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Um, I think you might have misinterpreted the chart.
Some seat tubes are externally butted - that way you can get the thickness you need around the seat - top tube joint for strength, and the seat post itself is supported via contact to the inner diameter for its full length. Best way to verify is to measure the ID of the tube.
You don't want to go too close to the transition zone on the butting in case you compromise the strength. That's why they give you a 245mm plain wall thickness to work with, you can change the seat tube length by up to 165mm without breaching their guidelines.
The red circle just lets you know which end is which - on an externally butted tube you can also tell because you can actually feel the transition on the external butt.
Mikenetic-
You don't want to go too close to the transition zone on the butting in case you compromise the strength. That's why they give you a 245mm plain wall thickness to work with, you can change the seat tube length by up to 165mm without breaching their guidelines.
Yeah that’s pretty much what I was saying. I need to leave 80mm of the plain gauge bit for strength.
I think I might need to order a different seattube for this build, I want to do a klunker style double top tube so I’d be better with a tube that isn’t externally butted.
M_V
Yup, dunno why I didn't think about that!
Goes in the painted red end.
Ceeway catalogue has this diagram for it
I think the saw symbol and the '165' means there is a 'trimming' area of 165mm but I can't see what that relates to in terms of the butting? It seems like there's 245mm of the thicker wall at the bottom and 40mm of taper. I'd have thought that meant pretty much 245mm of safe trimming area no? Should you not be heating the tube that close to the taper/thin wall bit? You want to leave like minimum 80mm of that thicker wall section in place?