# Stripped my crank arm



## Smid (Jul 13, 2015)

Riding yesterday I felt my pedal get real loose all of a sudden and by I could stop it was too late and I stripped my crank arm out. I'm looking to replace it but totally clueless on what I actually need or need to look out for sizing wise. Bike is a scott genius plus 2017 model. 
A friend has an eagle xx crank and chainring like new for $100 but he said he needed to know spindle size to see it it'll work and I don't even know what that is lol. I'm assuming I'll need to switch out rear shifter and derailleur as well as convert to an xd driver, from what I've kind gathered but not really sure. I do know that I want to go 1x instead of the 2x setup I have but don't have a ton of money to throw at it for the time being. 
I'm also assuming the eagle won't work with my shimano rear components, but again that's just assuming. Any help or suggestions would be appreciated


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## V1Rotate (Jun 29, 2017)

I'll preface my reply with the fact that I don't own/have never owned a Scott Genius OR an eagle crank, but I am currently looking at a genius/spark and have dug up just a little info. From what I can gather your biggest hurdle when trying to fit that eagle crankset on your bike is that your current bottom bracket most likely isn't the right size to accept the sram crank. If research serves me correctly you have a BB71 bottom bracket on your genius that has measurements of 41mm (cup diameter) X 89.5mm (cup width) using the shimano hollow tech II standard. Looking at the eagle XX crankset it appears to fit to a BB30 bottom bracket which will have a width of 73mm. Not having any experience with the bike I can't tell you off hand whether you could just pop out the old BB71 and replace it with a BB30 bottom bracket then install the eagle crank as I don't know if the frame will accept it (unsure if the outside diameter of the press fit portion that goes into the frame is a common standard size or not). With that said you shouldn't have an issue with your current shifter/derailleur/cassette combo just because you swap to a sram crankset. Others may chime in otherwise but that stuff is mostly cross-compatible


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## life behind bars (May 24, 2014)

Heli-coil the pedal hole.


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## V1Rotate (Jun 29, 2017)

life behind bars said:


> Heli-coil the pedal hole.


I second this... it would be way less work and less expensive. Plus you could still go 1X with your current setup


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## Smid (Jul 13, 2015)

Thanks for the response V1, just looked at the specs on my bottom bracket and you are correct so that's out the window


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## Smid (Jul 13, 2015)

I've never heard of a helicoil but that looks like a good option. Is there a reccommonded kit or brand to go with?


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## Procter (Feb 3, 2012)

Sorry, but what exactly is the model of the current crank? Unclear from post.

Anyways: 
1) Personally I wouldn't trust a helicoil on a pedal, too much leverage involved, I'd be worried it would break away at peak stress (you know, landing something . . . when you need it most). Wear full face. And a cup.

2) Secondly, isn't this bike under warranty? Take it to the Scot dealer.

3) RE: Friend's 'eagle XX' crank - there is no such thing, its either an old 2x _Sram XX_ crank, or its a 1x _Sram Eagle XX1_ crank. You definitely do not want to go from a 1x to a 2x, that's both a downgrade and a lot of money, to, as you suspected, swap out the rear with 10x (derailleur, shifter, cassette).


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## root (Jan 24, 2006)

As long as theres enough material around it, its my experIence that helicoIls end up being stronger than original. One thing to watch out, one side is reverse threaded, hard to find a helicoil for that at hardware store. A well equiped bike shop repair department should tho.


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## Harold (Dec 23, 2003)

root said:


> A well equiped bike shop repair department should tho.


Most shops I know don't trust helicoils for pedals


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

Smid said:


> I've never heard of a helicoil but that looks like a good option. Is there a reccommonded kit or brand to go with?


I'm one of the ones who doesn't trust them for pedals either, I've seen too many failures. It's only about a $20 gamble if you want to go for it though, the bushing kits may be better than the helicoil ones.


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## Lone Rager (Dec 13, 2013)

Helicoils are fine, and usually stronger than without *if installed correctly*, which is tough to do unless the crank arm is fixtured in drill press or mill to ensure the hole is drilled and tapped true prior to inserting the helicoil insert. Most LBSs are not equipped to do this.


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

Lone Rager said:


> Helicoils are fine, and usually stronger than without *if installed correctly*, which is tough to do unless the crank arm is fixtured in drill press or mill to ensure the hole is drilled and tapped true prior to inserting the helicoil insert. Most LBSs are not equipped to do this.


How would you secure the left crankarm in a manner that would allow you to drill a hole exactly perpendicular to the crankarm? Every pedal helicoil or bushing kit I've used has come with a reamer/tap combo that requires no drilling.

