I'm currently rebuilding an old steel frame. After measuring it I was sure it'd take a 27.2mm seat post (see picture) so that's what i bought. Sadly it doesn't fit (despite some "encouragement" with a mallet). So I'm now in the position of needing to buy another.
Anyone got any suggestions on what size I should go for. You can see from the picture how it's measuring and I don't want to go wrong again.....
Usually at the "Cut out" in the top of the seat tube, they measure a little bigger than the actual size of the post. I'd say your best bet is to try several sizes at your Local Bike Shop, there even used to be a tool to measure it, but it's rather hard to find these days. If I had to guess, it's probably a 27.0 or 26.8 just judging by your measurement.
The seat clamp is stretched a little so your measurement is off.
You can use a vernier caliper to measure a bit further down inside the seat post. Definitely a tool to buy if you can. Even cheap plastic ones are handy.
Another rough estimate is to use a flexible tape measure around the seat tube, to get the outside circumference. Then divide that measure by Pi to get the diameter. Then subtract 2 x the wall thickness (which looks like about 1.2mm)
I have a bunch of old short seat posts to help figure out how big a seat tube is inside.