Bicycle Mechanics - How Much Can a Seat Tube Be Enlarged?

I just bought a old Schwinn with a 26.6mm seat tube ID. Didn't come with a seat post and it seems like there are a million 27.2mm seat posts. Can I remove 6mm?? For reference here it is Columbus Tenex double butted steel.

I'd think it would be much safer (and easier) to remove metal from the seat post than the frame tube.

- Mark

You can find a 26.6mm OD seat post for around $10. Reaming out the seat tube will likely cost more unless you already have the tools.

Please don't ream or sculpt. Reaming, especially with thin double-butted chromoly like what your frame is made out of, is likely to compromise the structural integrity of the frame if you could even manage to take off that much material without completely cutting through the tube. By the way, that Columbus Tenax tubing is cosmetically blemished Columbus SL or SP that was sold to Schwinn for a lower price. (Is this a Tempo or Prelude? It should make a pretty sweet ride, so please don't hurt that frame.)

As for sculpting the seatpost, you don't want to end up in this situation (http://www.bikeforums.net/showthread.php/807903-Stuck-seatpost-success-story), and trying to shape the tube or the post sounds like an extremely easy way to end up there.

Best solution would be to get a hold of the exact right size 26.6mm seatpost. 26.6 is a fairly standard size, and you can buy one from Nashbar (http://www.nashbar.com/bikes/Product_10053_10052_175494_-1___), but I also guarantee you'll have no problem finding older, smaller seatposts for very cheap in the spare parts bin at your local bike shop. Get a hold of a smaller seatpost -- 1" (25.4mm) are extremely common -- and shim it with the correct shim (http://www.nashbar.com/bikes/Product_10053_10052_175458_-1___) or even with aluminum from a soda or beer can (coke cans = .1mm, tiny juice, giant Foster's, or Red Bull cans .2mm, or oil cans .4mm).

Or go here:

http://www.mybiketinley.com/Kalloy-266-X-350MM-Black_p_13179.html

$10. And if you order over $49, I think shipping is free.

Don't cut your frame nor an oversized seatpost, nor shim an undersized seatpost, just get the right sized one. They are cheap and easily available and there are too many downsides to the other alternatives. Once you remove metal you can never put it back. Don't forget to grease your new post so that it doesn't get stuck in the frame.

Thanks for the advice. I was going to hit the bike coop today and see what I could find. I tried CL and Nashbar and couldn't find anything. Although when I looked at Nashbar I looked under road not mtb seats and posts. I even checked Ebay and didn't find much.

BTW, it is an '86 Schwinn Passage. The frame seems to be straight but the paint is a bit scratched.

http://www.lmgtfy.com/?q=26.6mm+seatpost

26.6 ID, in a 28.6 OD seat tube indicates it is not butted. its a straight gage with 1mm wall thickness,
maybe the other 2 main tubes, if at all, are butted.

as others are saying , get another 26.6 seat post.

though I have used a shim machined by the UK , USE company.
they make a 25.0 premium quality seat post and shims to use it up to 27.2 ID frames.

Have a Campagnolo seat post off an AlAn I had in the 80's
thick wall standard OD aluminum, so I used it in a steelframe bike for a while..

Nashbar carries a relatively small selection of seat posts. Either go to the LBS, or ebay. $10 or so.

I did buy a Tempo recently, where the PO had somehow crammed in a 27.2 mm Thomson seat post. Of course, the seat post ovalled, then snapped off inside the seat tube. That was one heck of a job getting that post out. Bought it just for parts anyway.

My local bike shop had the Schwinn sized seat posts in stock, so go check at your local shop. I think they are the old mtb size as well. My 1980ish Specialized Hard Rock has the same seat post size.

... Can I remove 6mm?? For reference here it is Columbus Tenex double butted steel.

Just out of curiosity, how were you intending to remove 6mm from the inside of the seat tube?


Tom

I went to the coop and they didn't have a 26.6 but they did have a 26.8 which I bought and sanded (and sanded) until it fit in well. I used calipers to measure as I went along to make sure that I was removing material evenly and that I was in a good spot. Besides taking a couple hours or so I am pleased with the results. Thanks for all of your input.

Just out of curiosity, how much money did you save by compromising that seat post?

I'm a curious guy as you can see.

Talk to me brother.


Tom

26.6 ID, in a 28.6 OD seat tube indicates it is not butted. its a straight gage with 1mm wall thickness,
maybe the other 2 main tubes, if at all, are butted.Tenax is re-badged Columbus SL/SP tubing made specifically for Schwinn. It comes in 1.0/0.7/1/0mm wall thickness.

The smaller ID of 26.6mm doesn't mean anything about butting along the length of the tube, just what the wall thickness is at that end. An unbutted tube of 1.0mm all the way through would also have 26.6mm ID. Similarly, a 27.2mm seatpost size is very common in the higher-strength steels and can indicate unbutted 0.7mm wall thickness all the way through OR butted 0.7/0.5/0.7mm tubing. The size of the seatpost does not say anything about whether the tubing is butted or not.

I went to the coop and they didn't have a 26.6 but they did have a 26.8 which I bought and sanded (and sanded) until it fit in well. I used calipers to measure as I went along to make sure that I was removing material evenly and that I was in a good spot. Besides taking a couple hours or so I am pleased with the results. Thanks for all of your input.I've spun down quite a few seatposts on my lathe over the years. The cheaper Kalloy or SR/LaPrade type posts are fine for this as they have A LOT of meat in them. Superlight posts such as American Classic, not such good candidates.