OTOH, 27.2 minus 26.4. is 0.8mm difference.. a hand reamer, used in the seat tube,
can shave out the 0.4mm of tube wall thickness from the seat tube ID
ro fit a 27, 2 seatpost into..

seat tube if 28,6 diameter outside , is having a 1.1mm tube wall , leaves 0.7 mm tube wall
reinforced with the seat post itself..

this is the common wall thickness of butted seat tubes, butt being the thicker, bottom end
where its brazed/welded onto the BB shell..

you have a frame using un butted seat tubing.. same top to bottom..


years ago I did a lesser amount of cutting, reamed from 27.0 to 27.2,
and I had the seat post in hand, stopped cutting as soon as it fit.

as a result its the best fitting of the fleet..






....

I have a 26.4mm Sakae Ringyo MTE-100 post. PM if interested and we can work something out.

The Velo Orange seat tube is ovaled on the inside to reduce weight.

You would be asking for a catastrophic failure if you took 0.8mm off of the OD!

And, with an 0.5mm shim sleeve you can use a 1" seatpost..

+1 The post! (and my immediate first thought when I saw this thread.) Excellent post but it does have drawbacks. There is a lot of it and it is not light. Saddlebags will sit very funky under it. I took off the quick-release on min and replaced it with a simple bolt (and I think a large washer but I would have to go out and look). I have that post on one bike and two TiCycles posts with almost the full setback of that post but not adjustable. (The TiCycle posts are a bunch lighter and likewise sexier. Also have the advantage of dropping wallet weight considerably. They can make you one of any setback you want or you can keep you home equity intact and contact John.

Avocet made posts of much less setback, but more than most. Stay clear! They break. (I've had mid-ride failures 3 times on the two post I've owned. (Edit: those are probably all 27.2 and not a possibility for you anyway.)

Ben

I have an old, 1991, steel frame bike that takes a 26.4 seat post and have been looking for another. The original snapped off just below the seat clamp 2 years ago. I have been using an Uno and it is heavy and does not look good on my bike. It is hard to find a quality 26.4 seat post. I have been looking for quite awhile on alot of different sites. I just purchased a low cost 25.4 carbon fiber post off e-bay and mounted it in the seat tube using shims and electric tape. It took a couple of tries to get it set up in a way that it would fit into the tube and stay put with the seat tube clamp tightened down. At this point I am not sure if I would do that change again. If I was to do it again I would try to get a post closer in size, like a 25.8 or 26.0/26.2.

I had one of these turned down by a machinist so I could use it on our old Gitane tandem.

Thanks Everyone!! I appreciate all the replies!

The Ticycle seatpost are crazy expensive!! I know you warned me but WOW!! I won't be heading down that avenue anytime soon, as a new frame would be less expensive.

I'm gonna try a few other options first and then maybe PM John!

Another possibility:
https://www.amazon.com/Kalloy-26-4-X...2JL&ref=plSrch

GT makes a chromoly one that is curved for setback with a very secure micro adjusting head. This come up fairly often on eBay for ~ 35$ or less. They are heavy but if durability and adjustability trump light weight - this one would be a good choice.

I formed this lay-back seat-post at work, for my Hercules, but it means using a candle type seat clamp...

By GT are you meaning a BMX seat post, like the one shown in the picture below? If so how would that translate over to an 80's road bike? I'm also thinking that maybe a double bolted Campagnolo seat post may have a bit more setback than my current seat post so may look at picking up a used one of those.