smontanaro 

I am building up a mid-80s Trek 770 frame. My understanding is that all Trek bikes of that era should have had 27.2mm seatposts. I bought a no-name seatpost which is clearly marked "27.2" but it simply won't fit. I don't currently have a set of calipers to figure out precisely which part is wrong. I'll be doing that soon. I did measure with a small metric ruler and figured the seat tube inside dimension was 27.2.

Am I mistaken about the 27.2mm seat tube dimension or is it likely the seatpost is
mismarked?

Trucker Dan

If its a 531 trek which is what a 770 should be then it will be a 27.2. I have a modern 27.2 thomson seatpost in my 84 720 right now.

illwafer

check to see that the seat tube wasn't squished by the previous owner using a smaller seatpost. a strong screwdriver can help.

miamijim

+1. You'd be surprised at how much a seat collar can collapse.

rhm

I would be pretty surprised if ALL Trek's from any era took 27.2 posts. I'm sure all the butted 531 frames did, and the Columbus frames as well, but bikes made from other kinds of tubing may have taken thinner posts. Lower end models, in particular, probably took thinner posts.

illwafer

my 1985 520 has 501 tubes and 27.2 seatpost.

realestvin7

I'd had some 27.2 blues lately as well. Different brands not fitting in the same frame, and all that jazz.

repechage

Got another bike with a 27.2? If so, remove the seat post from it and test THAT post in your reluctant Trek, and check the "no name" in the other frame.
Good check to make sure you do not get stuck with a frozen post in your other bike too.

smontanaro 

Well, the caliper tells the tale, though I'm still a bit perplexed. Side-to-side the seat collar does appear to be 27.2. It's smaller front-to-back, as small as 26.9. Wouldn't clamping a smaller seatpost make it smaller side-to-side? It doesn't help that the seatpost seems to be as large as 27.3 in some directions (okay across other diameters).

cudak888 

Clamping a smaller seatpost would pull the lug ears together and forward, effectively reducing the seattube size as you describe.

-Kurt

smontanaro 

Any suggestions about how I could bend the seat tube back into shape?

JohnDThompson 

The Trek mountain bikes (830 & 850) used 26.8mm posts.

JohnDThompson 

Take a smaller diameter post (27.0 or 26.8), insert it into the lug and use it as a lever to spread the ears of the lug apart.

jgedwa

I have an 85 620 with butted 531 that takes a 26.8

jim

3speed

Since this mystery is already solved and the OP knows his seat tube is screwed up, I'd like to ask another question.

It's some how bad for the bike to use a smaller post, right? I've got a 531 frame that takes a 27.2 and a really nice 26.8 post. It's a shame to have it laying around and have a lesser quality post in my bike, but I'm guessing there's a reason it would be bad to use it?

realestvin7

So you've had botha 26.8 and 27.2 in the same bike?

look171

screw driver time. If not, I think this is a better method, cut a sliver or wedge of wood(to protect the paint and not do damage if you slip) and drive the split open just a hair. Grease the 27.2 post, it should slip in there without too much effort. Tighten the seat post binder and you should be good to go.

3speed

As in - It's a 27.2 now. I have a nice 26.8 I'd like to use, but it's probably bad to clamp it down tight enough to use the smaller size, right?

strock 

The problem with the smaller seat post is that clamping it down tightly deforms the seat tube, the apparent source of the OP's problem. It also puts you at somewhat greater risk of the seat post slipping down in the seat tube when you really don't want it to, although if you clamp the collar tightly enough, I suspect you'd be OK. If you really want to use the smaller seat post, I'd recommend using a shim. I don't think you can get a 26.8 to 27.2 shim (I'd be happy to be proved wrong), so you'd need to do the ol' beer can trick or something similar. Some people think it's a bit risky, but plenty of others swear by it.

Italuminium

to answer the original question in the most literal sense: when you let Italians do the measurements. My 27.2 campagnolo post perfectly fits my 1990 concorde, but a nice Aero Selcof post in the same nominal diameter doesn't.

3speed

[QUOTE=strock;12741764]The problem... so you'd need to do the ol' beer can trick.QUOTE]

Of course! Why didn't i think of that? Guess I can use this post afterall.

realestvin7

[QUOTE=3speed;12741816]What is it? You can send it to me. I have plenty of 27.2 posts.

3speed

It's a RaceFace Revolution in 26.8mm. If you've got one, or a quality post of similar design in 27.2, I'd be up for a trade. I really like the post, though, so if the shim works then I'd probably only want to trade for the same post in 27.2.

realestvin7

No thanks. It's not C&V proper. Haha.

smontanaro 

Thanks. The wedge trick worked. Couldn't get a wood wedge to hold in place, but an old screwdriver did...