Andy_K 

The seatpost that was on my '84 Pinarello when I bought it is a 27.2 Superbe Pro. It took a lot of work to get it out and a scratched it up pretty badly in the process. Installing another 27.2 seatpost was similarly difficult. It didn't want to go in and once it was in it didn't want to come out. Once it's in a bit it will slide several inches at a time if I put any body weight on it (e.g. leaning my torso on the saddle). I pulled the seatpost and measured the hole at 27.18mm. I believe that this is a Columbus SL frame so I think 27.2 should be the right size, but I'm open to the idea that I'm wrong about this.

Does this sound more like I'm trying to squeeze a 27.2 seatpost in a 27.0 frame or like the frame is pinched at the collar (possibly from someone having tightened it down on a 27.0 seatpost in the past)?

3speedslow

Could be. A good honing of the seat tube might clean some gunk on the walls out plus take possible burr down. When trying these posts, did you grease them ?

USAZorro

You wouldn't be able to slide a 27.2 post into a 27.0 tube.

Either the seat tube is deformed from over-tightening, or honed to a wider opening, or it is bigger than 27.2.

Andy_K 

Yes, I greased the seatposts. I also scrubbed around the top of the seat tube with a wire brush. I don't feel anything protruding there.

Spaghetti Legs 

I've had that issue on a bike or two and a little cleaning of the seat tube and some grease on the post does the trick. Columbus SP has a 27.0 and my one bike with that measured right at 27.0.

Andy_K 

OK, so I guess I'll scrub at it a bit more the next time I pull the seatpost. I just wanted to make sure that I wasn't going to damage the frame by riding it with the wrong size seatpost.

eschlwc

the problem is a very common one from my experience.

usually, by the time i polish the seatpost (and therefore take a tiny bit of diameter off it), it slides in better and snugs up well with the binder bolt secured.

i'm unsure if the above occurs with the binder bolt secured. if so, that's certainly a problem. if not, a little seatpost polishing is all you'll need.

Andy_K 

To be honest, I discovered that accidentally by trying to sit on it with the binder bolt not secured.

But, no, with the binder bolt secured it works just fine other than seriously discouraging me from making minor adjustments.

satbuilder 

You could also use some 320 grit paper after you clean it. Make sure everything around the slot in the seat lug is smooth.
Do you have a set of calipers? You could take several measurements of the inside diameter in several locations. That should tell you a lot.

DIMcyclist

Indeed. The ST on my Torelli is spec'd at 27.0, and- measured- actually IS 27.0. However, a Nitto 27.0 seatpost was such a ridiculously tight fit that (rather than damage either the post or the ST) I swapped it out for a Miche 26.8 Aero: it fit quite snugly & works like a charm.

verktyg 

Andy_K - Everything you ever wanted or needed to know about seat tubes & seatpost fits...

https://www.bikeforums.net/classic-vi...l#post18086687

https://www.bikeforums.net/classic-vi...l#post18088452

One other thing (mentioned in the above links), when reinstalling the seatpost put a small glop of grease in the seat tube and spread it around. Do the same with the lower part of the seatpost. NO BRAINER!!! It saves lots of hassles in the future and allows for easy readjustment.

Same thing works for stems!

verktyg

Chas.

The Golden Boy 

My Voyageur SP has a 27.2 seat post. It came with an SR Four 'Sir 27.2 marked seatpost. I can't get any other 27.2 seat post in that thing. Not that I've tried very hard-

I thought I was misremembering seeing the 27.2 on the seatpost- but, no- it's there. I'm quite OK with that seatpost anyway...

Scooper

What model '85 Pinarello is it?

The only '85 model that didn't use SL or SLX tubing was the Catena Lusso which used Aelle R tubing and took a 26.8mm seatpost. All the other models (Montello 85 SLX, Super Record Special, Treviso N.R., Treviso Victory, and Triathlon Victory) took 27.2mm seatposts.

Surface rust inside the seat tube can make it damn near impossible to get the proper size post inserted. Every few years, I use a Cyclus seat tube reamer on the seat tubes of my frames that take a 27.2mm seatpost to clean up the internal surface rust and ensure the seat tube is round.

