Shimano RX 100

[carpenter]

Not sure whether I should put this in vintage, but here we go:

I have an FCA 500 crank (RX100 Biopace) and an FCA 501 left arm, both 170mm any differences/reason not to use them together ?
I think that 500 may have been suitable for 113mm BB and 501 for 107mm BB?
The original FCA 500 left hand arm is "buggered" - threads for extraction gone

Thanks in advance

 

[Ajax Bay]

Not a problem. Need the right BB spindle length for the RX100 crank. I bought a bike for my daughter with a Tiagra crank (right side) and didn't realise till she came back from uni 3 years later that the left hand crank was a Shimano 600. (Nor did she, not that she'd have registered the difference.) Now using an FSA Vision (with matching left and right cranks):