Hi everyone,

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.

So I'm confused and I don't know what to order (and where). Do you have any advice, please?

Thank you!

PS: I can't share with you photos abd the URL of the bike from the Trek's, because it's my first post.

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.

Thank you, that was very helpful. I printed the 4-bolt Chainwheel Guide, and I'm very confident the BCD is 120.

Yes, without counting the ones that have worn off, there are five teeth missing on the 42. When I shift gears, the chain takes some time to grab a tooth, and sometimes it jumps when I apply more pressure on the pedals. Also, while looking at the chainring while pedaling, I can see it wobbling a bit. I have no issues with the small chainring.

It is very unusual for a larger chainring to wear out faster than a smaller one
Pictures would help
It is easy to mistake shorter teeth on a chainring designed to aid shifting for missing or worn out ones

I hope the OP can tell the difference between the search result mentioned, a 42T crank, and the one on their actual bike (42/28)

As to ring wear rates- I guaranty the ring you use more will wear out.

This forum has a10 posts before photos can be linked to or included, to filter out bad people. I believe one can post to the forum's gallery and one of us can repost that here. Andy

it looks similar to this bike, right? color may not match, but the front sprocket setup is the same, right?
https://media.trekbikes.com/image/up...996_A_Portrait

nice bike, btw... i'm jealous.

I would like to share, but like Andrew mentioned, I can't until I post 10 messages.

Yes, that's the one! The only difference with mine is that it doesn't have the SRAM logo on the crank. I can't share the URL of the product, but there are more pictures with a better view of the chainring.

Thank you! I love it! It's a shame they stopped producing it.

Basic search on Ebay or the web: "42T 120 BCD outer chain ring"

here's the exact replacement ring... Ebay Search: SRAM/Truvativ X0 X9 GXP Chainring 42T 120 BCD Aluminum Black Use with 28T

remember that ANY outer ring you order for your bike, as it is now set up, Must have Shift Pins and Ramps, or you won't enjoy it one bit, ok?

Awesome, thank you! I'll buy this one. However this a pin at the bottom of the chainring, but I don't know if the current one have one. I'll check that later tonight.

That pin is to keep the chain from getting stuck between the crank arm and chainring.. and also shows How to Orient the Chainring for best shift characteristics. .

So on the current, there is none. But there is some room for the pin between the crank and the chainring. I'll still buy this one and try it.

Thank you for help. I'll let you know the result!

All my large chainrings wear out first. Probably depends how hilly your area is :-)

And to be honest, I store my bike in the basement, and there is a concrete step in the small doorway. I may have hit the chainring on the step a few times... I try my best to be careful, but you know...