I have a zero offset Origin8 2-bolt seatpost, nominal diameter 25.8mm, measured diameter 28mm. It works, but the saddle is slammed as far back as the rails will allow. An offset post would probably be better. I've fiddled with seat angle, and the last post that I had was a 1-bolt offset that none of the notches were the right angle. I think I need 20/25mm offset, infinite adjust, 25.8mm seatpost, but that's not turning up anything. What should I be looking for?

How is it both 25.8 and 28.0?

I was confused by that too, until I just realized that I was misreading my dial caliper. It's measuring 1.010" to 1.019", or 25.65mm to 25.88mm. 25.8mm is almost certainly right.

First decide what size it really is, because 0.2 mm is a big difference with seatpins and 25.8 is weird - did you mean 26.8 or maybe 28.6 mm? Those are sizes I've heard of. If it's an uncommon size you might want to consider using a smaller common size like 25.4 or 27.2 with a shim, for wider choice. I like these they adjust easily with no notchiness and don't move once they're tightened, also a decent amount of layback (I usually slam my saddles back).

P.S. I just checked, that style seatpost is available in 25.8 mm like here

That's helpful, now I need to find out where that's available in the US. The initial confusion on diameter was solely due to my error in reading a dial caliper. The 0.2mm variation is probably due to the caliper being worn out, it will move that much mostly depending on how tight I squeeze it.

When I did a search they were all micro index like an Uno. Which you might consider trying.

The one from England is a good find.

The micro index is, I'm fairly certain, the same adjustability as I had trouble with. One notch puts too much pressure in front, and the other I'm sliding off the nose. I'm not sure why it would apparently work fine for a lot of people but not for me, but I've fine tuned the 2-bolt to the point that 1/2 a revolution of the screws in either direction is a problem.

https://www.modernbike.com/seatposts+diameteris25.6-

and there is a really high probability that the actual post size is 25.4mm... measure the seat tube again, with a better caliper or micrometer. ignore the bad caliper, and look for an uneven slot in the frame tube...
i also measure in the=ree directions on the OUTSIDE of the frame tube, at two different heights... near top, and about 4 inches down from the top. this can expose an Over-Clamp situation.

there are 8 or 9 bikes listed on Sheldon Brown's Database of seat post sizes that used a 25.8mm seat post... none used a 25.6mm post.
three of the 25.8 posts fit old/mid school schwinns, AKA: pre-takeover.schwinns.
a couple fujis, a miyata, an "Ancient" Iron Horse, and a diamond back were also list... and one trek MIGHT have used a 25.8 post....
look up your bike.. if it's an older one.
M-Z .. there is also A-L...
https://sheldonbrown.com/seatpost-sizes-m-z.html#m
someone may have sanded the frame tube to death trying to get out rust, or the top part is flared from whatever, and a larger post got stuffed in...
25.4mm is the second most common seat post size.... behind the super-common road bike post of 27.2mm
the next occasionally used size, above 25.4mm, typically found, is a 26.0mm.. 26.2-.4-.6-.8 are also seen around here... i've never seen a 25.6 or 25.8, ever.

I'll check the outer tubing dimensions this evening. The bike is a roughly 1972 Jupiter Beat Super Deluxe, which is not on that list. Every reference I can find to that brand and model mentions hexagonal tubing, but this has all round tubing. Paint and decals, except for the checkerboard stripes, are identical to this one. As far as I can tell, I think all the original components match that one as well.

Edit: I don't have a better caliper handy. At the bottom of the lug, the R-L measurement is 1.114", about 3" below that at the top of the decal it measures 1.128".

On closer inspection, that slot does look a bit uneven. I bet it is supposed to have a bigger seatpost.

The name micro notch implies finer gradiations per click, which might be fine enough compared to whatever you had in the past. Forging and castings are more accurate now than they were in the 80s.

Most bike shops have the Uno so you could compare the mechanism to you old post.

Juniper Beat, wow! How much does it weigh? Does it still have all the original steel?

Take a pic of it let’s see the icon of ‘72

Original frame, fork , and headset, and the front cable stop. Everything else is replaced. 28.5 lbs as pictured, built up with 3x9 mostly Shimano 105.

