Rethread a crank arm? Possible? Worth it?
 

So, my bike got run over by a car recently (thankfully, it was locked up and I was nowhere in range), and I've gotten over my grieving and have started assessing the damage. What at first looked like a bent drive-side crank arm now appears to just be a badly bent pedal spindle. The pedal is pretty well jammed in there though, so I imagine I'll bugger up the threads on the crank arm in the process of removing the pedal. So...

Will it be possible to rethread the crankarm after this so I can salvage the crankset? In case it matters, it's an SRAM S300 crank, 6061 aluminum, about $150 to replace.

Thanks guys.

Distorted but otherwise intact threads can be chased to repair them. Badly torn up threads can be repaired by drilling the hole oversize and rethreading for a Helicoil. A well equipped bike shop can do the job but be sure that's all that's damaged and that the job won't cost more than the crank is worth.

Also remember that pedals will align to a degree with the arm's face as they get fully tightened. So if the arm's face is unevenly gauged up the straight spindled pedal could seat askew and still feel bent. Pedals with a flanged seat do better here. Andy.

first things first. 1) remove the pedal. 2) asses thread damage. on the bright side, there might not be a 3). :)

Also, the pedal spindle is likely steel and the crank aluminum. The short steel lever may have more resistance to bending than the long aluminum crank arm so you may have the possibility of a bent crank arm too... You are going to have to get that old pedal out first to assess what is damaged and repairable.

Did the driver stop and leave a note identifying him/herself? If so, avoid the repair and let him/her|his/her insurance company foot the bill.

How much weaker will making the hole this much larger make the crank arm? Isn't a crankarm designed to be as light as possible, thus not to have excess metal?

If it was a CF crank with a bonded-in metal sleeve I might have concerns. But on an all-metal crank, no.
The usual development when using heli-coils it that it makes the item STRONGER. The coil essentially means you're using one size larger threads. And in a soft material like aluminium, that's almost always a good thing.

Make sure that you closely examine the crank arm for cracks or other distortions before you proceed. Use a bright light an a magnifying glass. Aluminum can develop small fractures that are not easily apparent until they let go suddenly. Having been rudely introduced to the pavement when this happens, I can attest that it is not much fun.

I'm hoping that your only damage is a bent pedal spindle, but OTOH I can't imagine a force strong enough to bend a hardened spindle without doing harm to the crank. Post some pics .

I'd be concerned just because this is aluminum. Do a good repair, forget about it and ride it for years and it it were me, I'd be riding it when it finally failed. I have witnessed 3 cranks braking at the pedal. Two were on the bikes whose wheels I was following racing, Both riders went down hard. I broke the 3rd a a perfect place, just about to crest a small hill. Put my foot down awkwardly on the road, sat down and coasted to a stop. Couldn't find my pedal. Oh, its clipped to my foot! I coasted a mile to my destination. I figured I've used up my good luck and won't mess around with shaky cranks again. (That one was an old Avocet tossed on to a newly set-up bike as temporary solution because they had the right chainrings.)

You have no idea what stress that crank saw when the car hit it. And with aluminum, that crack starter event will not show a bend or dent as so often happens with steel. Unless you are very methodical and check before every ride, any crack that starts will result in failure while riding. And it will be the crank just snapping and the pedal falling off, no warning at all.

Ben

A customer comes in and asks the shop to repair the stripped pedal thread, its always worth it , because that is what shops do , fix stuff for a fee.
Cost estimate too much for Your budget.. your call ..:rolleyes:


sometimes the replacement right crank is = to the whole both cost . bent? brand new pedal feels odd?. this is replacement time.

Thanks for all the input y'all. Unfortunately this happened sometime around 1:00 am, no witnesses or anything, and the driver took off, so I'm footing the bill.

i guess the thing to do now is to just bring the thing down to a shop and have them check it out. Luckily, in addition to bad drivers, Austin has plenty of great shops.

A Heli-coil™ is stronger than T6061 alumin(i)um; the pedal will more-firmly attach to the crankarm: that's not the issue. The crankarm's strength where the pedal threads into it is weaker when I drill a larger hole out of it.

...we do this for people at the co-op here, mostly as a service at cost.

You can no longer buy the heli-coil thingies and installation kit for pedal threading, but I think maybe you can get a spark plug one that is RH threaded in 9/16-20.

What you can buy is a kit with inserts that are stamped out of sheet metal, and a combination tool that reams the hole oversized a bit, then rethreads it to accept the inserts. These are available in both LH and RH threading. It's too pricey to buy just to do one crank, and the replacement inserts are about 5 or 6 bucks each.

I have no concerns that we are removing enough material to weaken the pedal eye, but I have some concerns about your description of the mode of damage and whether this is still an option.