I inherited an old Trek 7100 hybrid bike, it looks like a 2007 based on the color scheme or maybe a 2006

https://archive.trekbikes.com/us/en/...k/7100/details

https://archive.trekbikes.com/us/en/...k/7100/details

At some point the seatpost and saddle were stolen and the owner replaced it with the wrong size that needed a thin shim.

The one that needed a shim is marked 30.9mm and it is quite loose inside the seat tube without the shim (thin sheet of aluminum).

I purchased a new seatpost that is 31.6mm and it doesn't fit. It takes all my strength just to jam it in there a little bit and then it gets good and stuck.

So what size of seatpost could I need for this bike? Is there like a 31.3?

Most likely 31.6mm that's been pinched closed a little. Most decent bike shops will have an adjustable reamer which can clean up the ID of the seat tube and allow your new post to fit.

I now have several bikes with Machined shim sleeves post is 27.2.. Ooodles of choices there.

Sleeves with a 27.2 ID and what ever your seat tubes ID are as Its OD work perfectly

[FWIW, 1.25" is , 31.75mm so rounding.. .6 under , .8 over....]

seat tube of frame could be distorted... a bike shop with an adjustable reamer can cure the misshapen frame tube back to round..

To make your 31.3, you can get a shim sleeve and take of 0.5mm from it and not damage the actual seat post ... (which it reads like you have already done)





Patience, grasshopper..
...

I would be very careful to use a reamer on a seat tube that has had the binder slot already cut (as opposed to during frame building reaming before the slot is cut) especially with an adjustable one that has straight blades. The blades can very easily catch on the slot and either cut poorly or break.

I don't have access to Trek's dealer site which has a lot of prior years specs, like the OEM post diameter. Any Trek shop can sign on and search to see if the records go that far back. Of course the expectation will be that you buy something from that shop.

The usual way to deal with this situation is to try to open up the binder area before any material removal. Better shops often have a range of different sized posts to both be used as prying devices as well as gages for tube ID. It's not hard to use a slightly undersized shaft and wallow out a squeezed in binder area. Only after getting REAL close to the OEM post size of fit would I consider honing or reaming. And only then to take off the few "high" points. If I were to ream I would bevel, relieve the slot's inside edges so that a reamer blade wouldn't catch so easily. Andy