plainview

I recently took possession of this 1990 Corsa Extra and was disappointed to find that a too-small seat post had obviously been installed prior. The opening measures right a 26.8 currently, and I know this bike should take a 27.2.

I imagine this can be re-expended to it's original dimension, but I'm not sure of the best method to do that (I'm not going to pry it with a screwdriver). I'm concerned it may not re-expand evenly on each side, causing it to always bind/scrape on one side when intalling/removing a post. I'm also concerned that the seat binder tabs may actually be bent, realiazing that it would take considerable torque on the binder bolt to get this frame to secure a too-small seat post. Opinions?

3speedslow

If you are not comfortable doing this yourself, do you have a qualified LBS close by.

Scooper

Find a local bike shop with an adjustable reamer.

smontanaro 

Reaming will just remove metal. You need to open it back up and preserve the metal you have. A split one-inch oak dowel wedge tool should do the trick. Start with a one-inch piece of oak dowel, about 12 inches long. Cut it lengthwise, but at an angle. You wind up with two wedges. Lubricate them, put in a seatpost bolt loosely, slide in one wedge, fat end down, maybe halfway in (hang onto the thin end!), then follow with the other wedge, fat side up. Slide it down until the two meet and are snug in the tube. Tap with a hammer to open things up a bit. Lather, rinse, repeat. Rotate the wedges to open up in different directions.

CroMo Mike 

You don't want to ream it, you want to expand it. Trouble with exhaust pipe expanders is that they are too big - starting at 1-1/8". If it were mine I'd go to the lathe and make up a tapered metal plug that starts at what size the hole is now and tapers to what size I want it to be, plus a fuzz more. I'd also drill and thread a hole in the end of the plug so I could yank it out again with a slap-hammer once I bump it in. With some good anti-seize or grease, of course.

plainview

Yikes - I definitely don't want to ream it out to 27.2 as it stands now, unless your thought is to use the reamer as a wedge rather than a cutting tool? Although even that would likely create some scoring at the sharp edges.

As for having a LBS do the work, I had the most well-known shop in Portland botch a headtube facing job on a new frame due to a facing tool that was too dull (requiring over-tightening it to get it to cut, resulting in a rough/uneven cut). So I'm really leary about having shops touch my bikes anymore. I worked in a shop years ago, and we would just pry this open with a screwdriver wrapped in something to try to prevent gouges. There has to be a better, more controlled method.

plainview

I think you've got the right idea, although it is probably going to be cost prohibitive for me to have a machine shop do that, considering I already paid a premium for the frame. I think I'll likely return it before getting to that point, although finding a decent Merckx in my size and desired vintage has been a challenge...

Dave Cutter

Interesting. I would have just removed the bolt and used an extra large screwdriver to re-expand the crushed expansion slot. I wonder what a LBS would do.

MetinUz

This is what I would do as well, use a large screwdriver, then check for roundness using a disposable seatpost. May need to follow up with a reamer.

DiabloScott

Yeah - screwdriver in the slot --- a sturdy one. but I like @Smonatanaro's idea too.

exmechanic89

It never ceases to amaze me how someone will take a high-end frame like that and force a wrong size seatpost into it instead fo paying just a few dollars to buy something that will fit..

IcecreamltDan

I'd have a go at it with smontanaro's method. Sounds like a good plan to me.

redneckwes

If this was a gas-pipe Raleigh or Peugeot I would carefully work it open with a wood dowel and a large screwdriver.

This being quite a bit higher end than my usual victim I'll defer to more experienced opinions.

plainview

I didn't see this response at first. I like your thinking here, but in practice I feel like the device will take the path of least resistence and slide further down the tube as a unit rather than exert the lateral force needed to expand the seat tube. I can't imagine being able to hold on to the skinny end to the extent needed to overcome that.

This is similar to the idea of using a quill stem as an expander (two inclined planes). I'd feel more comfortable with the stem idea if the diameter matched the seat tube better.

72Paramount

I would take a triangular wedge put it at the top of the slot and gently hammer down until square maybe one tap beyond.

xiaoman1 

Good points about screwdrivers, I always use 2 screw drivers slipped both into the slot and use one to pry against the other.....using only one can deform the opposite side when prying against it.
There is also a tool used to check the dia. of a seat tube for sizing that is similar to ones used by a jeweler to check ring sizes (tapered)....you might see if your LBS has one and use it to open the tube that way.
Just don't jam it in all at once to open up the tube because you may run the risk of the "stuck" seat post syndrome.....it also has markings on it that indicated dia. 26.2 etc. etc.
JM2C's Ben

Bikerider007

I might try something wood like a broom handle and move up to something like a shovel or other yard handle. Grease it, put a rag on it or something so as not to rub or scratch the paint. If you are in cold weather I would put a hair dryer on it, seems like that might make it a little more malleable. Just my .02, don't blame me if you break it though.

If you do the screwdriver thing, wrap the tip with something rubber or at least cloth.