1mm change at the slot equals .3mm diameter change.....

a machinist' telescope gauge would be a handy tool for you, about now.... measuring a bit below the slot.

the thing i've run into while resizing seat tubes is that the slot only wants to bend in such a way that the spreading isn't evenly distributed in the diameter, and just partially flattens/straightens the tubing near the slot... i've had some better results by leaving the pinch bolt in place, but kinda loose, then spreading the slot some, then using a 1 inch or so tubing in the frame to pry and work the diameter more, etc... techniques can vary from material to material. just don't get in a huury or greedy with the effort... clamp ears have a nasty reputation for cracking!
on some frames, the material was very stretchy, so i over-bent the slot, then used vise grips to re-close the ears to a better place... they had been WAY over-pinched before i began repairing....

28.5 lb. puts that bike in the decent MTB weight class... very rideable!

I bet my brother has better tools for measuring. The slot at the top of the lug measures 0.088", and 0.120" at the bottom, which would add 0.2mm to the diameter if you assume the 0.088" is 25.8mm.

The bike is definitely slower than my 20.5 lb road bike, but much more flexible. This is the one I'll be riding for Gravel Nationals, aiming to finish better than last place in 35-39 men.

Looking at the picture of the bike, all signs point to it being a bottom-end bike from a Japanese factory: the laid-back frame angles (indicating that the bike was built using the jigs for the factory's one-speed and three-speed frames, which would have been the bulk of their business), the square-cut lugs, the need for a claw for the rear derailleur, etc.

The seat post size therefore is almost certainly 25.4.

Low-end Japanese bike is almost certainly correct. If the current 25.8mm seatpost is the wrong size, it's not too big.

The pea and the pedaler....

The Kalloy model number is SP-248 while Ergotec calls it the Hook seatpost. It might be easiest to get the 25.4 mm and shim it to suit whatever size your frame likes. If you do use a shim be sure it reaches at least below the lower edge of the top tube.

Thanks for posting the picture of the Beat. Slightly surprised that it didn’t come down to sub 27lbs.

With how you are using the Juniper I would not use a shim. That will give you one more thing that could go wrong. Looking at your binder pinch I would go the next size up.

Did you have to adjust the rear hub for clearance of the derailer hanger?

Very few concessions were made to weight, and I'm probably running a bit more sealant than I need to. I'm sure it wouldn't be hard to bring in quite a bit lighter, but I don't think the frame is worth putting all that much into.

I did have to spread the rear triangle to fit a modern hub. I've got a standard derailer hanger for a slot dropout, and a second one on the non drive side so that the axle is straight when it's at the back of the slot on both sides.

Surprised that you're surprised at the weight. It's a standard-issue price-point frame, built with the kind of tubing that's often referred to on Bike Forums as "gas pipe." Low-strength steel, but lots of it to compensate.

Shims are fine. I have a soda-can shim in a mid-'60's Reynolds 531 Peugeot track bike, holding a too-small Campy seat post in place.

Comment on the click thing - there are two bolt seatposts that make getting the exact saddle tilt easy. They can be adjusted easily on the road with no measuring tools, the new tilt ridden, then brought back to exactly where you started or say half way between. Several manufacturers. Nitto makes several. Campy has made thousands and lots are still around. Access to those bolts is a challenge but posters here can help you with solutions. I probably can still find my Campy made wrench. You might also be able to find an old SunTour. The Thompson seatposts have excellent 2-bolt clamps and will sell those clamps to framebuilders to be incorporated with posts of any setback. (I like to keep the clamps centered in the saddle rails and have two custom posts with 60mm setback to do that on bikes with steep seat tube angles.

Get the diameter dialed in first! Home Depot has calipers that will do that just fine and being digital, give you metric and English in both decimal and fractions.

Gaspipe is at least a seamless tube, albeit ordinary carbon steel - better than the ERW used in some junk frames.

Interesting. According to the results of a search, the pipes used for transporting gas over long distances are seamless, but the ones used within homes are often or usually seamed.