Securing helicoil into crank

[RickH]

I had a crank with a damaged pedal thread repaired with a helicoil. The repair was fine & seemed to be holding up OK. But when I wanted to take off the pedal the insert unwound itself with the pedal & mangled itself a bit in the process. The threads in the crank seem to be fine so fitting a new insert shouldn't be a problem (famous last words).

I wanted your opinion on the best options for keeping a replacement insert in place & being able to still remove the pedal if possible.

I'm presuming threadlock of some sort on the insert threads & grease on the pedal threads. I've no idea on different threadlock to know what is likely to be most suitable (& available in small quantities as I'll probably only ever use it for this in the foreseeable future). Or would I be best with something like epoxy to hold the insert in place (I can do Araldite ).

Of course I could always just get the LBS to put a new one in (it wasn't an expensive job - they only charged me £5 to fit the original) &, possibly, either forget about pedal removal or treat the inserts as a consumable item.

Your thoughts would be appreciated.

[Jdsk]

I suggest a high strength thread locking compound. You don't need the high temperature type.

https://www.parktool.com/blog/repair-help/pedal-bushing-repair-kit-procedure
https://www.henkel-adhesives.com/uk/en/products/industrial-adhesives/threadlockers.html

Jonathan

[Brucey]

threadlock is a must. An oft-omitted feature of this repair is that the insert should be set underflush in the crank by about 1/2mm.

cheers

[rogerzilla]

Some grades of Loctite (e.g. 603) are very oil-resistant and need 250 deg C heat to remove the fitting later. That would be ideal.

[videoman]

I used to run a mobile thread repair business predominately to the motor trade and industry and have literally helicoiled hundreds of threads in all sizes from 3mm to 20mm and never used any type of thread locking compound or epoxy resin and only very occasionally have I known a helicoil to come out.

Sometimes cheap copy helicoils fail (ask me how I know) and I would ask whoever completed the repair to try another one

[Brucey]

in all fairness the service loadings on pedal threads are quite unlike those found on pretty much every other thread you might want to repair using a thread insert. Also this thread title is a misnomer; pedal thread repair inserts are usually 'solid' rather than (heli-) coil-like; I don't know which were most commonly used by you, but I'm guessing that many (most?) would have been coli-like. So it wouldn't surprise me to find that they behave somewhat differently to other thread repairs too.

IME the insert is rarely a really tight fit in a freshly tapped crank and without being set slightly underflush and with some kind of adhesive the insert is quite likely to work loose. Hence the instructions in the Park Tool link.

cheers

[AndyA]

I used to run a mobile thread repair business predominately to the motor trade and industry and have literally helicoiled hundreds of threads in all sizes from 3mm to 20mm and never used any type of thread locking compound or epoxy resin and only very occasionally have I known a helicoil to come out.

Sometimes cheap copy helicoils fail (ask me how I know) and I would ask whoever completed the repair to try another one

Usually, this sort of repair on crank threads is done with a bush rather than a helicoil. These need red loctite or epoxy if they're to remain in the crank rather than stick to the pedal. There are two common thread sizes of pedal bush, 5/8"x20tpi and 5/8"x24 ti. I'd take it back to the bike shop that fitted the bush/helicoil and ask them to fit with epoxy. This year has been unusually busy for crank thread repairs for me, probably done 50. Usually it's more cost effective to replace the crankset if both sides are done - but between sold out spares and unobtainable e-bike weirdness there's been a lot of repairs

[Jdsk]

I had a crank with a damaged pedal thread repaired with a helicoil.

The name "Helicoil" is often used generically. Do you know exactly what was inserted?

Jonathan

[RickH]

Thanks for the teplies

Mine seems to be a helicoil (or copy) - it is (like) a spring & was quite difficult to unwind from the pedal threads, as it needed gripping firmly to turn it but not too tight to jam it against the pedal threads.

Some grades of Loctite (e.g. 603) are very oil-resistant and need 250 deg C heat to remove the fitting later. That would be ideal.

If I get a new insert in I hope to never need to remove it. Would 250°C damage the crank? It is a Race Face Aeffect crank which is "6066 Aluminum" according to Race Face. Otherwise, is there a similarly grease resistant version that won't require such high temperatures?

threadlock is a must. An oft-omitted feature of this repair is that the insert should be set underflush in the crank by about 1/2mm.

Just to clarify - do you mean the insert should be 1/2mm below the outside face of the crank on the pedal side? How important is that on cranks that use pedal washers by default?

Thinking out loud, the ideal insert would be something like an enlarged chainring nut that secures with a flange at the rear of the crank & clamps the pedal in place. Does such a thing exist?

