I have a Giant ATX 760 and need to replace the middle front cog after installing a new chain which is now slipping on that cog.

I went to a local bike co-op and the volunteer there said these are not one size fit all parts. From that I take it that you can't just go on-line and buy any old cog with the same number of teeth. You have to find the part for that specific bike.

How would you go about finding the right cog for your bike?

5 spoke crank, right? 110 bcd, most likely... run a search on Ebay for a "middle triple xx t 110 bcd" chain ring with same tooth count as present ring... place your tooth count where the x's are in the search i typed...

4 spoke crank? 104bcd.... is the ring attached with RIVETS, instead of an allen screw/slotted nut? you'll need to get an entire crank assembly with same tooth counts on the 3 rings.... if it has screws/nuts, then you can replace just the middle ring...

the screw mounted outer and middle rings can be removed/replaced without pulling the crank arm... inner ring will require removing the crank arm... the small inner rings rarely wear out, btw...

the "BCD" (base circle diameter) measurement is sometimes listed on the drive side crank arm, and might also be stamped into the chain rings... look at both sides of the chain rings... some are stamped on the outside, some on the inside....

Thanks for the explanation. Yes it's a 5 spoke crank.

I went to the bike co-op again today (The Bikery) and they found a spare chain ring that fit, and it was indeed a 110 bcd. 36 teeth. They had me replace it. Great value too. It only cost $15, voluntary donation.

excellent! i run a community bike shop... and buy most things i don't have donated from two other co-ops i frequent... found a 46 tooth ring for a CX racer at one of those shops just two days ago... it had just come in... no price tag... got it for $10.... a nearly new SRAM ring with ramps/pins...