This isn’t actually a unicycle-related question but I thought unicyclists have more experience of drilling extra pedal holes in cranks 
I’m fitting kiddy cranks to the back of my tandem and the tidiest way to do it would be with the chainset on the left. Tandem cranks with spider on the left don’t seem to be available in short lengths (I need something around the 120mm length). So I’m thinking about getting a normal adult chainset and getting the cranks redrilled at 120mm and tapped left handed.
So, what square taper bike cranks (preferably with spider although BMX style would do) have people successfully retapped for pedals at that sort of length? I know about the Sinz ones and Kooka ones (not available any more AFAIK, and expensive). In theory any solid cranks with a flattish shape would do, but I’m just trying to get some opinions and narrow my choices a bit.
They need to be longer cranks that need shortening and drilling, rather than already short (I know the Sinz are theoretically available in short lengths) because I need to reverse the pedal threads.
Rob
I don’t have specific products to recommend, but I’ll point out that you can tap & thread them for 120mm while still keeping the outer holes with the regular threading. They shouldn’t be in the way…
I got SINZ cranks for MountianUni and the second thing I did with them (first thing was test out the disk) was drill some more holes.
The longer cranks (160 and 165mm) have a bit of a grove which prevents you from going much shorter than about 130mm but the shorter 155s that I got do not and I was able to drill holes all the way down their length.
If you want holes drilled at 120mm I would probably go with 140mm SINZ cranks, If you can’t find 140mm cranks go longer but don’t go any longer than 155mm.
Thanks Eric, that’s useful to know. I was sort of hoping to start with full-length cranks so I can end up with useable reverse-thread holes at about 120 and 145 or similar, to allow for a bit of growth. I know the normal holes won’t get in the way, but they’d be pretty redundant in this application. So it sounds like Sinz cranks may not be ideal because I’d have to start with short ones and end up with only one useable position.
I saw a picture on the web of some Shimano cranks that were nice and flat and had three sets of holes drilled, but I think they’d be a bit hard to track down now (it was an oldish photo).
I’m sure there must be some flattish solid (i.e. not hollowtech or milled out at the back) cranks around - I’ll keep looking. Any more hints welcome
BTW, Eric - did you drill the holes yourself? If so, how did you jig them up to get the holes parallel with the axle? If not, how much did you have to pay the machine shop? I’m sure it’s been mentioned on here before, so forgive my laziness!
Rob
Apparently I didn’t take a picture when I was drilling but I do have a pic of when I was tapping a hole and the finished product when I did my 160s.
I tapped each hole right after drilling using a small bit in the drill chuck to line up the thread tap. The first time I drilled a set of cranks I did not do this and had strait holes with crooked threads.
And this is what the looked like finished. 160, 143, 127
As you can see 127 was pushing the limits of how short you can go on that crank.
The crank also has a convex shape so I think I will sand them flat with a belt sander to get the pedals to seat properly. I had problems with the pedals coming loos my first few rides with in the new holes.
Edit: If you want reverse threads at both 120 and 145 you could probably get 145mm cranks and use a helicoil to reverse the existing thread.
This is exactly what you need to do to ensure straight threads… except using a proper centering pin. (Good improvisation, though, Eric!)
You can use a dial indicator mounted onto (or, ideally, inserted into) the chuck to check your setup. Thread a pedal spindle into the existing hole and use it as your reference. Of course, you’ll want to be sure the pedal spindle you use is true before continuing 
As for how to hold the part in place… that depends on its geometry. Post up a pic of your crank if you’re having problems and we can help you out!
Thanks. I don’t have access to a pillar drill at the moment, so I’ll be needing to get somebody else to do it for me. Those cranks Eric posted look promising though - holes at 125 and 145 may do - although 120 would be better really (my daughter’s own bike has 114s so I don’t want to make her pedal anything too much longer if I can help it).
Perhaps 150s or 140s and helicoil the existing thread like you suggested - assuming I can find somebody with helicoils in that thread (isn’t 9/16 pedal thread a weird size pretty much unique to bike pedals?)
Rob