Andy_K 

It's an '84 Gran Turismo -- just scraps of the Columbus decal left, but it seems to indicate Columbus SL and that's what T-Mar said it is.

It really doesn't have much sign of rust anywhere on it, but I suppose it wouldn't take much. I'll have a closer look at some point. The seat post is in there at the right height now so I'm not especially motivated to remove it.

gaucho777 

I don't have a brake cylinder hone or a seat tube reamer. However, I did have some scrap wood and various types of steel wool when I can across this problem. My DIY reamer: I took a long piece of ~3/4"x3/4" wood, cut out a piece of medium/coarse steel wool with wire cutters, and screwed on the steel wool to then end with a washer and a wood screw. It worked well to clear off some gunk and perhaps some rust spots. I then took some 400 grit sand paper and smoothed out the edges at the top of the seat tube to make sure there were no scratch-inducing burrs. Best to do it without the BB installed or with something (rag, paper towell) stuffed between the BB and the top of the seat tube so no debris falls into the BB area. That said, based on the tightness you describe, it sounds like you may also have a slightly deformed seat tube as described in verktyg's informative post above.

Andy_K 

I missed this earlier. Those are very informative posts! Thank you.

The slot on my seat tube is narrower at the top (almost touching) than at the bottom, so I guess that is pinched. I may take it to a shop for this if I can't fix it with a little scrubbing and some gentle prying.

Lascauxcaveman 

I've honed more than one seat tube with a piece of sandpaper wrapped around a wooden dowel. Starting with probably 100 grit then working down to 330 or 420, wetsanding. since you only need to get down in the seat post 4 or 6 inches, it goes pretty fast. Followed up by a good swabbing out all the way down with wd-40, then greasing the seat tube and the post before re-inserting.

I de-ovalized one seat clamp area that had been pinched down on a too-small post. Did that with a custom hardwood dowel I shaped down on my bench belt sander. Bang it in, wiggle it out, rotate a bit, bang it in again, etc. Got it nice and round and just a tiny bit too large at the top; a problem that remedied itself the first time clamped in the proper size seat post. Worth the effort.

verktyg 

Mssr. Homme des cavernes,

Do you chip flint tools too?


The wood dowel is a neat solution... Whey didn't I think of that?


verktyg

Chas.

rhm

Just as you wouldn't necessarily believe everything you read in every bike forums post, you shouldn't believe everything you read on every seat post. I had a 26.8 mm SR Laprade post on my Claud Butler, and when I went to replace it with a 26.8 Zeus post, the Zeus post wouldn't fit. Not even a little bit. So I put the Zeus post on my Lambert instead.

verktyg 

Agreed, I've found a lot of seatposts, even Campys that were a) not round, b) had tapered or inconsistent diameters, and/or 3) undersized or oversized.

verktyg

Chas.

rootboy 

One of the best things I ever did in this regard was acquire a tool with which to measure the tube down BELOW the seat cluster/lug.
Readings at or near the opening can be misleading.

smontanaro 

I had a PX-10 frame over to my buddy with all the tools (Billy) over the weekend. I have a 26.4 Simplex seatpost for it. The seat tube was slightly ovalized as well, probably from some past owner using a too small seatpost at some point. After a bunch of work with his reamer, it almost fit. (We had to take a break as both had places to be, so didn't quite finish.)

Billy said in the bike boom era, some frame prep we take for granted (reaming seat tubes, facing bottom bracket shells, etc) often just wasn't done. After all, they were in a hurry. The Americans wanted more bikes. Billy's speculation about seatposts was that they probably had an assortment of different sizes sitting around and just grabbed the first one which fit. So, if your nominal 27.2 Columbus SL seat tube was either warped slightly by the brazing process or installed upside down (with the butt at the top end), when built, the bike might have shipped with a 27.0 or 26.8 seatpost. My 1970 Atala Super Pro (full chrome frame) had clearly never had the bottom bracket shell faced, as the chrome extended over the edge of the shell, and it was lumpy. Had clearly never been cut.

I think it just comes with the territory.