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I don’t think you can fit the Si cranks to a BSA frame - the axle is too short to cover the use of external cups as you have discovered or if you get an axle long enough the chainline goes out of whack and the FD will not work.
Admittedly it was much simpler when they only made the 104mm axle for 68mm shells and it was crystal clear that they would only ever work in a proper BB30 shell.
The BB30a shell for which the 109mm axle was made has 5mm added to it making it 73mm... but it’s added to the non drive side - the spider is in the same plane as it always was on the og 104mm axle.
Tl;dr cranks won’t work in your frame
Howard
Hello, I'm going mad trying to fit new press fit cranks on a caad 9 frame, with threaded BB shell. I have bought a rotor BB/ converter which I have fitted to the frame, https://www.wiggle.co.uk/rotor-bsa30-bottom-bracket-cups-steel/?lang=en&curr=GBP&dest=1&sku=5360740730&kpid=5360740730&utm_source=google&utm_term=&utm_campaign=Shopping+-+All+Products&utm_medium=base&utm_content=mckv|ssR2w6YUC_dm|mcrid|295292317327|mkw||mmt||mrd|5360740730uk|mslid||&mkwid=ssR2w6YUC_dm&pcrid=295292317327&prd=5360740730uk&pgrid=58852352866&ptaid=pla-522561790471&gclid=Cj0KCQjww47nBRDlARIsAEJ34bk0OsoG7N0aZ4zq58zQ46E1Xv3y6Tj_qb7CzApnJuVQwShdlXHX8hwaAt_UEALw_wcB
Trying to fit some cannondale SI spider ring cranks with 109mm spindle, however this comes up too short once the rotor cups are on.
Also it seems that the drive side would sit quite far apart from the frame, making the small chain ring nearly an inch off the stays, which seems weird.
I'd be grateful if anyone could shed some light on this. Do I just need to get a longer spindle? What's the deal with the ring so far off? I don't mind having to look for different cranks if necessary. Thanks.