Bashbike

Hi all,

I damaged the thread in the right crank arm and need to replace it. About my bike:

Front shifting:

• Derailleur: Sora FD-3403 Tripple for 9 speed
• Shifter: Sora ST-3400 Triple for 9 speed
• Crankset: no-name triple ring 52/42/30T, 175mm

Rear shifting:

• Derailleur: Tiagra RD-4500GS for 9 speed
• Shifter: Sora ST-3400 Triple for 9 speed
• Cassette: 9 speed, 12-25T

It is not clear to me how to find a right crank arm on Amazon or Ebay. What needs to match? The length 175mm, yes. What about the precise number of teeth 52/42/30? What about the front speed (triple)? What about the rear speed (9-speed)? What else?

MudPie

The features you must match:
1) bottom bracket design. The bottom bracket could be a square taper, or a splined interface.
2) crank arm must accommodate 3 chainrings.
3) if the cranks do not come with chainrings and you want to use your old chainrings, then the bolt pattern of the chainrings and crankarm mount locations must be the same. This is specified by the BCD (bolt center diameter) of the mounting bolts and number of mounting bolts. For example, a 110 mm / 4 bolt pattern means there are 4 bolt locations and those locations will lie on a 110 mm circle drawn around the center of the crank arm spindle.
4) crank arm length, but you mentioned that in the original post.

The features that are nice to match:
A) if you buy the crank arm with chainrinngs provided, match the number of teeth 52/42/30 as in your original set up. This way, you won't have to adjust your chain length. But the number of teeth is more personal preference and need not necessarily match. If you get one with less than 52T as your big ring, you might have to shorten your chain
B) Finish, purely aesthetics but crankarms of different color or finish (black/silver/gray...matte/shiny....) Some find this detail important, others could care less.

dsbrantjr

If you are referring to the pedal threads, they can be repaired by tapping to a larger size and insertion of a Heli-Coil (or other type) insert. The resulting threading will be stronger than the original.

sweeks

And... if the damage isn't too severe, you might be able to use the crank anyway. If only the first few threads are distorted, preventing the pedal from threading in, try threading the pedal in from the back side of the crank to clear the way for the pedal to screw in normally. If the pedal's threads aren't long enough to reach the damaged area, you can have a shop use a tap to chase the threads. Of course, if the damage is bad enough, you can take dsbrantjr's advice and have a helicoil installed.
It probably doesn't hurt to add that any time a steel fastener is being threaded into a softer metal such as aluminum, the fastener should be threaded in *by hand* as far as possible before applying a wrench.

fietsbob

In general if you need a right crank arm the cost may be less for whole crankset . or very close.