How to find replacement crank arm?

Question

recently the crank arms on my Nishiki Colorado Comp 27.5 bike failed. I must’ve struck an obstacle with my pedals during my ride yesterday, because on the way home I noticed the right pedal was bent and when I removed it the threads were torn up. I installed this pedal myself; I confirmed it went in with the correct thread direction, though I settled on hand tightening.

So obviously I need a new crank of about 8” (200mm) and with 4 bolts. But I’m not sure how to find the proper crank, nor how to avoid this crank destruction in the future.

Answer 1

To find a replacement, there are a few details that you need to get from your current crankset. The most important one is to find what the bottom bracket of your bike is. You may need to remove the crank arms to work this out.

Next, you need to know the bcd of the chainring to ensure that it will fit on your replacement crankset. Also verify that the four bolts are evenly spaced on the chainring, as some brands like Shimano change their bolt spacing in a way that is only compatible with Shimano cranksets or a select few third party cranksets designed to take Shimano chainrings.

Lastly, purely for bike fit reasons, you'll want to ensure that the replacement crank arms are the same length. While the arm of your current crankset might be 200 mm, that is not the measurement that you need for this. To measure crank length, you need to measure the outside of the crank arm from the centre of the spindle to the centre of the hole for the pedal. This measurement should be between 170 and 175 mm on almost all standard bikes.

Answer 2

Replace the entire crankset (both crank arms, and possibly the chainring too). You'll have a hard time finding a matching right side arm.

From the photos I can see you have a '3 piece' crankset where the axle is fixed in the bottom bracket and is separate from the cranks (the bolt in the crank arm shows this). There are a few crank-axle interface types: square taper, and a few splined - 'Octalink', 'Powerspline' and 'ISIS' types. You'll need to find out which one you have.

It's quite likely the interface type is square taper, if you remove the central bolt and washer, you should be able to see the end of the axle. If it's square you have a square taper crank.

To find a replacement you will also need to know the length of the crank arm from bottom bracket axle center to pedal axle center. These come in 2.5mm increments, 'default' size on a MTB is 172.5 or 175mm.

If you want to use you existing chainring ring you'll need the chainring bolt circle diameter (BCD) and pattern - yours look evenly spaced, some have pairs of bolts closer together. Or, you can get a crankset with a new chainring, in which case you just need one with a chainring with the same number of teeth.

Replacing square taper cranks is not a hard job, but does require a special tool to pull the cranks off the bottom bracket axle. Also, the new cranks should be tightened onto the axle with a torque wrench to ensure sufficient torque is applied (but they are not over-torqued). You do not want the alloy cranks to loosen on the steel axle as this can ruin the square hole in the crank in short order.

Answer 3

Some bicycle makers have web sites where you can order the replacement parts for the specific bikes they make.

I also bent the pedal and stripped the threads on my crank arm on my Nishiki Pueblo and the retailer (Dick's Sporting Goods) was unhelpful; they insisted that I drive 150 miles(240 km) to their nearest store so their service department can do the work.