# Should I re-thread my crank arms or just buy new cranks ?



## dmypubmail (Jul 9, 2017)

After many years, my crank arm threads have become trashed. I'm wondering if I should try to get a kit to re-thread the crank arms or whether I should just buy new crank arms. Thanks for any pointers - I don't have a lot of experience working on bikes.


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## jestep (Jul 23, 2004)

Buy new or buy used even. Not something I think should be repaired if it's even possible. There's just too much stress on them and they're too important of a component to mess with unless you're a machinist and really know what you're doing.


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## dmypubmail (Jul 9, 2017)

*what's a good site to shop for new crank arms and pedals ?*



dmypubmail said:


> After many years, my crank arm threads have become trashed....


where can I shop for new crank arms and pedals for mid 90's fisher aquila ?

thanks


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## Lone Rager (Dec 13, 2013)

Nothing wrong with a properly installed helicoil. It's stronger than the original threads. You do need to fixture the crank arm to drill and tap it straight.


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## RAKC Ind (Jan 27, 2017)

Ive used helicoils probably 100 times for different things. Even fixing stripped caliper mount posts on fork lowers.

I wouldnt even hesitate to fix a crank arm with them. Install with a little blue loctite (liquid version), put the pedal back on forget about it.

Those that are against it have never actually used them (or used them properly) for repairs. Like when using the tap to cut the new threads you need to spray wd40 in the hole first. Never cut or especially try to cut threads in aluminum without it. Go slow, back the tap back out and clean off about half way in (for this case, deeper the hole the more times you need to back it out and clean the tap). Finish threading, back tap out, clean, run it down one more time (should all bit spin through with your fingers) to clean out and shavings and burs.

Gently thread the helicoil in after putting on a little loctite. Right after that install the pedal.

Sounds worse than it is, any helicoil repair i do in aluminum it takes longer for me to dig the stuff out than it does to do the repair.

If your still not convinced they have threaded inserts. Threaded inside in the size you need but you drill and tap an oversized hole just like doing a helicoil.

Difference is you end up with a fully steel threaded hole for the pedal.

The insert route is done industrially on things much more expensive, dangerous and sees 10000× the loads put on them. Never failing.

Either version done correctly (just being slow and steady with the drill and tap by hand only) Ill put my entire bank account and life on the fact the repair will outlast a new crank arm 10x over. Unless you get cranks that already have the steel insert from the factory (shimano saint/zee for instance).

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## RAKC Ind (Jan 27, 2017)

Oh and almost forgot, my fat bike stem, lower hole on steerer clamp, helicoiled because i got in a rush trying to get forks installed and ham-fisted it, pulled the threads....

Almost 2 yrs now of my fat, 285lb ass beating the crap outta the bike.

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## fishwrinkle (Jul 11, 2012)

helicoils saved my butt on a stripped alternator mount and exhaust manifold. 125k miles later still going strong


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## life behind bars (May 24, 2014)

Heli-Coil.


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## l'oiseau (May 5, 2015)

In my last job, we used to religiously use helicoils on all of our test fixtures. Why? Because after being assembled and disassembled hundreds of times, the threads would gall on aluminum parts. We never had any issues in terms of stress and stiffness. It honestly just like using a bit larger diameter screw for that portion.

For pedals, make sure it goes in straight to the stop flange. Theads are not strong for bending loads, so you want to make sure the flange on the pedal fully seats on the crank to take the bending load.


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## root (Jan 24, 2006)

Dont forget one side is reverse threaded. Aside from that, heli(or other brand) coils are plenty strong. That said, are the cranks worth the time, effort and cost to repair? The coil
Itself is cheap. The drill bit and tap will cost a little bit more.


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## J.B. Weld (Aug 13, 2012)

I've seen a lot of pro-helicoil threads and I wonder how many people have done them on pedals and how many times? I've done a lot of them using the proper tools and meticulously followed correct procedures and had way too high of a percentage of them fail and end up back in the shop. I don't know exactly what percentage but I'm sure it was higher than 10% which is unacceptable and why I stopped doing them.

The right kit will probably cost at least $20 and it's a risk IME, put the money towards a new crank.


