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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.

  1. 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.

  2. 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

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The reason for plastic here is that you'd rather ruin the (cheaper) nut than the (more expensive) bottom bracket. Properly installed, the plastic is more then enough...the tool that you use for installing the nut only lets you do a tiny bit more than "hand tight". –  Ken Hiatt Sep 13 '12 at 16:43
    
Not for your exact model, but all the same parts and a decent reference: bike.shimano.com/media/techdocs/content/cycle/SI/Alfine/FC-S500/… –  Ken Hiatt Sep 13 '12 at 16:47
    
Thanks Ken, interestingly the document you link suggests periodically retorquing the retaining nut, as though it coming loose is an expected part of its service life. Re material strength, I would hope that aluminium would still shear off before the harder steel. Nonetheless since they are all relatively cheap and before I had any responses to this I shelled out a few dollars for a titanium cap that should still be softer than the steel. Hopefully this doesn't turn into a big mistake. Also nice that it has a hex drive. –  J Collins Sep 13 '12 at 18:07
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2 Answers

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I grease the splines. I prefer to put a bit of blue loctite on my fixing bolt to be sure it doesn't loosen up if you run in to issues with this happening. I probably would be a bit wary about using loctite with a non-metal fixing bolt.

I've seen aluminium fixing bolts on ebay, just search for them. Here's one set of results.

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Thanks Benzo, I will probably loctite the pinch bolts, haven't decided on the cap just because the diameter is large and once it sets it'll take an act of war to detach. Have you ever had problems? As I mentioned to Ken above, I went ahead and bought a metallic cap (titanium) so the bond should be effective. –  J Collins Sep 13 '12 at 18:10
    
You could always use a lower strength threadlocker, loctite 222 - purple, which shouldn't need to be heated to remove. I havn't had any issues myself, but there are probably some cases where it gets tough to remove with standard tools. –  Benzo Sep 13 '12 at 18:35
    
I can attest to blue Loctite causing a part to be difficult to remove. I had a bike with an older square taper bottom bracket that kept coming loose. Blue Loctite solved the problem, but when it finally was time to take it off I had mostly disassemble the bike, put the bottom bracket tool into a vice, put the frame and bottom bracket on the tool, and turn the entire bike around the vice and tool. That just barely got me enough leverage to get it off. I don't use Loctite too much anymore. Definitely wouldn't put it on a fixing bolt. Never tried purple though... –  jimirings Sep 17 '12 at 13:42
    
Wow jimirings you mean you put Loctite on the taper itself? I can't actually believe you got it apart at all! I would be expecting to need something like a 5 ton press to get that apart, good to hear it didn't break anything. –  J Collins Oct 10 '12 at 15:13
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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.

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That is pretty much what I assumed originally, the nut is a piece of plastic. Having reassembled the system by the book (with the same old plastic nut as I'm still waiting on the Ti version) it seems to be holding together okay. One detail, since the shaft is a slip fit in the bearings it seems that the torque on the nut is intended to preload the bearings laterally enough to force them to turn with the shaft. From the original install I can see the shaft had polished on the (plastic!) bearing surfaces. Still the pinch bolts should be enough after it is assembled to maintain that preload... –  J Collins Oct 5 '12 at 13:30
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