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Looking at a frame on ebay to build up a winter hack.
The seller says it needs a 27.9 seatpost. Does such a thing exist?
The seller doesn't know what he or she is doing.
If it's a modern frame it's most likely 27.2.
The seller says it needs a 27.9 seatpost
My guess is that's the seat collar size (i.e. the diameter of the exterior of the seat tube). It'll be a 27.2 seatpost.
My guess is that’s the seat collar size (i.e. the diameter of the exterior of the seat tube). It’ll be a 27.2 seatpost
The walls can't be that thin, if the frame takes a seperate seat collar it's usually 30.0 or 30.2 for a 27.2 seatpost.
It's probably slightly expanded at the top and/or the seller can't use a calliper. Both a pretty common. 😀
Is it Planet X?
Better to google the frame specs and go with that or ask if they have a post they want to include or tell you the one they have been using
Better to google the frame specs and go with that or ask if they have a post they want to include or tell you the one they have been using
I'd go with looking it up, even if you do get a post or advice from the seller.  I scooped up an early 90's mtb as a hack bike a few years ago. The seatpost head and bolts were corroded, so I bought a new one to match the old one. Turned out the old one was the wrong size (oxidation made it fit!) and I had to shim the new one with a bit of Tango can (steel is real).
If you do need an odd size look at BBB. Cheap and loads of sizes: 25.0, 25.4, 25.6, 25.8, 26.0, 26.2, 26.4, 26.6, 26.8, 27.0, 27.2, 28.6, 28.8, 29.0, 29.2, 29.4, 29.6, 29.8, 30.0, 30.2, 30.4, 30.6, 30.8, 30.9, 31.0, 31.2, 31.4, 31.6, 32.0 and 34.9
Or get a 27.2 and shim it, then you won't have an almost brand new seatpost that's a size no-one wants when the frame snaps a few months later. for example.
Edit: nothing between 27.2 and 28.8, though! Looks like 27.2 and a drinks can ftw. Or just the 27.2 if the seller's wrong!