Anyone with firsthand experience using Helicoils to convert regular cranks for tandem

[swimdad]

I am thinking about making my own set of tandem cranks from several pairs of regular Dura Ace cranks I have around and have seen a couple of references about using a 9/16 -20 helicoil insert to accomplish this but I can't find anyone who has actually done this. I also can't find the 9/16-20 helicoils either.

Any help would be greatly appreciated.

JM :)

[csm]

I've used helicoils before. not too difficult if you go slow and steady.

[swimdad]

Thanks for the reply. where did you get the 9/16 -20 Helicoils from? Did you use a left hand helicoil or insert a right hand one in backwards? Thanks

[Dave Wages]

Any bike shop should be able to source the helicoil and tap, the left hand thread is kinda bike specific.

If you're doing the timing cranks, you'll want to use a left hand threaded helicoil.

[csm]

I worked at a bike shop and we sourced them from a tool co.

[zap]

Comotion long ago ('92-'93) offered Mavic road groupo for their Double E tandem which we purchased. The Mavic crank was modified by them.

We (I) did strip the left crank helicoil but our trusty mechanic (Coombe prior to his pedal days) fixed that and it's been solid 'til I retired the whole mavic kit two years ago.

The process looked simple but Coombe was one hell of a mechanic.

[RPS]

Did you use a left hand helicoil or insert a right hand one in backwards?Backwards doesn't change thread direction. Isn't it like turning a nut over?

[R2D2]

Try MSC Direct or McMasterCarr to source.

[DfCas]

Bike tools etc has some options:

http://www.biketoolsetc.com/index.cgi

search "pedal taps"

[mschol17]

Backwards doesn't change thread direction. Isn't it like turning a nut over?

Yes

[Ti Designs]

I've done exactly what you're thinking for my own tandem and a few other clients. My tandem has a set of Dura-Ace 9-speed tandem cranks made of three sets of triple cranks with two reverse threaded right side arms and one forward threaded left arm. For something as simple as that, ELDI makes both the inserts and the taps. These are inserts, not Helicoils. A Helicoil is a wire winding that threads into a larger thread cut from an STI tap. The Helicoil gains it's holding strength as it expands instead of backing itself out. Pedal spindles see cycles of lateral force, but the rotation tightens them, so a Helicoil would have a tendency of working it's way out the back of the crank arm. ELDI makes solid inserts which can be bonded in place. The Dura-Ace triple crank is ideal for this purpose as the spider only has outward facing chainring tabs like a track crank, so the timing chainrings work out perfectly. Three sets of triple cranks will get you a set of tandem cranks and two spare sets of chainrings - Dura-Ace triple shifts better than anything else I've ever found, but it uses a special chainring set, and being that it's 9-speed they're going to be hard to find going forward...

The real trick is doing the work. You need to machine and tap the hole perpendicular to the crank, which isn't something you want to be doing by hand. I would suggest using a milling machine or large drill press for the job. The drive cranks are easy to mount as the spider sits on the table. The left arm is a bit harder. I have a 3MT to Shimano square taper adaptor that I made - there's a tool that's never gonna pay for itself!

[dvs cycles]

A friend did a D/A 9sp set a couple years ago by taking them to Santana.