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I've bought an old second hand bike frame and I'm trying to build it up into a road bike. Things have been going quite well, but I've got a bit confused by the seat tube. At the top of the seat tube in the frame, there's the text "θ 261 E" (it might be 264 - the "1" is a little bashed up). I assumed that meant I needed a 26.1mm seat post, but they seem impossible to find.

I also used a pair of calipers to measure the inside of the tube and it seems to be about 25.5mm (presumably it might squeeze down to less).

Does anyone know what this marking means, and what size seat post I actually need?

Many thanks.

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    Get better calipers - there are a bunch of seat tube sizes, and 25.4mm (ie 1 inch) is a common older size. Theta is a common symbol for "internal diameter" so I think you're looking at a seat tube measurement that has been stamped on but got beat up a little.
    – Criggie ♦
    Commented Feb 7, 2018 at 10:10
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    Also note that measuring the hole in the frame is hard - its generally not round at the top where you can get to with calipers. Try and measure as far inside the frame as possible.
    – Criggie ♦
    Commented Feb 7, 2018 at 10:11
  • It's possible the 6 is actually a 5, as well as the 1 being a 4, so that would give you 25.4mm i.e. 1". Perfectly conceivable the seat tube internal diameter is 0.1mm oversize to admit a 25.4mm seat post.
    – Argenti Apparatus
    Commented Feb 7, 2018 at 14:24
  • 26.1mm seat posts are an old standard from before WWII but they can be found. Got one last year. But the E imho points towards an external 26.1mm for the seat-tube for the size of the clamp.
    – Carel
    Commented Feb 7, 2018 at 18:54
  • What is the (approximate) make/model/year of the frame? It may help folks on here narrow down the possibilities. (I was putting together an old Peugeot frame a while back; ended up having to order a custom 23.xx mm seat post. Le sacré français!)
    – sam
    Commented Feb 7, 2018 at 19:01

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Well, the seat tube is definitely 25.4" (thanks for the help, Criggie): I've bought one that seems to fit. I still don't know what the stamped text means, but I guess it doesn't matter.

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