[Brucey]

….Would 250°C damage the crank? It is a Race Face Aeffect crank which is "6066 Aluminum"

threadlock is a must. An oft-omitted feature of this repair is that the insert should be set underflush in the crank by about 1/2mm.

Just to clarify - do you mean the insert should be 1/2mm below the outside face of the crank on the pedal side?...

anything over ~200C will start alter the structure of the crank if it is held for any length of time. Most epoxy resins soften at lower temperatures than this, so are probably a better choice for this job.

The insert needs to be underflush whether there is a pedal washer or not. Park Tools recommend that the insert needs to be set more underflush than 1/2mm.

….Thinking out loud, the ideal insert would be something like an enlarged chainring nut that secures with a flange at the rear of the crank & clamps the pedal in place. Does such a thing exist?

I've never seen such a thing; I also wonder if it would be reliable; the loads on a pedal thread are very odd indeed, and any diametral clearance would probably start an evil chain of events.

cheers

[rogerzilla]

The loads on a pedal aren't hugely different to those seen by car wheel bolts (or nuts, which are just the other way around). Those used to be left-hand threaded on one side of the vehicle, but the problem was later solved by conical (most manufacturers) or spherical (Honda, for example) seats. You could do this on pedals, which would allow a RH thread on both sides and stop the pedal gradually eating into the crank, but (oddly, given the proliferation in BB and headset standards) no-one seems willing to change the 9/16" thread and flat seat. One disadvantage would be a thicker crank at the pedal eye, to accommodate the recessed seat.

[Brucey]

The loads on a pedal aren't hugely different to those seen by car wheel bolts (or nuts, which are just the other way around). Those used to be left-hand threaded on one side of the vehicle, but the problem was later solved by conical (most manufacturers) or spherical (Honda, for example) seats. You could do this on pedals, which would allow a RH thread on both sides and stop the pedal gradually eating into the crank, but (oddly, given the proliferation in BB and headset standards) no-one seems willing to change the 9/16" thread and flat seat. One disadvantage would be a thicker crank at the pedal eye, to accommodate the recessed seat.

Modern car wheels are 'hubcentric' which -in the simplest terms) means that not only are the wheels more likely to run true and be properly balanced, but those service loads which might overcome simple friction in the bolted interface are resisted by the hub spigot. This also changes the nature and extent of the loading on wheel studs/nuts vs older 'lugcentric' designs. I have always supposed that conical seats (which appeared before hubcentric designs because widespread) mainly improved wheel centring and therefore wheel balance.

In any event whilst both pedal threads and car wheel fasteners are liable to precession, I think that is pretty much where any similarity ends in terms of loading; pedal threads are primarily loaded in a cantilever bend (rather than an orbiting lateral thrust ) , something which is strenuously avoided in other engineering applications because it usually causes complications at least and insoluble problems at worst.

If you used a conical seating on pedal threads, it would improve matters (possibly) but it would not allow a RH thread on both sides. Jobst Brandt claimed that it would, but his analysis/comparisons appear to consider only the orbiting lateral thrust, not the strong cantilever loading present in real pedals. To make conical seatings work with RH threads both sides, you might have to incorporate a conical 'nut' on the back of the crank too. And in any event the radial pressure arising from the conical seatings would cause the stresses in the pedal eye to increase dramatically, necessitating a bigger, thicker pedal eye. So it is not clear that any advantage would be gained.

We might have ended up with handed 9/16" pedal threads almost by accident (it was the best of the schemes that was tried a hundred-odd years ago) but it causes remarkably few problems, even when the concept is abused e.g. with pedals that don't have full shoulders on the spindle.

cheers

[RickH]

A brief update.

The LBS has given me 2 inserts FOC so I will attempt a repair in my own time. (I've put a spare set of cranks on so the bike is still operational.). I will give all contact parts a thorough clean & degrease before assembly. I'll use some araldite on the insert threads & a little fresh grease on the pedal threads so it won't (shouldn't) get stuck but can leave the pedal fitted while things set.

The inserts are actually solid, but very thin. The original must have peeled apart in the process of damaging & then removing it to make it appear like a coiled spring.

[Brucey]

apologies if this is obvious but

a) you need to coat both parts with glue before you assemble, else you can't be sure that there is glue everywhere in the joint once it is assembled. Yes this uses twice as much glue as necessary, but cleaning up the excess is the lesser of two evils here.

b) unless you get lucky with an easy-turning insert or make a suitable tool (eg by fitting a small OD spacer to the pedal spindle) before winding the thing in, then you won't easily get the insert in underflush as is required.

cheers

[alexnharvey]

The inserts are actually solid, but very thin. The original must have peeled apart in the process of damaging & then removing it to make it appear like a coiled spring.

Maybe, but I think it's more likely that it was a coiled wire type insert, of the helicoil type.