I'm replacing the chain ring on an old Trek 1200...probably from around 1990. I had the chain ring replaced a few years back at a used bike shop and I think I got an obscure part. There are two front rings with 52 and 42 teeth and 7 rings on the cassette. I'm planning on replacing it (the front rings not the cassette) as the teeth are worn down, but I'm worried I'm going to replace it with some chain ring that is somehow incompatible with my gear system. Any tips?

Thanks!

Sheldon Brown's Bicycle Crank/Chainring Bolt Circle Diameter Crib Sheet Measure.

You need to match the bolt circle diameter (aka BCD), the number of bolts and the tooth count you want. I expect yours is a 130 or, possibly 110 mm, BCD 5-bolt crank and chainrings to fit these are easy to find. Ask for 7/8-speed chainrings to maintain the gap between them needed for your chain as 9-speed and above are spaced somewhat closer together.

Seems like that bike came stock with 53/42 but 52/42 was also common on 7 speed bikes. Are you happy with the current setup or do you want to change gearing?