm than those for small journal rods.
Does anybody know why this should be?
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It's a bit late to answer this but here goes anyway (while my tea is brewing *lol* ).
My guess is that BSA decided as that if the journal on the new crank was larger in diameter, hence had more bearing surface, it could then be narrower, being narrower of course reduces the bearing surface.
I've not tried to work out which journal/bearing type has the greater surface area, I doubt there is much difference.
I don't know where I heard it from but the crank design change was initiated due to small journal cranks breaking at the join between journal and web (or at least flexing too much), as opposed to big end failure, if true BSA would have concentrated on strengthening that area rather than making the big end last longer by having greater surface area.
The small journal crank seems to be generally accepted as fit for purpose provided the engine is not highly tuned eg high compression Pistons, 357 cam etc.
Time for tea *woo*
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I'd also point out that the issue with the small journal cranks wasnt their bearing area, but that the cranks snapped if the motors were tuned. The LJ cranks resolved crank failures not an inadequate bearing surface.
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Yep, snapped a few. 14:1 at 8 grand *bash* *pull hair out* *sad*
Cheers