Seatpost size: 27.0 or 27.2 mm
 

I have a vintage Italian-made Bianchi mountain bike from 1992. The original seat post was too short and rusty. I replaced it with a 27.2mm diameter post. The post fit but it did require quite a bit effort, some light pounding and twisting. Now its is too tight and hard to remove (not that I need to remove nor adjust). Unfortunately, I did not check the diameter mark on the original post before I discard it. From the manufacturer's specs, I see that they used 26.8, 27.0 or 27.2 mm seat post for that bike. Should I buy a 27.0 mm seat post to replace the one that I already installed? Would 27.2 mm seat post fit if the original size was 27.0 mm? Would 27.0 mm slip now that I installed/forced 27.2 mm into it? Thank you for your feedback. I kept thinking about this since my install.

I wouldn't want to guess I would just want to use a pair of vernier calipers and measure things and confirm before I start buying things. If you have to hammer in a seatpost that is usually a good sign of it is being the wrong size but I would loosen everything and measure it and always do that first and take a couple measurements around the circumference and make sure you are getting similar results.

I might consider taking it to your local shop and letting them help out as they can hone out the old seat tube and take measurements and hopefully get the correct seatpost.

Bike parts rarely if ever need to be forced and if you feel like forcing it that is a good sign it is wrong and you need to stop immediately and figure out the issue without any force. If you feel like all you can do is force it take it to a professional mechanic and let them tackle it.

Consider how rusty that old seat post is... the Seat Tube will be likewise rusted.
the tube needs to be cleaned up and carefully measured with an Accurate Device and practiced hands.
If you don't have a way to do this, take it to someone that does.

For measuring the ID of a seat tube, use a bore gauge (then use the calipers on that). Start there.

I’m wondering why a 1992 Bianchi seatpost would be rusty.

Were any still made of steel by that point?

A steel frame will transfer to an Al post a surface amount of rust. Unsure if that's what is the case here.
I'd rather see rust on an Al post then AlOxide:) Andy

Since you are not in a hurry to replace or remove the seat post, you can apply a "penetrating" lubricant daily or weekly until you are able to remove it. Lesson learned, caliper correct size next time so excessive force and possible tube damage does not result.

Thank you for your prompt help. Appreciated. This bike has a big sentimental value, which I have been using in the past 30 years. The bike was not use much in the past 5-6 years sitting in a house 200 yards from the Ocean with high salt and corrosive environment. I started restoring the bike last summer. I only have access to the bike over summers, where finding parts, bicycle repair shops are very difficult. Therefore, I take the parts with me. I hope to complete the restoration this summer, which will involve also sanding, priming and touching up corroded paint on the frame. I have attached the photo showing the condition of the old seat post that I have discarded. This was taken 3 years ago before my restoration project started. I also attached catalogue image of the bike (in Italian). I don't think this bike ever exported to North America.

Removing the old post was difficult. I had to apply penetrating oil. Installing the new one was not more difficult than removing the old one. I am not a person who forces things. I tend to tighten gently. The new post was inserted until the minimum insertion line. After riding a few days, to my surprise, the post slipped. I am 180 lbs. Luckily, I had ParkTool SuperGrip Compound with me. I removed the post, applied this and resolved the issue. You may ask: what is your problem then? I ahve a few other bikes and I adjust seat posts from time to time. But this one feels super tight making me think that I should have bought 27.0 dia post, not 27.2 mm. I can still spend (or waste) $15-20 to get a 27.0 dia post take it with me to see whether it will be better.

NOPE! Seriously stop, if you just joined here to answer a single question and are not interested in the forums, why take all that time to create an account when you have google with no sign ups needed that you can search easily all the time. If you are trying to get your post count up JOIN IN ON ACTIVE THREADS! This is a bike forum, we all love talking about bikes and we have a ton of active threads spread across 37 sub-forums. Plenty to get involved in within all sorts of different topics within cycling. Just making junk posts is not a way to get help but being active around here is. It is a fun place but when people make posts over and over in their own thread just to post a picture instead of getting active it ruins that fun a bit.

You have some interest in bikes otherwise you wouldn't be here so get active and pretty soon those 10 posts will fly by and you can get into triple and quadruple digits easily.

It's VERY simple. If a 27.2mm post fit at all, it's the right size and a 27.0 will be too small. It is physically impossible to insert a post which is 0.2mm oversize.

The tightness may be because of ovalizing at the seat clamp area, or simply a tolerance issue. FWIW- the tolerances for seatposts are that the tube should be the nominal plus 0.00-0.05mm, and the post should be the nominal minus 0.00-0.05mm. This kind of tolerance is designed around reasonably achievable tolerances, ensuring that the post will fit without jamming or being sloppy.

