Bicycle Mechanics - Left crank inserts less deeply into square taper spindle

I've worked with several square tapers and have never encountered this issue.

I have a MTB with square taper spindle fastened by a bolt (not a nut). The left crankarm fell off during use. Upon inspection, the crank bolt was stripped because only 3-4mm of the threading penetrated into the spindle before it was tight. I thought this was odd and possibly due to a slightly deformed crank arm.

So I bought a new Shimano Alivio crankset. The same thing happens! The right crank arm (along with chainrings) fits onto both the left and right end of the spindle quite well, giving plenty of room for the bolt threads to grasp. But using the left crank arm on either side, it only goes in half way before it is tight, leaving the bolt very little threads to grasp the spindle.

This has been true for 2 cranksets already. So I pulled the left crank arm off another MTB. This one fits perfectly into the spindle! And the 2 cranksets' left crank arm have trouble with this MTB spindle as well.

I feel I must be doing something wrong. How can the left crank arm leave so much space while the right crank arm (of the same crankset) fit perfectly... for 2 different cranksets?

If what I've described is not clear, I can take photos. In short, the spindle only fits halfway into the left crankarm shaft, but all the way into the right.

Are you absolutely certain that the left side arm is being installed with the correct orientation? If it's installed with the inside on the outside, your description fits perfectly. Try flipping it over and see what happens. Might be a run of left arms with the taper machining done from the wrong side of the arm.

So, the old BB spindle happened to be the correct length for the new crankset? If not, buy a new sealed bearing BB with the matching spindle length.

rccardr: Yes, the left crank arm is installed with the correct orientation. It can't be reversed because it's curvy on the outside (see the pic below).

JiveTurkey: I bought a crankset that would give me the correct chainline. The right, drive-side crankset fits on just fine, giving me a 50mm chainline.

I've attached a photo below to illustrate the point. I'm using a nut-type square taper spindle as an example because it makes the issue clearer. As you can see, the first two crank arms engages deeply enough to allow the threading to protrude out. The last photo is the Shimano Alivio I just purchased. No protrusion. Has anyone seen this before?

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It's the ISO/JIS thing, I suspect .. taper angle is similar.
but width at end of taper is wider/narrower...

so JIS BB and JIS crank are needed.

It's the ISO/JIS thing, I suspect .. taper angle is similar.
but width at end of taper is wider/narrower...

so JIS BB and JIS crank are needed.

i think he's saying that the left crank arm taper and the right crank arm taper are different. it may well be that they are jis/iso. but he questions how TWO cranksets can be mismatched. he wonders how unlucky he can be?
i may be wrong here, but i reread the OP to make sense of what he was. please feel free to lambaste me for being dense, if i'm wrong here.

...not that anyone needs permission, or cause for that matter.:)

edit: it's possible, i suppose, that the left crank arm that "fell off" damaged the spindle in doing so, and that is causing the problem. OTOH you mentioned, i think, that you successfully mounted a crankset from another bike sucessfully so the chances are slim to none. but to test, attempt mounting the right arm on left, or left arm on right. that should tell you a lot as regards an asymmetry of the spindle, damage, or mismatched crank arms.

The crankset is supposed to be JIS. I have swapped the left/right crank arms. The right crank arm works well on both sides. The left crank arm doesn't sit well on either side.

I took the new crankset and tried it on my two hybrids that use square-taper nut-type spindles. No matter where I put it, the left crank arm just doesn't go in nearly as deep as the the right crank arm or any of the other original left crank arms.

On one of the hybrids, I did have just enough threading for the nut to partially grasp the spindle. I then took a 14mm socket wrench and tightened the hell out of it, forcing the left crank arm onto the spindle. In all likelihood, it was tightened beyond spec. I know this is probably a bad idea since it risks splitting the crank arm, but it did seem to engage the spindle better. I then put this back on the bike it was intended to go on and excessively tightened the 8mm crank bolt. It's still not as deep or as effortless as the right crank arm, but I do seem to have a better spindle engagement now. I wonder if this is poorly machined or if some new cranks are just odd like this. I've never had prior issues with square tapers.

Just bought a new left crank arm from fleabay and tried to put them on a 105 BB from late 1980s. only goes half way in.
then i found this
http://www.sheldonbrown.com/bbsize.html#square


Yet another source claimed that the squares of Campa, Mavic and
Stronglight (ISO) are smaller than Shimano and Suntour (J.I.S.) with the
result that J.I.S. cranks will move about 4.5 mm further in.

is this the shimano alivio FC-M410 ?
I have one of those and it is having the same issue, non-drive crank appears to be tighter and not want to seat as deep as drive side.

symmetrical spindle, issue is def with the cranks themselves.
i wonder if shimano has a bad manufacturing run?

On the other hand, non-drive cranks tend to be under more stress than drive side, perhaps their intention is for us to tighten that side more? doubtful

is this the shimano alivio FC-M410 ?
I have one of those and it is having the same issue, non-drive crank appears to be tighter and not want to seat as deep as drive side.

symmetrical spindle, issue is def with the cranks themselves.
i wonder if shimano has a bad manufacturing run?
On the other hand, non-drive cranks tend to be under more stress than drive side, perhaps their intention is for us to tighten that side more? doubtful

That's what it sounds like.