Left hand crank arm fell off whilst riding home Sunday, on close inspection the threads on the plastic preload cap see very worn/shallow.

Bit surprised as I would have thought the pinch bolts would be enough to hold it on, but even tightening them right down (as far as I’m happy to go anyway) still allows the arm to slide off with very little encouragement.

Although not good in practice, I thought it would be theoretically okay to ride without the preload in much the same way as riding without a top cap (although removing it may be difficult once the pinch bolts are torqued).

Anything I’m overlooking?

No, you’re right. BB/cranks may be ready for a change though.

Okay ta, just torqued them up a bit higher and it seems to be holding. Bit tighter than I’d normally be happy with though.

Should be enough to squeeze an hour on the turbo anyway…

Remove a spacer so they go further onto the splines…