Slightly out-of-round seat tube posting opening
 

I am setting up a bike -- an old Peugeot Prestige in 531! Nice! Except the seat tube post opening (at the seat post clamp) seems slightly out-of-round. The seat post doesn't slip in without force (which I have learned not to do.)

any DIY tricks for correcting this condition?
or is this a frame shop reaming job?

someone used the pinch bolt to distort the seat tube [blind guess]

if you can find a steel seatpost of the right size, [or a machined shim and a smaller post]

perhaps it can be cold set back..

but without data in thousandths of an inch [fractions of mm]
of difference of the oval axis , its a blind guess

if you choose to ream oversize the seatpost should be bought first, then slowly ream the ID
untill the post just fits. or it will be sloppy fit ..
and you go back a square on the game board.

I made my 27.0 tube road frame take a 27.2mm seat post with an adjustable reamer,
and 45 minutes of fussy labor.
so bore is also smooth.

An actual reaming job with the cutting of metal would be an absolute last case option. If the frame was ever built up at some point then it must have been successfullly assembled with a seat post. It's far more likely that someone cinched down a too small post and egg shaped the upper part where the relief cut is but the rest is OK. What you can do is figure out where the tight places are by offering up the end of the post to the frame and looking carefully. With those spots marked on a wrap of masking tape you put around the top of the tube use a one or two size smaller seat post or a close fitting but slightly loose pipe as a try tool to open up the clamping area back to round. To get good leverage but ensure proper support you want to insert it so the end goes in only about 1/2 to 3/4 inch past the end of the relief slot. Then pry the post to one side towards the tight spots as marked on the masking tape. The idea is to gently bend those areas back out to round. Obviously start with firm but limited pressure and use the proper size of post to check your progress often. If the steel flexes and springs back then increase the amount of pressure you're using until it bends back out to round. When you can insert the post to the end of the clamping slot with the usual effort then likely you'll find that the post slips in all the way without undue force being needed. Don't worry about going very slightly too far with this process. The steel will happily reform to a proper fit the first time you clamp the correct size post back in.

And keep in mind that a seat post is seldom a drop in situation. Many a bike has just enough distortion in the frame tubes that some slight twisting action is typically needed. But of course if your post goes to the end of the reformed and slightly enlarged clamping area and then needs more than a light but firm push or just plain jams again then perhaps your post is one size too big for the frame.

many thanks
I was surprised this opening was out of round as the bike arrived carefully packed and wrapped, with the seat and post carefully wrapped and positioned and tied to the frame.
the pinch bolt was loose and so I can't understand what might have happened between removing the seat post and it arriving here. (though the seat post is badly scratched and so that is a signal)
the answer now that I think about it is probably "nothing". the post would come out of an out of round slot more easily than it would go back in. I expect it was held open by the seatpost and then once it was removed it sprung (sprang? hmmm.) shut like a trap door.
I'll follow your suggestions and report.

Yeah, they most likely forced the post in there and it held due to the out-of-round clamp. BCrider's method of expanding the oval back into a circle is right on. You'll need to find a smaller-diameter post for the procedure, or a pipe of some sort.

Once you've expanded the top back to round, make sure the post fits snugly all the way down without rocking. It could be that the post the bike came with is too small and tightening the clamp-bolt caused it to distort.

this by the way is a Peugeot Prestige in Reynolds 531 circa.... late 70s?
so apparently that original seatpost was (per Sheldon Brown) a 26.4mm.
which my calipers basically confirm.
anyway, I did get a seat post back in there last night. she rides nicely. alas she feels a little small for me. but it was fun to get her on the road.

A dozen years later and thank you.
I just fixed the slightly out of round seatpost on, what else, a Peugeot. I wrapped the bender post in 400 grit sandpaper to get a close fit, and the end of the bender post was about even with the bottom of the tip of the seatpost lug, protecting the seat tube.

Before doing anything, carefully eyeball the area and let the frame guide you.

First of all understand that the lug/tube could not have been distorted with correct size post in place. So it's either transit damage, or the prior was using an undersized post, or it's only slightly under and will spring open to accept the post with a bit of force.

The best indicator is to look at the slot in back. If the top is narrowed it needs to be spread to parallel. OTOH if it is parallel, then odds are that it only wants a bit more force than you were using.

Whether it's to slightly spring it open, or to correct one that's pinched, the best place to spread is at the slot itself. One way might be to use a wedge or screwdriver, starting at the top and tapping it down to spread the slot.

If all seems OK but you don't want to mar your post, gently spring the ears out a bit to give a bit of breathing room.

A dozen years later and you are very welcome! Congrats on the fix.

And Lo It Came to Pass,
Once the Seat Tube was Rounde Agane,

The #$%&ing 26 mm Seatpost is Loosey #$%&ing Goosey Because Of Course It Is.