I have a late '80s Bianchi Strada LX that I'm renovating and I'd like to replace the seat post. Are they all the same diameter or is it head tubes that vary? I just want to be sure that I'm ordering (online) the correct size.

There's a lot of different seat post sizes out there. Before you order anything, remove the current seat post and it should have the diameter engraved on it near the bottom, right around the minimum insertion line. For example, the one in the pic is 27.2mm.

They come in all different diameters.
This guy thinks his is 26.4.(bottom of page)

I thought taking that approach would work for me until I discovered that the PO replaced the post with one the next size down! Checked the specs on the bike against the post size. If needed use digital calipers. I was wondering why I couldn't keep the post from slipping!

All good responses.

I think the OP means seat tubes, not head tubes.

For a given O.D. like 28.6mm, different tubing used for the seat tube have different wall thickness, and require different seat post diameters. So no, they're not all the same.

Most high quality 28.6mm seat tubes have 0.6mm wall thickness, and so take 27.2mm seat posts (2 x 0.6 = 1.2 and 28.6 - 1.2 = 27.4. Allowing 0.2mm of tolerance between the seat post O.D. and the seat tube I.D., you wind up with a 27.2 seat post). Seat tube wall thickness varies with the strength of the tubing used, and some 1020 or 1030 carbon steel seat tubes have 1.0mm wall thickness, so they take 28.6 - 2.0 = 26.6 - 0.2 tolerance = 26.4mm seat post.

The seat tubing wall thickness can vary between 0.5mm and more than a millimeter, so there is a range of seat post sizes.

OK, thanks guys. On the weekend, I looked at a PDF of a Bianchi catalog for that year or there abouts (there was a Bianchi Strada LX in the catalog) but the seat tube diameter wasn't mentioned in the specs. I'll assume 26.4 but I'll also see if the size is engraved on it to confirm.

Thanks again.

And please post it on here. Your data could help the next person who has this question about this particular bike.

I understand that in some areas, there's no LBS close, but if at all possible, get the help of your LBS. I took mine in and had it sized up in no time. If you're spending this much time trying to determine the size, it might be more worth it to support the locals and make new friends.

I apologize if there's an obvious answer for why you're ordering online, just thought I would throw my two cents out there.

Most bike shops have the J.A. Stein seatpost sizing rods (or similar tool) to help determine the seatpost diameter for a given seat tube.

I would suspect a late '80s Strada would be 26.8 and I agree starting with what is printed on the current post is always your best bet, unless it fits extremely loose or tight.

Scooper those are great tools. I don't use mine often but much better than the caliper approach.

Ordinary can aluminum can make a great shim sizing up a post too small-

maybe not the best permanent fix, but it works.

also ... if your current post works properly, but is merely ugly through misuse, use 3m auto sandpaper and a metal file to polish it up to better than new condition.