Shimano Hollowtech Crank Install - How to confirm 180 degree install
 

I am simply replacing a left crank arm with a power meter crank arm and wondering how to confirm a perfect 180 degree install between the left and right arms. I might be over analyzing this, but after install it seems the cranks may be a few degrees off from 180 degree...it could also be an optical illusion given no part is perfectly straight anyways. Is it event possible to be this close to 180, but not spot on?

The splines on the BB spindle don't really permit you to get "close" to 180 degrees, and I have never had a problem with this. You could rotate it one spline/notch in each direction to confirm it's right.

edit: it's been awhile since I installed mine, but I think the little "safety plate" can only hook onto the spindle if the crankarm is in the correct position.

You cannot install the non-drive side (left) crank arm incorrectly. The splines are keyed, meaning they are not equally sized and spaced, to ensure that the left arm can only be installed onto the spindle at 180° from the drive-side crank arm.

Above two are right of course. Nothing more to be said about this topic specifically.

But I'd like to say that this is a FANTASTIC example of how good design improves lives with almost no drawbacks. Shimano could *easily* have chosen to design a splined interface that's radially symmetrical and thus requires careful installation to align. And then if you screwed up the install, Shimano could blame you, the user, for not following their instructions or whatever.

Remember this next time a company tries to blame the user.

Poka-yoke.

I'd agree with this. And the reason I asked the question is when I was new to biking, installing my first crank, I didn't even think about positioning (I guess assuming you shouldn't be apply to install in the wrong position)...I tightened the bolt and the crank positioning was all messed up. Makes much more sense to do it this way!

You guys have all read the studies that prove that crank arms 180 degrees apart delivers maximum performance from a bike? Yeah, nah. You probably still use round chainrings too.

Well actually... You can totally install it at 0 degrees if you like, and folks who pedal with their arms do. It'd be nice if Shimano would put another hole for the check pin for these peeps...

Interesting...

Though, I admit, I've never tried to do this...but, now you've got me curious. :foo:

You can't install it incorrectly unless you hammer it in. The safety pin won't go in if the left arm is not slid into the spindle correctly.

agreed that Shimano was smart in keying the crank arm just like its cassettes/freewheels
Saved countless hours of aggravation by garage mechanics like myself.

The keying isn't what ensures you can't do it wrong, it's the splines.

Any HT2 cranksets I have lying around off a bike are stored at 0 degrees, they take up a lot less space!

Besides the stopper plate and it's pin needing to in the hole in the crank axle how hard is it to just look at the damn things and know instantly?

I had exactly the same thing happen
 

I could have written your post. I have been staring at them since my friend installed my Stages left crank arm. I had even asked him if the design was such that you couldn't install them wrong and he said no you could. Now from this thread I see that he was wrong and they have to be straight, no matter how much I stare at them. Thank you putting up this post!!

I'd hazard a guess that the OP is installing an FC-9100 era crank or later -- the way those arms are shaped makes it easy to second guess whether they are 180 degrees apart.

Exactly. The drive-side (right) crank arm is asymmetric on HT2 road cranks, but this is only obvious when viewed from the drive-side. When one installs the non-drive-side crank arm, the asymmetric profile of the drive-side crank arm throws off one's perception of the crank angle from the non-drive-side. It took me a while to figure this out when I installed an FC-R7000 crank set. No such issue with GRX cranks.