Originally Posted by
Erouan
I have a 920 Trek bike that I use daily. I bought it 5 years ago and the big chainring needs to be replaced (It has missing teeth). In the service tab, the description of the item crank is the following: "ProWheel forged alloy, 42/28, 104mm BCD, 2-piece, 175mm length. Uses standard Shimano compatible Hollowtech II BB". But in the other hand, one of the picture from the Trek's website shows a different brand and BCD: 42T 10SPD TRUVATIV 120 BCD.
Sheldon Brown has
a page on this topic with relevant measurements.
But are you sure the teeth are missing? Some teeth on a chainring are usually lower than others to enable smoother shifting. Others may be ground down by rocks or kerbs. Neither of these conditions mean that you need a new chainring, as long as the remaining teeth are engaging the chain properly.