Bicycle Mechanics - shortening crank arms

I want to shorten crankarms from 170mm to 145mm for a child tandem stoker. Where can I get the taps or have them re-tepped. I saw a PARK tool tap set for $40. It says it is for cleaning up and retapping damaged pedal threads. Will it also work to tap virgin threads in a newly drilled hole?

Ben, there are ready made kits for child stokers.

if you really want ot tap your own, include the price of the drill and the tapping bar if it is not included

Is this not an option?


http://www.bikemannetwork.com/biking/p/COMPCRNKTDM/CR8870

It should work fine. If you don't know how to correctly use a tap, and most people don't, do a search on the web first. It will make your life easier and less expensive.

I want to shorten the existing crankarms, not add a child stoker kit.
I don't want the bolt-on shorteners because I've read they are bad for kids joints because they make they increase the distance between the pedals creating a bow-legged riding position. So if anyone has info about the taps, please let me know. Thanks:) :)

I want to shorten crankarms from 170mm to 145mm for a child tandem stoker. Where can I get the taps or have them re-tepped. I saw a PARK tool tap set for $40. It says it is for cleaning up and retapping damaged pedal threads. Will it also work to tap virgin threads in a newly drilled hole?

I shorten cranks regularly for friends and for myself. I use Park taps, but they can only be used on alu cranks. Here's the procedure:

You need: a cheap pair of donor pedals with axles in good condition (i.e. not bend), a 13mm drill bit, a 19mm or 20mm mill bit, a drill press with a vise or a milling machine.

1 - extract the pedal axles from the donor pedals

2 - pinpoint the location of the new holes in the cranks

3 - for each crank: screw the corresponding pedal axle, mount the axle in the head, align the crank with the vise and carefully tighten the vise without introducing efforts in the pedal axle. Then release the axle from the head and unscrew the pedal axle. This way, the new hole you'll make in the crank will be exactly parallel to the old one.

4 - mount the 13mm drill bit and drill the new hole

5 - mount the 19mm mill bit and flat-spot the hole. You need to do this because virtually no crank has a perfectly flat outer surface, and you need to create a circular surface parallel to the hole for the pedal axle to rest upon when it's screwed on, otherwise you risk fretting and damaging the threads when pedaling.

6 - Tap the hole with the correct tap for this crank. If you're good, do it without guide. If you're like me, mount a spring-loaded cone in the head and use it to guide the tap vertically (it has a little hole at the back for exactly that purpose)

7 - Re-mount the 19mm mill bit and flatten the area around the hole again, as the tap will have pushed the metal out.

8 - Remove the crank from the vise, chamfer the other side of the hole with a file or a knife, so it's clean and doesn't cut.

If you don't use the proper tools, that is, if you drill the new hole with a hand drill, I guarantee you your new hole won't be aligned properly and the pedal will feel all weird. It doesn't take much at all: the first cranks I made, I did the holes right (with a mill) but I tapped the hole without guide, and it turned out slightly out of alignment and I could distinctly feel one of the pedals wobble underfoot.

I see no reason why the park taps wouldn't work on a freshly-drilled hole.

Call up a machine shop and ask them how much they would charge. I don't think it would be over $20. It would probably take them about ten minutes to do the job, and you would have it professionally done.

Keep in mind that you need TWO taps for pedals, a right-hand thread for the drive side pedal and a left hand thread for the non-drive side pedal.

Very good instructions from ppc. I've had 2 sets of cranks shortened to 140 mm for my own use by a proffesional machinist and there are a few things to note regarding drilling the new holes. Before I forget I'm sure those taps will do the job as well.

Anyway the crankarms you put new holes in need to have enough thickness and width of alloy to start with. Many modern crankarms are scolloped mid-length making them unsuitable candidates for new pedal threads. Very importantly you need to drill new holes absolutely parrallel to the BB axis otherwise you will get pedal wobble. Now this isn't that hard with most cranks as you bolt the inside face of the crank to the machine table and support the arm your drilling. This is straight forward on a milling machine but if your just using a drilling press then maybe not. Anyway talk to an engineer because the cost of having a proffesional do it may not be that high although even the proffesionals may not have the right taps in their collection to re-tap the holes because they are very much cycling specific.