If you feel it is being forced too much and not happening, then take it elsewhere but these are made to expand and close to a point.

jeirvine 

I like the oak idea, but I would start with a one inch dowel (25.4 mm), split exactly in half. Then drive a wedge of some sort into the slot in the wood. A large flat head screwdriver might be a good wedge. Using a screwdriver in the seat tube slot would mar the paint, if it's not already marred. Please report back once you proceed with something.

thinktubes 

Maybe a ring sizing mandrel would do the trick? Can be had for less than $10.

Salamandrine 

For best results, wedge it open using one of the methods described, and ream it out.

The reamer is just to fix the slight distortion that will be there if the seat tube has been squished in and opened up again. You are just chasing it, only a tiny amount of metal should be removed. If you don't do this, you won't get a nice round tight fit, and the seatpost won't clamp properly, and it will get scratched up too.

So yeah, take it to the LBS or buy the tool. Unfortunately, this is a PITA repair, one I always dreaded.

smontanaro 

From John Hurley, over on CR:

https://www.flickr.com/photos/7665899@N05/880185603/

I was describing John's scheme from memory, because I couldn't find it at first. He said he also posted it to the old CR in 2007, which I've yet to find in the search.bikelist.org archives. If you are a current member of Classic Rendezvous, his message from last February us in Google Groups:

https://groups.google.com/forum/m/#!searchin/classic-rendezvous-lightweight-vintage-bicycles/Seat$20tube$20clamp$20area/classic-rendezvous-lightweight-vintage-bicycles/_Ra4rXtbOP8

His message is #6 in the thread.

Scooper

I guess I need to clarify my suggestion to use a variable reamer.

If the seat tube is supposed to accommodate a 27.2mm seatpost, it is likely to have 0.6mm walls (28.6mm - 2 x 0.6 = 27.4mm). The 0.2mm difference between the I.D. of the seat tube and the O.D. of the seatpost is to allow for differences in manufacturing tolerances and distortion of the seat tube from torch heat while brazing the seat cluster. Seat tube reaming is usually done after a frame is brazed in order to mitigate the distortion and ensure the inside of the seat tube is round and provides clearance for the seatpost.

Since some hamfisted clown tightened the binder bolt to snugly fit a 26.8mm seatpost, a variable reamer beginning with reaming the seat tube to 26.8mm and gradually expanding to 27.2mm in the non-cutting counter-clockwise direction would restore the seat tube to its original I.D. and roundness without removing much - if any - material.

It should be possible, since you're restoring the I.D. to its original size, to do this twisting the reamer counter-clockwise (IOW, not rotating the reamer blades in the cutting direction, but in the blunt direction to essentially cold set the seat tube back to its original diameter).

dazevedo

I had that issue on a frame once. Went to the co-op we have and used a seatpost sizing rod and a rubber mallet. Just put a piece of tape at the point you need to stop and grease up the rod and hammer it to size.
[h=1][/h]

dddd

Definitely it is a matter of cold-setting the clamp to a wider opening.

I normally do this by pushing in a near proper-sized, thickwalled seatpost, using one that has enough of a taper or radius at the end if needed to center up and slide in without damaging paint.

I put the post in about 1" deep only, then simply tilt the post rearward just forcefully enough to spread the ears back to where they need to be.

The bolt gets removed first of course, and within a few tries the post is sliding through normally.

If the clamp has any tendency to bind or dig into the post after the bolt has subsequently been tightened and then removed, only then will I use a half-round file to remove the burred edges inside of the lug, to either side of the slot.

Some of the chromed steel seatposts slide into a damaged clamp opening most easily, and it isn't really necessary for the post used for restoring the opening to be an exact size match.

A longer post makes the job easier and more controllable!

plainview

You guys have armed me with some excellent ideas. dazevedo's and dddd's ideas appeal to me the most in terms of practicality and controllability. It might take a combination of both. I'm also intrigued by Scooper's reverse reamer idea, but I'm not inclined to invest in a reamer, and I wouldn't trust a shop to execute that move (Perhaps a co-op will have one. Havn't tried the co-op thing).

Salamandrine's point of needing a final reamer touch-up to restore factory insertion smoothness is also well taken.

The seller has given me a window of a few weeks to explore the repair and decide. I'm going to sit on this a bit and see if I get super-lucky and find another early 90's Merckx that isn't damaged (my dream bike is actually a Century, but they seem more rare and I know I'm late to the party). If this one were otherwise pristine it would be an easy decision, but it has a number of paint touch-ups that are kind of sloppy (at least it's rust and dent-free). I wouldn't try to return it after attempting the repair, regardless of outcome ("you tweak it, you keep it").

If I decide to keep it and attempt the repair, I'll first check with the seller to see if he'll do a partial refund. It only seems fair, as this damage was not disclosed in the ebay ad (and the seller said he owned the frame for seven years).

Thanks again for all the help - what a great forum!