Also I'm curious how many pedal helicoil kits you've installed and what you're success rate has been. I'm not suggesting that I've done them all perfectly but I've done my best to follow instructions precisely and have experienced several failures.


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## Procter (Feb 3, 2012)

Lone Rager said:


> Helicoils are fine, and usually stronger than without *if installed correctly*, which is tough to do unless the crank arm is fixtured in drill press or mill to ensure the hole is drilled and tapped true prior to inserting the helicoil insert. Most LBSs are not equipped to do this.


I don't get how they could be stronger than original, if that's what you mean - You are replacing 1 threaded interface, with 2 threaded interfaces . . . right?

Any threaded interface has an imperfect contact surface ... there is always some wiggle room. This portends its eventual failure, whether that's quick or slow. the thread interface on the outside of the helicoil will probably be the stronger one, since the tap and the helicoil are designed together to have the closest tolerance possible. But there will still be wiggle room.

On the pedal-helicoil side, despite thread standards, they were manufactured independently - even with tight tolerances it's an imperfect interface.

I wouldn't feel comfortable with that on a single point of failure under very high leverage, like a pedal.


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## mack_turtle (Jan 6, 2009)

Who was the last person to put a wrench on that pedal? Pedals don't just strip out cranks and it's extremely difficult to strip by over-tightening.

I would be willing to bet that someone did not tighten it hard enough at some point, which allowed it to work itself loose and strip out. If it came from the shop like that, a mechanic is at fault. Might not help your situation now, but helpful for future reference. Tighten those pedals!


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## root (Jan 24, 2006)

My personal experience has been withbautoparts, I haven't helicoiled any cranks, so i guessmy thoughts on this are less valid. Helicoils end up often stronger befause the coils are steel threaded into bigger threads in the aluminum. But to bore out the hole bigger you need to have enough material. Also for pedals the face where the pedal shoulder butts to the crank needs to be in good shape. Using a coil takes away some of that and if pedal doesnt have a good flange or washer, it could contribute to comingnloose again. Id ask my friend who works as a bike wrench how many failures hes seen, but i think they dont do crank repairs. Easier and less open to liabilty to just replace.


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## Lone Rager (Dec 13, 2013)

^^^ Yes,

As for fixturing a crank arm here's one way. Attach it to a spindle. Clamp the spindle in a V-block held down to the milling machine table. This puts the axis of the pedal threads parallel to the mill spindle. Place support the under far end of the crank arm. Locate the center of the pedal threads visually or with a bore locating scope. Use a dril bit or reamer to open the hole to the correct size. Chuck up the tap in a tapping adapter in the mill. Tap the hole. 

What you don't want is to wiggle the drill or reamer, or wiggle the tap so that remove more material and open the hole more than the specified amount, or make it conical rather than cylindrical.

There automotive and aviation designs where helicoils are used from the outset because they can be stronger and also last through more maintenance cycles.


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## Procter (Feb 3, 2012)

Lone Rager said:


> There automotive and aviation designs where helicoils are used from the outset because they can be stronger and also last through more maintenance cycles.


Stronger than what though? That makes no sense. Why would you use a thread, inside another thread - you'd just make the inner screw (the pedal spindle in this case) larger, to achieve the same result.

Root's point might be valid for a re-work application (like we have here) but I'm still a little skeptical: Your helicoil thread is steel, so you have (larger) steel threads up against aluminum. But then you have your original aluminum threads up against imperfectly aligned steel helicoil threads. So its plus some strength, then minus some.

But for original work (not re-work) it seems like it would be much better to just: 
- Make the screw bigger, to fit directly into the bigger threads
- Make the screw steel, if the application called for it


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## root (Jan 24, 2006)

Same size bolt threaded directly into aluminum vs threaded into a helicoil (helicoil btw is a brand, there are other thread inserts, but the name is so strongly associated) inserted into the aluminum. The part in the aluminum is larger area. Also it greatly reduces problem with galling since the bolt when beingbtightened is moving against the steel coil insert. Once installed the coil shouldnt move within the aluminum.


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## dr hoo (Jan 12, 2004)

mack_turtle said:


> I would be willing to bet that someone did not tighten it hard enough at some point, which allowed it to work itself loose and strip out.


Pedaling (when everything is mechanically sound) will tighten pedals. https://en.wikipedia.org/wiki/Precession_(mechanical)#Bicycle_pedals

I would guess installation borked the threads in some way, or the threads were borked from manufacture and not corrected.


