I'm stumped by this, hope someone can help!
The left crank on an SLX crankset came loose on the bike during ride. The splines in the crank were fairly buggered from wobbling around loose so I bought a replacement - the drive side was still OK as it seems to be the stronger metal of the two.
I've come to install it, and I can't get enough of the drive side to come through the bottom bracket to get the safety pin to engage on the crank.
I've fitted tonnes of bottom brackets and cranksets before, the bike was faced last year and still looks smooth, and I've measured the BB spacers and they are 2.5mm as they should be (2 of them on the RHS, one on the LHS - it's a 68mm BB).
I can only think the bearings have expanded somehow, or the drive side is damaged and not engaging fully - but I can't see anything obvious wrong with it...
This photo shows the drive side fully engaged (after hitting with a rubber mallet to be sure!) - it doesn't look far enough through to my eyes: