Specialized/Altus crank question

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I have an older (not sure how old...thrift store bike) Specialized RockHopper that came with Altus components. The pedal on the drive side crank arm has loosened, augering the hole out and rendering the drive side arm unusable. Couldn't find any local shop willing to tap an insert into the arm. I used to do it commonly when wrenching at shops in my teens.
Is this not common now?


The arms are Altus 170s with a marking of FC-10 on the back side.

My question is: What arms are compatable with the bike? I am purchasing a set of like arms and chainrings off Ebay today, but the markings on the back of those arms say FC-CT91.

Will they work or have I bought yet another set of expensive door stops?


-Charles in Colorado

i've never discovered what the fc numbers on the cranks are for, but the 170 is the crankarm length; do the new arms have anything like that? if not, just put them side-by-side, as close as you can get, and see if there's any difference. if you're still not sure, lbs should be able to tell you. other than the length (BTW: i ran 170 driveside/175 non-drive on a "frankenstein" bike all summer & couldn't feel the diff), the critical dimensions are BB interface (square taper, i'm guessing?) and chainring size; almost all 4-bolt rings are 64/104. 5-bolt rings are all over the map (but pretty much obsolete anymore, too)


hope this helps....

You need to find some MTB repair books perhaps at the library which will identify the bottom bracket type. Unless it looks like a square, tapered shaft going from the bottom bracket to the pedal; in which case it is a-you guessed it-square taper BB and will take almost any mtb crankset. You need to tell us what you've got.