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

dr hoo said:


> Pedaling (when everything is mechanically sound) will tighten pedals. https://en.wikipedia.org/wiki/Precession_(mechanical)#Bicycle_pedals


In theory maybe, in the real world though it doesn't seem to happen and under-tightened pedals are the #1 cause of stripped pedal threads IME.


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## mack_turtle (Jan 6, 2009)

Try this experiment:
1. Install your pedals to "finger tight"
2. Ride trails for an hour or two and allow the magic of precession to "tighten" your pedals for you
3. Report back with your findings


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## dr hoo (Jan 12, 2004)

J.B. Weld said:


> In theory maybe, in the real world though it doesn't seem to happen and under-tightened pedals are the #1 cause of stripped pedal threads IME.


Is that your experience? I wonder how many of those pedals have you put on before they stripped? Because it is pretty much impossible to know what they were torqued to unless you did it yourself.

In my experience (30 years of cycling), just snugging the pedals, I have NEVER had a pedal back out, much less strip. That's a lot of pedals on a lot of bikes (mine and others), and includes a short stint of paid wrenching.


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## dr hoo (Jan 12, 2004)

mack_turtle said:


> Try this experiment:
> 1. Install your pedals to "finger tight"
> 2. Ride trails for an hour or two and allow the magic of precession to "tighten" your pedals for you
> 3. Report back with your findings


I would suggest changing #1 to "torque to 1 ft lb". If you use a beam type torque wrench you can read the torque upon removal to quantify any tightening. One calibrated in in lbs would be best.

Finger tight is not always actually seated fully, especially if the parts are not very clean. The pedal must be seated, obviously. I am certainly not saying putting the pedals on half way and going for a ride will tighten them. Not at all.

BTW, I probably go for more like 2ft lbs myself (snugged, basically), but 1 would make for a better test. Have fun with it.


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

dr hoo said:


> Is that your experience? I wonder how many of those pedals have you put on before they stripped? Because it is pretty much impossible to know what they were torqued to unless you did it yourself.


You're right, I don't know for sure but I think it's a decent educated guess, I've seen the beginnings of thread damage on crankarms that came into the shop with a loose pedal on numerous occasions. More often though I've fixed creaks and clunky feeling cranks by tightening a slightly loose pedal, we did a lot of repairs and this was nearly a daily occurrence for me.

This subject seems to come up a lot, I know lot's of people get by with just snugging their pedals and that's fine with me but IMO there's very good reason that Shimano and other crankarm manufactures specify 307 in. pounds *minimum* torque for pedals.


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## mack_turtle (Jan 6, 2009)

I worked in bike shops for several years and saw lots of stripped crank arms. Once in a while, they has been cross-threaded or the wrong L/R pedal was forced into arm. Most of the time, though, it was a pedal that had not been tightened enough and the threads were rattled into dust as a result. I think 30 ft-lbs was the torque I always used with hex socket or crows-foot pedal wrench adapter.


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## zacklawhon (Aug 23, 2017)

2 options
1: If it is the non-drive side crank, you can have any bike shop order a replacement arm on the cheap. 
2: If it is the drive side, just go on ebay and buy any Shimano crank that has a Thru axle spindle, XT, XTR, SLX, Deore, ZEE or Saint. It ill install into your stock BB set.


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## Grassington (Jun 24, 2017)

Just a slightly dodgy thought, but it's possible only the last few threads got mangled as the pedal rattled off, in which case it might me possible to clean them up a bit by threading in the pedal from the inside of the crank to re-form the chewed section. The damaged threads will be weakened, but it's a good get-you-home fettle if it happens on the trail.

Yeah, pedal torques are pretty high. IIRC the last XT cranks I dealt with spec'd 40 Nm, which is unlikely to be achieved with a hex key or short spanner.


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## zacklawhon (Aug 23, 2017)

Park Tool makes a tap just for pedals. Any shop should have one and offer that service.


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## FJSnoozer (Mar 3, 2015)

1. someone local may have a crank arm or crank set for you that would work. Maybe you can trade for beer to someone who has upgraded their crank. 

3. look on ebay for the correct crankarm to match yours. You MUST know what you are ordering. you need to know your crank arm length too. 

2. your friend probably has an eagle "x1" crankset which is the new aluminum OEM style crankset with no direct chainring mount option. This is fine as long as you purchase the correct Sram Bottom Bracket to go with it whether it is BB30 or GXP. if it is BB30, you MUST make sure your frame can accept it. With this you can go 1x and can merely drop your shifter and shorten your chain if you are currently 2x. This whole ordeal will wind up costing you around 200 bucks or maybe a little more.


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