patc

How tight are seat-post tolerances? My old seat-post is 27.0mm (and is loose enough to fall in when the bolt is loosened). The one I would like to buy comes in two size: 27.2mm or 25.4 + shim. Can I likely get away with using 27.2mm? Does that 0.2mm really matter?

Thanks.

mechBgon

Is the frame definitely built for 27.0?

patc

As far as I know - that is the stock post.

brokenrobot

Your description of the seatpost falling in makes me think your bike was originally supposed to take a 27.2 anyway... 0.2mm isn't much - I suspect you'd be fine, if the current seatpost is as loose as you describe.

well biked

If you really want the 27.2mm post, I'd go ahead and get it. You'll probably have to sand the portion of the post that goes into the seat tube some, but no big deal to take off .2mm. I've done it, no problems. And it sounds like if your 27.0 post will "fall in," it's kind of a loose fit anyway. If you get the 27.2 post, and it won't go, don't do something like pound it in with a hammer, just sand away enough material so that it will go in with reasonable effort. Snug is okay, having to pound on it is too tight.

Grand Bois

I'd get the 27.2. If its too tight , you can sand it and repolish it. I had to do that to a vintage Simplex post yesterday, even though it was supposed to be the correct size.

patc

Thanks for all the replies, folks. Will get the 27.2 in my next order.

Mothra

Sounds like your bike may have been made with 27.2mm anyway. Slipping seatposts are one sign of using an undersized post. Your clamp is probably squeezed down too far now, so I'd use a screwdriver to spread the clamp back out to its original size, then the 27.2mm post should fit without any problems... and no slipping.

Grand Bois

Be careful. I hate to admit that I did such a stupid thing, but I tore a hole in a Reynolds 531 seat tube that way.

rmfnla

Have a decent shop check the size.
If it really is 27.0 they can hone it out to accept 27.2 (remember; I said a decent shop).

phantomcow2

somebody might be able to let you borrow some calipers. I would think most automotive shops would have these, maybe your bike shop.

12XU

Our shop has two separate seatpost sizers to figure out exactly what size seatpost is for your bike. We've had a guy try to squeeze .4-.6 mm out of his seat tube which is NUTS, especially considering it was a custom frame. Most good shops should have these sizers (or calipers as previously mentioned), so go and ask them to measure your frame!

AnthonyG

I have squeezed a 27.2 mm seat post into a 27 mm seat tube on an old Reynolds 531 steel frame. It fitted but the seatpost was VERY scratched up from fitting or adjusting and it was realy difficult to move.

Regards, Anthony