I've just bought Graham Weigh road/race bike which has a rare Reynolds 731os Race tubing frame. Although the ITM seat post wasn't seized, I've just had to clamp the stem in a vice, and gradually use the frame to twist the stem out of the frame. The stem measures 27.2mm, I'm not sure if its original, but as it was so tight I have my doubts. To save having the same problem again, would it be better to use a 27mm stem?

the old stem was probably hard to remove due to corrosion, not because it was the wrong size. if it fit in there, that's the right size. start by cleaning out the seat tube, make sure to remove any old crud and rust. then get a new 27.2 seatpost and make sure to apply grease to the seatpost and inside of the seat tube.

if you use a smaller seatpost, it will be prone to shifting under weight and can also damage the seatpost clamp.

Lots of 72.2 ID bored frames out there, it's a common size.

you can get a ball burnisher, its looking like a grape cluster.

chuck it up in your drill,

that will clean and smooth the inside of the frame tube.

Grease your seat post, before inserting it, and remove it
and re apply a thin wipe of grease occasionally,
to keep it from seizing in place in the future..

Clean out the seat tube, as Catnap suggested. Now, your ITM post, is it all scratched up? If so, smooth it out with fine sandpaper, like 600 or 1000 grit. Wet sand the whole thing. You can sand for a long time before you get it down to 27.0, but you don't need to. Dry it off, grease it up, and see how it fits.

Thanks for the advice, good point about cleaning out the seat tube. Unfortunately, the stem was black coated alloy and now scuffed and scraped, probably one for the bin but at least I know another 27.2 is the right choice, thanks.

If you're going to trash the post anyway, then *definitely* hit it with sandpaper, smooth it down, and make sure it fits before you buy a new 27.2.

KeS

There are lots of ways to remove a black coating. If it's anodized, then oven cleaner works well. I usually use Drano drain cleaner (sodium hydroxide, also known as lye), but that can be a bit dangerous. If you do that, and then a little sanding, your ITM post will probably fit just fine.

Its worth the time to clean out the tube - Using the right diameter seat post is a must - Again take the time to get it right...