Think about this as a 2 lane road with a double line down the center. The nominal is the double yellow, and the fog lines are the extreme limits. As a practical matter makers try to stay centered in their "lanes" without crossing the centerline,or running off onto the shoulder.

In your case, it's a tight fit because one or both are crowding the double yellow.

If you are uncomfortable with the fit, you can consider reaming the frame or trying another post, but do not ream the frame before confirming that the post is on spec. I see too many instances of oversize seat tubes, which is a far more expensive issue than buying another seatpost.

i've only had a couple Bianchi MTBs thru my place.. both were heavy tanks hung with low-mid grade parts. i'm guessing from memory, but i think the posts were chromed steel... i recall thinking how remarkably similar they were to the more common Peugeot MTBs from back then...also, seats get swapped with a post attached most often.... most likely a 27.2mm, but many older MTBs used 27.0 and 26.8 posts due to thicker tube sets specified for strength on these old MTBs... my DB Ascent uses a 26.8mm post, 1990 vintage... so does my '83 DB Ridgerunner... just checked a Corsaro project frame.. also 26.8mm.... this is why the OP needsto get that frame measured, or go to a shop that can do so.

just a guess... you did mean Telescope Gauge,, correct? Bore gauges are EXPENSIVE and rarely seen in anything but Machine shops. they also have limited use per Gauge since their range of operation is limited to a rather narrow Diameter variation....

I would use sandpaper on the post to remove 0.1mm all the way around. That will be 0.2mm in diameter.

Very clear and logical explanation! In the meantime, I did an online search and found in one site that states "A rule of thumb is that the widest post that slides in without being forced (other than pushing or twisting by hand) is the right one. If it drops in, with play, before the pinch bolt is tightened, the seatpost is probably too narrow." This supports your logic. I am guessing 27.2 mm is the right size for me. Either the seat tube has some corrosion or impurities inside, or perhaps the tolerances of the seat post is not that great. I bought it from Amazon for about $20 (FOMTOR Bicycle Seat Post). I will check inside the seat tube as well as measure the seat post diameter to confirm. Thanks very much!

I said I will post pictures but I could not due to being a new member. Some of you might be curious. I should be able to post tomorrow after I have reached the required posting threshold.

Well said! This is also heavy with thick tubing. I am 99.9% sure the seat tube is chrome-plated steel. SunTour groupset with indexed shifting which I enjoyed using with no issues. The bike has a unique look. I will post the catalogue page tomorrow.

It also helps to let the bike itself tell you what's happening.

If a post is tight from the beginning, and doesn't seem to get tighter as you go deeper, then the issue is distortion of the seat lug, or sometimes a burr in the slot.

OTOH if it starts OK and gets tighter after a few inches, then it's an overall size/tolerance issue.

Again don't quote yourself to get your post counts up JOIN IN ON ACTIVE THREADS THAT AREN'T YOUR OWN. Be a member here it is fun, I promise after 8 years here I am still an active member and some have been here longer. It is a great place but only great if people join in.

https://cimg8.ibsrv.net/gimg/bikefor...d3c3272eb7.jpg
This is the photo of the post that I discarded without much thought. It appears this is chrome-coated steel. My current summer project is to restore the bike frame and parts from rust.

https://cimg8.ibsrv.net/gimg/bikefor...ba4c7bc073.png
This is the catalogue page of the bike. I have enjoyed riding this.

BioPace, U brakes, "X" frame design... a classic example of the early 1990s attempts to better the MtB experience. At least Bianchi saw the reason to not install the rear U brake on the chainstay

Interesting that the catalog shows an Al post and the OP's bike has a steel one... Andy

Good point. The catalogue version might have shown another, more expensive seat post. This bike has a rather heavy frame with thick steel tubing, but it is one of the softest and comfortable hard-tail bikes that I have ridden. I found a post from an Italian source (translated to English) which explains the logic behind this design. I am not sure if it is hype or truth but feel comfortable for sure.

https://cimg7.ibsrv.net/gimg/bikefor...05d1cf8448.png

More proof that a picture is worth a thousand words. Both the problem and solution are now obvious.

The problem is the rust, and the post will fit perfectly with a light sanding to remove it. Or replace it with an aluminum 27.2 post.

BTW - be careful and do not overnighter the clamp. The ears seem to be bent inward, probably from overtightening in the past. Keep it up and you might Crack the tube, or bend them in enough to touch.

I have a bike that had a fairly tight seatpost that was tighter than it should have been. I wrapped some wet/dry paper around a thick wood dowel and carefully sanded off the built up gunk on the inside of the seattube. Now that seatpost fits just like it should.

Cheers