Seatpost Won't Fit
 

I bought a 2001 Bianchi Pista frame the other day and am building it up with some of the parts I had on another bike. When trying to transfer the seatpost, I ran into some trouble.

My 27.2mm seatpost wouldn't fit. I checked the specs for this bike online, and Bianchi claims that it takes a 27.2mm (Reynolds 520 tubing TIG welded). I tried 2 other 27.2mm seatposts I have lying around and on other bikes, and none of the 3 worked.

Any ideas?

If you have one handy or can borrow one you could try 27.0 but a 27.2 post should fit that frame.

It is possible that someone else has distorted the shape of the seat tube somehow, possibly by trying to tighten it down onto an undersized post. If this is the case and you try to use another undersized post you may further damage it by clamping and also the post will not be properly supported below the clamp area. I am uncertain what to suggest if this is the case, but neither trying to force the correct size post in, or removing material by reaming is the right approach, IMO. Perhaps consultation with a frame expert would be the prudent course.

If you have a good set of calipers, measure the ID of the seat lug area at different angles. this can tell you if the seat tube near the clamp is ovalized. Some folks have used a large diameter plumb-bob (metal) to insert and tap to get the hole round. Some have inserted a large, flat screwdriver or small pry bar into the rear slot to spread the top of the seat tube at the clamp and this can also bend back out the clamp rings which may no longer be flush. This can be a tricky operation if the metal is already fatigued and cracked. You could ruin a seat lug, so it isn't without risk. When the sides are too narrow and the rear is elongated, I have taken a hammer to the clamp with sacrificial 6mm binder bolt inserted and nutted and used a flat, dull chisel to strike the pin to flatten the rear seat lug area a little to get it back into round with success. But these were often with TIG welded, cheap steel frames with large welded or brazed surface area where clamp rings attach to the seat lug or were cast as part of the lug. And NOT a fancy lugged road frame that takes a Campy style 19 - 22 mm seat post binder bolt. If the frame is really nice, I will prep the area, heat it and bend it, then quench it, and ream it and then repaint it.

Mind you, maybe the specs are wrong. If the measurements on the caliper and consistent and the hole is round and it takes a 26.8 or other seat post and the clamp rims don't seemed overly bent, then go with the smaller seat post.

if the slot behind the seat tube tapers in at the top, its been messed with,
in the last 11 years, by prior mechanics fitting an undersized seatpost, perhaps..

straight gage seat tube, wall thickness is constant, then it may be a 26.8.

27.2 is commonly fitted in a single butted tube wall thickness less at the top,
more at the bottom .. main tubes DB, so thinner wall in the center.

OD remains the same.

+1

Check the slot at the back of the seat tube. It should be consistent in width along its length. Sounds like the classic case of someone crushing the tube to fit a smaller (wrong) sized post.

I had to open the ears of my Gary Fisher's seat tube quite a bit to fit the proper 26.6 post in. Some idiot had clamped it down on a 25-something post.

The slot is an even width for the length of it. I'm going to take the frame to my LBS and have them measure the ID of the tube and make sure it's even, etc because I don't have the tools for that.

I'm able to get a seatpost in a few inches, but then the resistance becomes too much and I can't get it the rest of the way in. It's so tight that, when I was trying to push it in, it literally shaved off the outer anodizing on one of my black seatposts...

Bike shop.. G0 .. seek live , not virtual help.

https://www.youtube.com/watch?v=dV5HuShISa8

check out what's used to hold the seat to the frame

Some have inserted a large, flat screwdriver or small pry bar into the rear slot to spread the top of the seat tube at the clamp and this can also bend back out the clamp rings which may no longer be flush. This can be a tricky operation if the metal is already fatigued and cracked. You could ruin a seat lug, so it isn't without risk. When the sides are too narrow and the rear is elongated, I have taken a hammer to the clamp with sacrificial 6mm binder bolt inserted and nutted and used a flat, dull chisel to strike the pin to flatten the rear seat lug area a little to get it back into round with success.

Check to see if round not oval ....if oval use 2 screwdrivers inserted into the slot at the top tube and "gently" open up the tube. 1 driver will often times close the opposite side of the tube causing more distortion. open a little and check....open and check...dot try to get it all the first time.
JM2C's :thumb:
Ben

Thread is 4 years old.

Brains...

If the post will insert part way, odds are that it's the right size. However there can be tolerance issues, or most likely with a welded frame some distortion that will call for some limited reaming to correct.

Yes old post but comments about the fix still relevant. JM2C's Ben

Indeed since I just found this on a google search and it will help me with the same problem. I am about 99.9% sure my frame uses a 27.2 post but it won't go in.

exactly...this is why answering old posts with additional information is relevant.
good luck with your fix.
Regards, Ben :thumb:

A good bike shop may be able to straighten out the frame tube circularity...

I've even used their adjustable Reamer to shave out 0.2mm of ID and put a 27.2 post in a 27.0 frame.

Making the frame precisely fit the post In Hand.

Yes, this is a common occurrence. The top tube is fixed at both ends, and expands when heated. Steel at brazing or welding temperatures is soft, and the expanding top tube can easily distort it. This also can happen at the head tube end, which is one reason why head tubes are reamed and faced before installing the headset. Most builders will run a reamer through the seat tube to fix this issue. The OP's LBS may have one to do the job.

wait you figure this out yet. I’m trying to get my seat post in my 2001 pista. (It 7 years later)

this thread started at ...... 03-22-12, 07:47 AM :innocent: