Bicycle Mechanics - 27.2 vs 27.0

Hi,

I have a Colnago Super from the late 70s with an unknown seat tube diameter. I wanted to swap seat posts, so I grabbed a 27.2 and got it in there but it was more difficult than I expected. I compared the old seat post and the new seat post by eye and touch before I made the swap and they looked the same, and I'm pretty sure I can see a 2mm difference. Would it be extremely difficult to get a 27.2 into a 27 seat tube? I know it can happen, but I hate to think I might be "one of those guys". I felt the seat tube afterwards for deformation and it didn't seem to be deformed.

Any tips on how I can verify I'm A OK? I guess I might be able to measure the circumference and then do the math to get the diameter, but I don't have a caliper. Could I take the seatpost out and just measure it with a tape measure accurately enough? I don't really wanna change it though cause it's kind of a pain and I got it dialed in at exactly the height I want.

Thanks,

Kyle

Go to your LBS. They can measure it for you.
I use a cheap $10 digital caliper from Harbor Freight that I keep in my bike tool chest. It's a great bike tool.

Any good bike shop should have a set of calipers to measure the seat tube I.D. Maybe you have a buddy that has some. It's really quite easy to measure. Whatever you find, buy the correct size seatpost. Good luck

Tim

yeah, over time .2mm off can be problematic.

sounds like a beautiful frame, worth getting it right.

Hi,

I compared the old seat post and the new seat post by eye and touch before I made the swap and they looked the same, and I'm pretty sure I can see a 2mm difference. Would it be extremely difficult to get a 27.2 into a 27 seat tube?

Kyle

You're talking roughly a .007" difference in diameter, and it's a small number that can make a big difference. measure it with calipers like the others said.

^.007" would be hard to measure on a caliper, but 0.07" - that's a significant change :).

I'll get a caliper. I don't like relying on the LBS. Any chance any of you know what the Super's seat tube size was off the top of your head? Sheldon says the super pista was 27 (eeks), but a later master was 27.2. I can't really say. I have a bad feeling now that I think about how easy my post is to get in to my track bike which is 27.2. Did I perhaps do any permanent damage or can I just take it out and be OK?

Just out of curiosity, how did you lube/grease the seat tube/post before installing. Seat posts can be pretty tough to get in, particularly at first and if you didn't use enough (or any) grease. Even if they're the correct diameter.

I used quite a bit of grease, and, incidentally, when I inserted it almost all of it was squeegied off, so to speak.

P.S. how can a caliper measure the seat tube? I can't even guarantee the previous post was the correct diameter. Then again, if it wasn't, the tube's probably now reamed to that dia. I guess the best bet is measure the other seat post and see if it's different. Aghh I wish I had a caliper now.. gotta know!

It could be a little rough in there due to having had a stuck seat post at one point tho. It was an alu SR Laprade that got oxidized in there. That's the one that was in there when I got it.

yeah, i wouldn't worry too much yet, if the post went in without a whole lot of force, it probably didn't do too much damage.

i understand about the self reliance thing, but even with calipers it's easy to be .2mm off when you're measuring a round tube. the shop probably has a tool that's designed for the job.

if there's rust in there might as well have them face and chase it.

The jaws on one side of the caliper measure outside diameters and the jaws on the other side measure inside diameters.

According to Sheldon Brown, it is 27.0mm.

i'd be very impressed if you got a 27.2 into a 27.0 without superhuman strength or a large mallet. if the Colnago was super thin tubing possibly, but still....

I've got a 27.0mm post, nice Sugino lamprade-style. Don't ever expect to use it. Came off a Centurion that should have taken 27.2mm post, which it has now (and yes, that 0.2mm matters).

http://www.unc.edu/~cupery/pics/bike_parts/eBay/Sugino_post/IMG_1548--Sugino_27_0_post_side.jpg

More pictures here (http://www.unc.edu/~cupery/pics/bike_parts/eBay/Sugino_post/). PM me if you want it; you can have it for the cost of shipping and my time in packaging it (so, not much).

MattP I checked Sheldon's site before coming here, but that refers to the pista, which I don't have. They certainly could be similar, but Sheldon's database can't really be considered complete enough for a certified answer.

timcupery, thanks for the offer. I'll consider it if I end up needing a new seatpost.

Bekologist, do you speak from experience or just estimation? It was fairly hard to get in, but I by no means have superhuman strength. I weigh ~135. I had the saddle on at the time, though, so I used some of my body weight to help it down.

BIG PHEW and a sigh of ***********. Guess how I solved this oh so pressing issue... I read the fu°Cking seatpost. Yup, under some grime and heavy scratching from removal, it turns out there's some engraving of the numbers. 27.2 baby, alright! Guess my eyes aren't so bad. ;) (or maybe it was just the feel that wasn't so terrible that I couldn't insert it.) I spent time trying to measure it and I was like .25 off either way depending on minor this that, so I can say a clothing measure is not sufficient for any reasonable accuracy, anyway.

Thanks for the advice, and sorry for wasting your time, but maybe some poor sap will read this and learn a thing or two about reading. I wonder if it's possible that it's still a 27.0 and the previous owner stuck the wrong post in there... It still was strangely hard. Regardless, I won't worry as much.

BIG PHEW and a sigh of ***********. Guess how I solved this oh so pressing issue... I read the fu°Cking seatpost. Yup, under some grime and heavy scratching from removal, it turns out there's some engraving of the numbers. 27.2 baby, alright! Guess my eyes aren't so bad. ;) (or maybe it was just the feel that wasn't so terrible that I couldn't insert it.) I spent time trying to measure it and I was like .25 off either way depending on minor this that, so I can say a clothing measure is not sufficient for any reasonable accuracy, anyway.

Thanks for the advice, and sorry for wasting your time, but maybe some poor sap will read this and learn a thing or two about reading. I wonder if it's possible that it's still a 27.0 and the previous owner stuck the wrong post in there... It still was strangely hard. Regardless, I won't worry as much.


I am glad you have solved the "problem". I was going to tell you that there is no way in the world you can fit a 27.2 mm seatpost in a 27.0 tube. I have tried before I realized that my Kona Kilauea uses a 27.0 seatpost. No, didn't break anything. But man, those .2 mm mean so much sometimes :)

Now, I wish I knew why was it so important for Kona to choose 27.0 instead of 27.2 - and force me to order a stupid seatpost all the way from USA.

Now, I wish I knew why was it so important for Kona to choose 27.0 instead of 27.2 - and force me to order a stupid seatpost all the way from USA.


Protectionist trade policies?

:D

.2mm in diameter makes a smaller tube... much lighter... yes... hehe j/k. Well thanks again guys, I'm riding in peace.