Help! Threaded my left hand crank pedal threads

[Brucey]

Helicoils were first made for imperial threads; they are not the only thread repair insert type however.

Helicoils are spring-like and are slightly oversize in their free state, hence the winding in tool etc. However not all thread repair inserts are like this, some are solid, and can (with care, they are still thin-walled) be wound in from the top. I've not used the VAR ones so I don't know how exactly they behave when they are installed; maybe they are solid?

There are innumerable ways it can all go horribly wrong but with a solid insert the #1 favourite is that the tapped hole is not of the exact correct tolerance for the insert/male thread inside it. Close behind is that the thread has some crud in it; the slightest fragment of swarf is enough to cause the installation torque to increase dramatically.

One of the reasons the spiral wire type (Helicoil) inserts are so popular is that they tolerate small variations in size and installation procedure rather better than alternatives. If it all goes horribly wrong, you have a length of wire that can be extracted from the hole without great difficulty in most cases. Solid inserts are somewhat less forgiving, even if the repair might be a bit stronger if it is done well.

cheers

[NATURAL ANKLING]

Hi,
Correct terminology is "Wire thread insert"
Helicoil is a trade name for such, and of course everyone understands what that is.

But back to the job in hand, if you have not inserted them before, not an easy task to get right first time.
Also as then tend to be stainless, if the threads of pedal are not quite right then there may develop interference between pedal and insert.
Either the threads get graunched and or the insert comes out when pedal is extracted or put in.
Done right it should give good service.
Solid inserts are probably left best to an engineering firm who can use best practices.
In the past I have always made my own inserts by filling damaged hole with a solid stud then re-drill and tap hole.
Though this can be done by hand with skill, best in a machine shop.

Tip-
For woodworkers dowels glued and lightly tapped into a drilled hole smaller than dowel just, and tapped with hammer into hole makes a permanent repair

[David9694]

I’ve ordered another - ahem - wire thread insert from SJS and I’ll take it more carefully this time, e.g. look for an intended right way for it to go.

It would be nice to screw the Helicoil in into the crank first but without a tang, it’s difficult to know how to grip it - it seems to be such a tight/ stiff fit.

Running the tap through again clears out any remaining broken bits of insert.

I’ve checked the size and pitch and they do match, at least in theory.

[nigelnightmare]

Are you SURE the thread was 24tpi and not the standard 20tpi as most 9/16" pedals are 20tpi, if so this could have been the cause of it not going into the newly tapped crank.

[Brucey]

Are you SURE the thread was 24tpi and not the standard 20tpi as most 9/16" pedals are 20tpi, if so this could have been the cause of it not going into the newly tapped crank.

good point!

I assumed that the tap would be 20 tpi (because that is basically the only tap that would be any good as a pedal tap) but as you say the OP has written 24tpi.

If the tap really is 24tpi then it is no good at all.

cheers

[fredN4]

cant believe all this “advice”. Buy a new crank {FFE - family-friendly edit }.

[David9694]

cant believe all this “advice”. Buy a new crank {FFE - family-friendly edit }.

True - I could also fit an electric motor to my bike as there are some pretty steep hills around here.

If you want to treat me, the link is below:

https://www.ebay.co.uk/itm/New-Shimano- ... SwDddbKBCk

[David9694]

https://www.sjscycles.co.uk/tools/var-p ... -tpi-left/

The SJS site refers to 24 TPI, but I note Sheldon's reference to 20 TPI. The helcoil is going to be one diameter on the outside and a smaller one on the inside, but the pitch (and the thread direction) must be the same?

Anyway, the good news is the second helicoil worked. I didn’t use any fluids. I wound it into the crank from the back on its own - it went about half way - one turn forward, half a turn back. Using the pedal, I installed it fully. I was then able to extract the pedal and insert it from the right side. I now have the pedal in, its shoulder abutting the crank and a couple of mms of helicoil sticking out on the inside. Not sure if the enlarged screw hole is parallel to the axle - but that could be my imagination.