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## J.B. Weld (Aug 13, 2012)

RAKC Ind said:


> Those that are against it have never actually used them (or used them properly) for repairs.


I have.


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## RAKC Ind (Jan 27, 2017)

That's shocking honestly. I've yet to have a helicoil repair of any sort fail.

But in that sense (you've worked on more bikes in a week than I ever have) threaded inserts is the fix. If that can't fix it the rider is bashing the hell out of those cranks and pedals, not even new cranks will survive lol.

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## Fleas (Jan 19, 2006)

The only bad part about the Heli-coil is that you have to remove material from the crank arm; material that was supposed to be holding your pedal in your crank arm. You fill that space with the Heli-coil, which is awesome for holding tension, but the crank arm itself is now weaker. I say pony up the $40 for new old crank arms.

-F


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## J.B. Weld (Aug 13, 2012)

Yeah, I know I'm far from perfect but I always took pride in doing quality work and it's the saddest sight to see a customer walk in the door with a pedal in one hand and a bike in the other a week after you fixed it. I just figure if someone who has the right stuff and does it a lot sees failures then a beginner who says they don't have much experience would be better off buying a new crank. That's what I'd do for sure.

Probably a bunch of ones I have fixed are still out there though, who knows? I used both helicoil and inserts btw.


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## noapathy (Jun 24, 2008)

I don't know what to think. If the OP has the tools, it's an option. Not sure it's a good one given the age of the bike. Given this and the OP's unknown mechanical ability and what tools if any may need purchased.

Option 1: Helicoil - maybe $20 for parts.
Option 2: New crankset - maybe $40, which could lead to more expenses if the chain/cassette are also very worn.
Option 3: New(er) used bike - $ to $$$.


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## l'oiseau (May 5, 2015)

I'd really bet the majority of failures for helicoils in this case are from not being square to the mounting face on the crank. This would be my #1 concern and I don't see an easy to way to fixture it.

Likely they face that surface drill and tap the threads in the same fixturing in production, so it's likely to be pretty square.

Without any reference to that face, how are you supposed to put the insert in square?

I'd say the issue is not with the part, but the process. I bet a good machinist who faced the recess in the same holding while drilling and tapping for the insert would have a very low failure rate.


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## J_Westy (Jan 7, 2009)

The OP seems to be saying both arms... meaning two kits

I'd go the ebay route for some similar replacement or "upgrade" to

Shimano Deore M590 9-Speed Triple Chainset | Chain Reaction Cycles


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## J.B. Weld (Aug 13, 2012)

l'oiseau said:


> I'd really bet the majority of failures for helicoils in this case are from not being square to the mounting face on the crank. This would be my #1 concern and I don't see an easy to way to fixture it.
> 
> Likely they face that surface drill and tap the threads in the same fixturing in production, so it's likely to be pretty square.
> 
> Without any reference to that face, how are you supposed to put the insert in square?


Getting the insert in perfectly square can be an issue which another reason I'm not a huge fan but I'm not sure that had anything to do with the failures I experienced. The best cranks do have a pretty square surface for the pedal to mount against but a lot of cheap ones aren't even close and there's rarely an issue with thread failure due to that because the spindle basically mills itself into the soft aluminum.

The kits I've used don't involve any drilling but have a tapered reamer/tap combo that is supposed to be more or less self aligning. Drilling squarely into a crankarm is a particularly tough challenge because you can't just lay it flat on a table. How do you secure it so it's lined up perpendicular to the bit? I could see using the chainring on the drive side but the non-drive?

You may be on to something tough because whenever there's a crankarm thread failure it does chew things up a bit, seems like the reamer/tap combos would be better if they included an attached milling tool to clean and square up the surface. It still wouldn't be worth the time IMO, unless it's a hard to replace vintage item.


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## l'oiseau (May 5, 2015)

Yeah... might not be worth it...

I'm just rationalizing based on the lack of failures of seen for axially loaded fasteners (and I used to use these inserts a lot in the automotive industry). Pedals are a totally different beast though... and the fastener is not loaded in an ideal way... therefore the thread and interface becomes more critical.


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