Machining a Square-Taper Crank

I just got a set of ProWheel alloy cranks from UDC to put on my son’s 26er XC muni. They won’t fit on, though: see the picture below.

The old crank had a ring around the square hole that was, I guess, just machined into it. This ring allows the crank to get pressed fully onto the taper. The new crank, obviously, doesn’t have this feature, and so won’t press all the way onto the taper (the frame and bearing holders get in the way).

So my question is whether this would be easy to fix by just machining the new crank to match the old one. I think the “ring” part would require removing around 3/16" of material to match up the old one. Any idea what tools would be needed to do this? Or, if I have to take it to a metalworker, about how much should I expect to pay to have this done?


Maybe a dremal if they make a bit that will work without taking forever… You could cut it down at an angle then with a grinding bit to even it out nicely. That’s not really efficient and I’m sure some people here will be able to help you more than this, but I try.

That’s strange - what frame are you using? I use prowheels on my 26" muni (UDC CrMo hub in Nimbus II frame, so standard pressed bearing holders) and there’s quite a big gap between the crank and the bearing holder. I suppose something like a KH frame with machined bearing holders would probably have a bit more thickness of metal outside the bearing, but I’d be surprised if it was enough to touch the crank - but perhaps it is. Or perhaps you’ve got a hub where the taper is a bit skinnier and the cranks go on a bit further.

That doesn’t really help with your problem though :o… I’d agree with the poster above and say if it’s only just touching just grind some away with a grinding wheel of some kind.


It’s a Telford. The space for the crank is only slightly bigger than the ring in my original crank. I love that curvy frame, and don’t want to replace it–but with 170 cranks, my kid complains about how slow it is to ride.

Keep the ideas coming! Thanks!

I’m sure gerblefranklin will chime in here as soon as he gets back from class, but I’ll throw in my two cents in the meantime.

It’s sad that UDC calls most every crank an “alloy” crank as that’s about as useful as calling it a “metal” crank. You have to guess what the composition of the crank is before you can do any work on it. I’m going to guess that it’s an aluminum crank.

Unfortunately aluminum doesn’t lend itself to grinding very well. Being a soft metal, it will quickly gum up your grinding wheel… shortly thereafter your wheel will be an aluminum wheel and it won’t do its job anymore. Bad news. If you choose to use a grinding wheel expect to throw out both your crank and your grinding wheel shortly after you begin your job.

On the flip side, since aluminum is soft, it’s easy to cut. If you have a drill press and a hole saw of the right diameter, you may be able to do this fix yourself… just make sure to cut very slowly (low RPM and low feed rate). I wouldn’t advise doing it if you aren’t confident with your shop skills, though.

That leaves you with a trip to your local machine shop. Yeah, it’ll cost you a bit to get these cranks reworked, but it should be a straightforward task for an experienced machinist. This way you can rest assured the task will be done correctly.

Sorry, I didn’t address these points…

Most cranks are cast… machining is quite expensive. You’ll know the difference between a cast and a machined part when you see the price tag :smiley:

Just off hand, most shop rates are about $100 an hour. It shouldn’t take an hour to get this done (I’m thinking 1/3 hour at most), but your local shop may have a minimum charge.

If I were in your shoes I’d show up at the local shop around closing time on Friday with a sixer of good beer, your cranks and a handful of cash. Maybe you can sweet-talk a machinist into doing your job “on the side” for a cut rate.

Edit: perhaps bring your old cranks too, so the machinist knows how much material to remove.

I’m sorry, I don’t quite understand this. I don’t see how the frame and bearing holders get in the way. Are you saying that the crank is too thick at the square taper and the taper is too deep? Then the crank touches the frame before the taper starts to get tight? If that’s right, you want someone to mill off the appropriate amount of material perpendicular to the taper axis. This will certainly cost more than the $22 per pair for the cranks unless you do it yourself or have a friend that can do it for you. From the standpoint of cost you may want to consider a different kind of crank.

Yes, pkittle, please clarify your issue. Because not all cranks fit on square tapers the same. Without describing the confounded technical design issues, some cranks may only bolt onto the outer half of the tapered surface. Other cranks may slide on darn near all the way. This variation is not an issue unless, as Greg suggests, the cranks slide on so far that they contact the bearing holders.

And if your cranks only slide onto the outer half of the taper, an appropriately-sized spacer between the cranks and the bearings helps to prevent any “bearing slide” due to lateral loads.

Or then there’s the cheap-o method, vice and file. I’ve had to do this with some old (worn) cranks and narrow unicycles. It might be a big pain with steel cranks, but the file cuts pretty fast into the aluminum/alloy.

4" angle grinder a few steel cutting discs a pair of goggles and a vice,prepare cold beer in readiness of completion.done.

Sorry for not being clear enough. The problem can’t be solved with a spacer, because the crank hits the bearing holders before getting even close to being tight on the taper; when they hit the holders, they can still wiggle back and forth.

Finding the right cranks to fit may be tough. The frame requires cranks with significant offset (to increase Q) or they hit the frame’s rear seat stay. So I suspect that Torker LX cranks, which I’m pretty sure are without offset, won’t work (at least, when I tried putting a DX wheel on that frame, the cranks hit the frame).

This is what the uni looks like:

Well then, looks like all the previous suggestions for grinding down the outer edge of the inside plane of the crank mounting area are in order. I have had to do that crank mod before, and I just used a belt sander and finished up with a fine hand file. A Dremel would result in a cruder finish, but would also do the job, as would John’s hand-file-in-a-vice.

True that Torker alloy cranks are straight out, no offset. So that’s not an option for your gorgeous frame.

Thanks, all! I’ll let you know what happens when I get this project finished.

uhm… the 04 and possibly 05 torker lx cranks were tapered.