Keefusb

I recently purchased a 1989 Eddy Merckx Corsa Extra frameset. Does anyone here know what seat post diameter this bike would take?

Germany_chris

probably 27.2 but could be 26.6 or 26.8

rccardr 

99.9% of these have a 27.2 seatpost.

Andrew R Stewart 

If the frame size is a large one that the maker might have used a seat tube with a slightly thicker wall thickness than the smaller sizes use. As example Columbus has made their iconic tube set is a SL and a SP version. SL has a .6mm wall and after the frame's been made generally takes a 27.2mm post. SP has a .7mm wall and thus usually has a 27mm post.

Do understand that the actual ID of the seat tube, at the seat lug/binder area, is also dependent on the builder's skills at not distorting the lug while brazing and the after brazing finishing work (reaming a heat deformed tube back to close to round). So as always the best advice is to make a guess and test before buying/using. This is where a LBS that has a series of tester seat posts is a big savings. Have them test fit a post and buy the right size from them as the payment for their tools and skills. Andy

bikemike73 

Plus 1 ^ Andrew !!!!!!

LBS should have many size seat posts to get the correct size.

Could use a micrometer as well.

I would use the LBS !!!!!!

himespau 

I bought one of these when it was on sale for $25. Haven't had to use it yet, but wished I'd had one when I bought a frame that was claimed to be SL and my micrometers told me it had a 27.2 mm opening, so I bought a really nice 27.2 post. Turns out, the opening was 27.2 (tubing - 60 cm Colnago Super Piu without tubing labels - is probably SP), but I could only fit a 27.0 post in there. Now I have a cheap tool to feel for what size post I actually need. Sure shops might have a more precisely milled metal gauge, but I may never actually use this, so I didn't want to drop $100 just to add to my store of "at least I'll have this the next time I need it" tools.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

JoeTBM 

Doesn't seem to go small enough, we bought and use one like this in the shop - https://www.ebay.com/itm/32466286828...0AAOSwcOZcLCms about $25 shipped

himespau 

There are 2 other rods tucked inside for smaller sizes down to 25.0 or lower if memory serves. I'd looked at the one you have, but was concerned that the sloping sides (instead of flat steps) would not identify spots preventing me from sliding in a post of a specific size.

Andrew R Stewart 

himespau has almost got it right. Yes, the tapered shape of some sizing sticks will only check the mouth of the seat tube. And if that mouth is not square/flat the taper won't even sit coaxial. While much better at gaging the seat tube's ID a stepped sizing stick still only checks to a depth of the stick's steps, generally less than an inch. Deformation can occur well below the step's depth. measuring the ID with a caliper is about the same WRT not reaching past the heat affected top/lug and one has to make a number of measurements around the ID to cover any ovalizing that might be present.

It is because of these lacking's (that well too many people fall into trusting) that some will reach for the sizing posts box and test full length seat posts in the ID. This method will cover the full depth that one's properly sized post will sit at, with no assumptions of the fit at the lug being the same a few inches in. Andy

veganbikes

This is the tool people are looking for:
https://steintool.com/portfolio-item...t-sizing-rods/

Measure things either using that tool or using a set of dial or digital calipers. Many shops especially those who have been around a bit will likely have that tool.

himespau 

That's just the metal version of what I have in plastic. A bit more sturdy (and potentially more precise as I trust Stein more than some random injection mold maker in China), but still the same issue of not detecting problems more than an inch below the top of the hole. Better than a pair of digital calipers in showing what will actually fit as opposed to what the size of the opening is, but it won't get everything (maybe most things you can't fix with a little reaming using sandpaper on a broom handle). As Andrew R Stewart says, a box of posts of different sizes is the best but it's also the most expensive (unless you're a shop that has them handy). Having spent more money than I should have on incorrectly-sized posts over the years, I have a few to try, but I'm hoping I don't have to buy any more.

JoeTBM 

Running a volunteer shop of rebuilding donated bikes has it's challenges and buying an $85 tool when we can get by with something less is part of our difficult daily decisions of how best to spend our limited money. The $60 difference means that maybe we can afford other needed parts or supplies. I don't need every tool that Park Tool or Stein makes to get the job done but I do need tires/tubes/cables etc. We are heading this year to hit 2,000 bikes repaired/rebuilt up about 15% from the last 2 years. I would love to have every "best" tool out there but it is just not practical for our operation.