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Besides eyeballing it and comparing the line to the seat tube, are there any tricks to getting 48 splined crank arms perfectly aligned? I must have spent a solid 15 minutes looking at them from every angle until I was satisfied.

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Use a short bubble level about 12 inches long. Put the cranks level and compare the two. – GKBMXer Apr 15 at 2:41
up vote 6 down vote accepted

If your bearings are decent, just take the non-drive pedal off and let the pedal weight pull the other side down, or slowly let it fall from slightly off center. This is how I could tell with my Bullseye cranks, besides hopping on and trying to pedal it. If that doesn't work then I'd try tying a heavy weight or looping a paint can around one of the pedals close to the crank.

You're looking for a 7.5 degree distance, so on 185mm cranks that's 24mm or nearly and inch of diference between splines. Weighting the opposite side enough should make it clear. I then took some nail polish and marked both the crank and axle so I didn't have to play this game again.

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good idea, you made me look up law of cosine to get the distance for 175mm cranks. lol, it's 22.89mm...I guess it's pretty noticeable if you are off. – dotjoe Apr 24 '12 at 17:39
You're going to hate me if you actually used the law of cosines for this comment, but you're making it more complicated than it needs to be. The hypotenuse is 175mm, and you're looking for the opposite length of the right triangle made from the angle, so it's just 175 * sin (360 / 48) in degrees. You also REALLY notice if you try to pedal like that. – Ehryk Apr 24 '12 at 19:33
lol, I knew there had to be simpler way :) my geometry is extremely rusty, so I used first thing I found on google. – dotjoe Apr 24 '12 at 19:49
Im gonna give you the check for pointing out how obvious it'd be off. If I had realized this, I wouldn't have been so worried about getting it perfect. – dotjoe Apr 24 '12 at 19:52

Some BMX cranks are keyed, so that they will fit only one way. If yours is not keyed, then, no, there is no better way to line it up than to do it by eye.

If you want to double check it, take a plumb bob, and place it across the center of the pedal threads on one crank. Turn the crank until the string lines up with the other pedals' thread center.

If the string follows the center of the crank arms, you are lined up.

As pointed out by @Ehryk, the difference is pretty visually noticeable. Probably not worth all the effort involved here, since you see an almost 1 inch variation by eye easily.

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thanks, I was unsuccessfully trying to use a drywall T-square at one point. Will try the plumb bob. – dotjoe Apr 24 '12 at 16:01

Besides the given suggestion(s), another possibility (although a bit overkill) could be to take a picture from afar (5m+), with the camera aligned to the bottom bracket axis, and analyze the alignment on the zoomed image, via eyeballing or some drawing/viewing program.

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I would try using trial-and-error. First I'd attach one crank arm and get the nut finger tight. Then I'd put the second on where I thought it should be, take it off and put it on one tooth away from that, then try again in the other direction. I can't help but imagine that (as Ehryk mentioned) when it's wrong, it would be very obviously wrong.

Once I was satisfied with that, I'd line up one of the arms with the seat post or the down tube, take a few steps back, and eyeball it.

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