Attaching crank arm with Shimano SLX M660 Firstly, apologies if this question has tones of frustration, my new crank set and bottom bracket has never been installed correctly, so I am going to do it myself.


Should the drive spline on the left crank be greased? It was assembled with grease but that seems to have helped it fall off twice now.
Can I get an aluminium substitute for the plastic tension nut and if so can someone identify the exact part number I need? I can see they exist, just not certain which one will be compatible.


Background: Originally the shop forgot to put in the bottom bracket spacers, so while the crank and shaft moved laterally making shifting a mess, I think it also allowed the crank to move and take tension off the tension nut allowing it to spin off and disappear. This allowed the crank to eventually just slide off the spline. When it was reattached with a new nut and (this time too many) spacers, it didn't have enough thread engagement and eventually sheared threads, allowing the pedal to slide off again. I never want this to happen again. I intend to get the tools to disassemble the bottom bracket, place all of the spacers correctly, then use an up-rated tension nut whose threads won't shear.

Shimano crank set technical document

Edit: Update
Back in 2012 I rebuilt this crank with grease and by the book. It used the replacement plastic retaining nut, blue loctite on the pinch bolts with correct torque and the correct spacer arrangement in the bottom bracket. This crank has held up perfectly and continues to, three years later. The titanium nut arrived but has never been needed.
The cap is like the star-fangled nut on a threadless headset - it's only there to hold the crank in place when you do up the pinch bolts.
If the pinch bolts aren't holding the crank in place on their own, something else is wrong, and you really need to fix that. If the crank splines are worn because the shop fitted it wrong in the first place, they should replace the crank.