using pi, I measure 28.96mm diameter. I'm guessing that rounds off to 29mm. Was this seatpost diameter standard on old italian bikes? -- My bike is from the 70s or 60s. Guessing its older, but maybe this seatpost diameter can dial in the vintage?

Where can I find longer (200mm+) seatposts in this diameter? -- or adapters for modern seatposts?
I have a lot of thinner alloy posts that are the right length. Would the world's easiest first carbon fibre build project be to wrap around CF sheets around an alloy post? Any other ways of adapting a thinner post?

P.S. were alloy cottered cranks ever made?

I recommend: stop using pi and borrow a digital caliper. I could be wrong, but I've never heard of a vintage '70s Italian bike using that size post. This MIGHT be a size you'd find on a Mtn. bike with OS tubing, but I think you have an inaccurate measurement, try fitting a post you know the size of (say with a 27.2 clearly marked on it) and see what goes/no-goes. I recall there was an early European cottered crank made of alloy, but it was a rare bird cause it really didn't hold up. Can't recall who made it: Nervar? TA? Durex? Somebody will chime in...

OK, so as per my usual MO, I did a little research AFTER I stuck out my neck, and found the following thanks to blackbird's site (it was a TA crank I was remembering): https://www.blackbirdsf.org/ta/ref1500.html
And I was wrong about it not holding up, I guess that TA just phased it out when the square tapered axle gained acceptance and their CycloTouriste crank took off to such wide acclaim. Plus much simpler manufacturing for them, I should think. This isn't a true "cottered" alloy crank, but a special hybrid design. I read that Weinmann made one that seems to be a true alloy cottered crank, and that has to be even more rare since I can't find any pics of it...and I bet THAT design didn't hold up or a big firm like Weinmann would have kept making them.

I agree with the caliper. You need to be accurate to .2 mm. Theres just no way to do that by measuring the circumference. If you use something thin enough to wrap tightly around the post, it will stretch too much to be accurate, and if you use something stiff, it won't sit close enough to the post. If all else fails, take the old post to a bike shop and let them measure it. Or take the frame and they should have a seatpost guage.

ok... turns out a seatpost that I think is 27.2 fit in perfectly into the bike Guess it wasn't 29.

thanks for the link on alloy cottered cranks, unworthy.
I thought all cottered cranks came with a big chainwheel attached. Would finding chainwheels for that be difficult? I guess it would be infinitely easier/cheaper to find an italian threaded bb and crank instead.

Any conceivable chance that a 1960s or 1970s Olmo does not have an italian threaded BB?

Given the size of that "spider" i would guess that chainrings would be extremly hard to come by. For the bottom bracket, if you measure the shell, italian will me 70mm wide, english is 68mm wide.

Actually it looks like the same mount as the regular TA Cyclotouriste crank and the Stronglight 49D series, so chainrings wouldn't be THAT hard to get...the Criterium crankset and spindle would be mighty hard to find and prolly very big $$$ if you did. I'd guess that any Olmo should have a 70mm Italian BB shell, unless it's an extremely early one. I've seen some very early Bianchis that had odd shell widths (like smaller than 68mm) and Olmos were made in early times, but...measure with that digital caliper (or an accurate metric scale) and you'll know for sure.

I feel comfortable with tape measure: It does indeed measure out to 70mm... where others tape out to 68mm.

thank you for the help.

2006 campy should match the bike fine