Crank amateurs and seasoned vets alike are lulled into complacency by the simplicity of a Shimano crank install. There are a few pitfalls that are easily avoided. Here's a big one: The plastic safety tab won't snap down, and what that means.
This little plastic piece right here come on and take a look that we're all familiar with who have Shimano cranks this little piece right here seems like a little throwaway item, but it is not. It's a very good indication that you might have a problem with your install if you can't push that down. Now let me show you here. You see how these splines are right here you can see them very clearly of course they correspond to the splines on this crank right here. Now an important thing to note is that the back side of here has no splines whatsoever now all the Shimano cranks road or mountain are like this they're not splined all the way through and what this means is that if you stick this crank arm on here you can actually have it look like it's splined but it's not. Then you can just barely get it splined like that and tighten this down and it would work but you hardly have any spline engagements you could shear the splines just right off of here just strip them right off now the real danger is when you get the crank right about there and you notice that I can't push this down all right I can't push this little thing down now you see it went down. That's because this needed to be on far enough for that to pop down into this hole now what is all this about. Well what this means is if your bottom bracket like this one right here if it's too wide this will not be able to push on far enough this is a common problem with bottom brackets that thread together and the reason for that is there's not a whole lot of meat on this edge right here okay this isn't even one of our bottom brackets and look at how thin that is if you start machining that out to make a place to hold it with a tool to turn it you're having to add to the width of the bottom bracket okay that's where you start running into crank arms not being splined on properly and because you're going wider your chain line is going to be off so all those intermediate detents you have on your Shimano really aren't going to work properly because the crank chain rings in space are not in the proper orientation for how the derailleur was designed to move through its very limited motion
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