Crank arm pedal threads are stripped

[antrossini]

It's an aluminum 3 piece crank, it is kinda faded but I think it is an SR Suntour if that means anything. It came on a Trek 3700 purchased in late 2006 or early 2007...
When I was riding it, it would loosen up... and I'd tighten it with my fingers as good as I could... but it wouldn't work too well for long rides... one day it fell out and it stripped the threads inside the right hand crank arm.
So I was wondering if there was any way to re-thread the hole... or buy a new arm... or something that is a fairly inexpensive fix. I can't afford a brand new crank.

[Alex Ramon]

There is a product called a helicoil. First you have to drill and tap the crank arm to a larger diameter so the helicoil can be threaded in. Some shops will do this for you, but often the cost is more than it would be to replace the crank arm.

[antrossini]

How do I tell what size to buy?
Do most bike shops have them or do I have to order them off the site?
What does tap mean?
How much would a new crank arm be?

[Alex Ramon]

Quote:How do I tell what size to buy?

To be honest I'm finding a lot of conflicting info online about the correct size to use. You likely have 9/16" pedals... I'll keep looking for more info. When I worked in the shop we always had a guy named Max who took care of helicoil repairs.

Quote:Do most bike shops have them or do I have to order them off the site?

A good bike shop should have one, or at lease be able to assess whether or not it is worth doing...

Quote:What does tap mean?

A 'tap' is a tool that cuts new threads inside a hole. <a href="http://www.diynetwork.com/diy/tl_metalworking/article/0,,DIY_14390_2270183,00.html">Here's an article</a> that describes the process.

Quote:How much would a new crank arm be?

You're probably looking at somewhere between $10-$30 for a single crank arm. Your best bet would be to take your bike down to the local bike shop to find a proper sized replacement.

[Anton]

Hi,
I have some problem with my Trek 3700 approximetly 2 years old. I think this staff is defective. I have found on internet a complete crankset for $99 but only in stores not shipping is available. Right now I am looking for a bike shop around to get an estimate? Pain in the ...., If you find some good solution please let me know.
Thank you,<br />
Anton

[ozzzyrick]

Hi Antrossini,
I'm familiar with the Helicoil re-threading and to be honest it will be expensive unless...
You know someone in the engineering business. I would look for a small engineering business that may have a helicoil repair kit.
To do it yourself requires perfect alignment of the original thread bore, using a special sized drill and tap for the helicoil and a specific tool for setting the new thread, which is stainless steel.
If you can get it done cheaper than a crank arm replacement the new thread will be stronger than the original.
Regards,
Rick.