Regards, Anthony

Keep in mind that you need TWO taps for pedals, a right-hand thread for the drive side pedal and a left hand thread for the non-drive side pedal.

Park Tools sells the pair of taps, I don't think they sell them individually.

Very importantly you need to drill new holes absolutely parrallel to the BB axis otherwise you will get pedal wobble. Now this isn't that hard with most cranks as you bolt the inside face of the crank to the machine table and support the arm your drilling.

None of a crank's outer surfaces is a suitable reference plane. Only the square taper or octalink, and the old pedal hole (which is supposedly drilled parallel to the BB axis by the manufacturer, one would hope) can be reliably used to align the new hole. Since I don't want to bother with octalink or square taper, I use the old pedal hole and it works well. But don't think lying the pedal flat on its back will point the hole perfectly vertical : I've seen several crank arms that were curving outward all the way to the pedal hole.

I'd take it to a machine shop (if there's one close by) if at this point you don't have any equipment unless you plan on using them in the future.

None of a crank's outer surfaces is a suitable reference plane. Only the square taper or octalink, and the old pedal hole (which is supposedly drilled parallel to the BB axis by the manufacturer, one would hope) can be reliably used to align the new hole. Since I don't want to bother with octalink or square taper, I use the old pedal hole and it works well. But don't think lying the pedal flat on its back will point the hole perfectly vertical : I've seen several crank arms that were curving outward all the way to the pedal hole.

Yes I agree with you in principle but in practice I have found that if you bolt the crank-arm with the inside surface (surface facing the BB) or the chainrings firmly onto a milling table then its good enough and I cant feel any pedal wobble.

Regards, Anthony

Highpath engineering do crank shortening
http://www.highpath.co.uk/cycles/products/specialneeds/shortcranks.html

One other warning. If these are relatively new Shimano cranks, Octalink or newer, the arms are hollow. (Shimano's marketing name is "Hollowtech") so there won't be any metal thickness to tap if you drill a hole above the current pedal hole.

There are multi hole cranks out there just for the application you want. It would be cheaper and safer to get a purpose made crank.

.....Here's the procedure:
1 - extract the pedal axles from the donor pedals
2 - pinpoint the location of the new holes in the cranks
3 - for each crank: screw the corresponding pedal axle, mount the axle in the head, align the crank with the vise and carefully tighten the vise without introducing efforts in the pedal axle. Then release the axle from the head and unscrew the pedal axle. This way, the new hole you'll make in the crank will be exactly parallel to the old one.
4 - mount the 13mm drill bit and drill the new hole
5 - mount the 19mm mill bit and flat-spot the hole. You need to do this because virtually no crank has a perfectly flat outer surface, and you need to create a circular surface parallel to the hole for the pedal axle to rest upon when it's screwed on, otherwise you risk fretting and damaging the threads when pedaling.
6 - Tap the hole with the correct tap for this crank. If you're good, do it without guide. If you're like me, mount a spring-loaded cone in the head and use it to guide the tap vertically (it has a little hole at the back for exactly that purpose)
7 - Re-mount the 19mm mill bit and flatten the area around the hole again, as the tap will have pushed the metal out.
8 - Remove the crank from the vise, chamfer the other side of the hole with a file or a knife, so it's clean and doesn't cut.
....

Thanks for the guidance, but I had to make this changeto #3 above, due to my special cranks (long story) and equipment limitations.
3) installed pedal threads of the donor pedal spindle into the collet of the mill head. Then clamp the pedal spindle into the mill-table-vise. Release the threaded end from the collet, leaving the spindle in the vise, and male pedal threads in perfect alignment with the mill head. Install the crank arm on the pedal threads. Put drill bit in the collet/chuck, locate the hole and drill.
One thing to watch out for, and add clamp to prevent, one of the cranks will be threaded such that the forces of the drill bit will try to loosen (unscrew) the crank from the spindle.

It wasn't pretty, but it worked! :)