Assuming its not a spline issue now as a result of incorrect fitting, you need to go back to installation with this one.
Did you fit it?, Did LBS fit it?
Ensure that your BB is fitted correctly as per your BB shell width.
Check the installation instructions for number of small driveline spacers required – use this as a starting point only. Any spacers i refer to from here on are the ones that fit over the crank spindle. There should be no need to adjust the BB spacers from here on.
Install cranks into BB with driveline spacers in place and tighten crank bolt until it bottoms out.
Heres your fault finding – IF:
1) crank bolt doesnt bolt out (doesn’t definately come to a dead stop at the bottom of the thread (forget about torque readings as I doubt ‘bastard tight’ is on the scale 😉 ) then you have too many driveline spacers in place. Remove ONE and re-install. try again to bottom out crank bolt. (also now check as per step 2)
2) crank bolt bottoms out but you notice wobbling or knocking then you have too FEW chainline spacers in place. Add ONE and reassemble. If you already removed one, then add a half width chainline spacer instead of a full one
3) crank bolt bottoms out, but cranks dont spin freely. replace full width spacer for half width spacer and reassemle.
Basically, you are trying to achieve a bottomed out crank bolt with a BB preload that will allow the crank arms to spin freely. Unless you bottom out your crank bolt.. you may end up with self loosening cranks. Effectively the bottom out acts a mechanical ‘thread lock’
a bottomed out crank bolt with too few chainline spacers (i.e. cranks wobble within BB) shouldn’t come loose, but will accelerate dirt ingress and BB wear.
I’ve seen many external RF cranksets installed badly whereby the crank bolt has been backed off in an effort to make a tight BB spin freely resulting in – self loosening cranks! I don’t doubt that bikes come from manufacturers or from LBS retrofits installed in this way.
Each of my RF crank installations take at least 2 or 3 install, remove reasseblies to get them spot on.. DH bikes with chainguides often more than 5 fettling refits. Kind of a pain, but then i’ve never had one loosen on me.
I should point out that I’m pretty anal with bike builds and it takes me a lot longer than it should 😉 I also agree that this is kind of a PITA.
This is my opinion, from experience and I hope it helps.