Re-drilling crank arms

Hi Guys,

I need to create some short crank arms for the bike I'm building for my 9 year old son, I bought some stronglight ones at 155mm but they will not fit even with a 127mm BB.

I have a spare '97 LX crankset so I thought I'd shorten the arms and re drill the holes for the pedals which leads me to 2 questions,

What thread diameter and pitch is correct.

How do I tap the left hand thread on one of the arms. I wondered about tapping through from the back but can't get my head around it at all :?

Hope you can help.

Cheers

Barry

You'll need a left hand thread tap. Pedal taps are availble but pricy I guess for doing this just the once. I'm unsure if its a standard size though I know they are 9/16. A good machine shop may have something or you could try highpath engineering, I'm sure they mess about with cranks.

http://www.highpath.net/cycles/special/shortcranks.html

looks pricey, but for the amount of work its pretty good.

The majority of pedals use a 9/16 screw with 20tpi, although kids cranks use 1/2", so you would need to take this into account dependant on the pedals you choose for your lad.

Threading from the back wont work, the thread remains right handed
You need a left hand threaded tap which you can get from Park Tools but these are only for cleaning threads out. For a new thread, your gonna need a taper tap to start the thread off.
the bay have a set of taps, but you'd need to hunt around for some left handed ones.

I've used Highpath, they did a grand job.

You need to jig the thing carefully to get the pedal spindle exactly in line with the BB.

done it to a few sets, and yes, setup is key.

If you want it clocked in the mill and all that jazz, yup, £40 is more than fair. . .
However, if you want meto buss it on the pillar drill, pretty good squareness, and then cut the arms down and vagiely linish a radius rather than spin it on the mill again, I'd do it for 18 return posted. PM me if thats useful! (you are in the uk. . .!>)

Have a look down your local industrial estate,i'd be very suprised if you dont find a few small engineering firms that have been around since the ships that used to be refitted down your way
They will have every bit of tooling needed to drill the hole then tap it.
You may run into probs on the reverse tread but if you can find out the thread/tap size im sure they will have one or can access one or even if your on good terms with the LBS they might lend you one
If the engineers have the tooling they wont ask for more than a tenner,for what is a ten min' job,beer money really

I dont think somebody suddenly came up with a whole new thread size just for pedals :? ,its taken from a thread size that was already around and in use

You dont have to do it all yourself ,pay someone else but look for the older firms where the average age of the staff is 60,those old fellas could identify the thread by sight

Thanks for all of your help guys,

Cranks duly sent to Highpath for shortening.

Thanks again,

Barry

If the cranks are for a child who will no doubt grow out of the bike soon and not have the weight to unscrew the pedals, why not thread both cranks right-handed? If you already have the correct right-handed taps, you'll probably find a replacement pedal is cheaper than the left-handed taps.

Then here's the dodgy bit: plenty of Loctite or perhaps even some Araldite on the thread? Will you honestly need to take the pedal off again?

Local engineering firms wouldn't necessarily have the tools required for the thread as 9/16 x 20tpi is seldom seem elsewhere