Unlocking the Secrets of Seatpost Diameter & Sizing

Replacing a well fitted seatpost, sizing is not a problem. Just check toward the bottom of the seat post to find the size usually written in mm.

However, replacing a seatpost that constantly slips or choosing one for a bike that the post is missing is another story. Seat tubes are not round or consistent in their diameter along the upper section. With the heat of brazing/welding, usually in at least a couple of different steps, and the presence of the binder slot and vent hole there's all kinds of distortions and warping potentially going on. And this miss shaping is greatest at the binder section, just where the caliper jaws contact the ID. One really needs to get a deeper check before really knowing what's the best post size.

Also remember that not all frames have had the same degree of after joining finish work. So, a burr could hinder a post from being inserted to begin with yet might fit well below the offending item. This is where a file may be need and, in severe cases, a reamer may be required. Also, it's important to clean the seat tube to remove and junk or old grease.

1. Make sure the edges of the slot in the back of the seat tube are parallel even gap from top to bottom. This is how the frame was made. The slot allows for tightening about 1-2 mm at most. Use a straight edge screw driver to pry the edges apart and make them parallel.
2. Check for burs usually seen at the top - use gloves. File if needed. Extensive burs or irregularities deeper in the seat tube may require re-boring at your local bike store.
3. Clean the inside of the seat tube. If I note a lot of corrosion or gunk, I'll use a DIY cleaner comprised at one end of a straightened portion of clothes hanger wrapped around some steel wool and straight at the other end. Use as shown with the steel wool and then a cloth with some ethyl alcohol. See our video    • Don’t get Stuck with a Frozen Seized Seatpost   on preventing seized seatpost at 1 min 30 sec
4. Individuals on various forums and videos have suggested several methods - some of which just don't work. The seatposts vary in diameter by 2 mm increments. Using a post that's the exact size as the seat tube may be hard to install???? Using one that's just 1-2mm smaller???
a. Measuring the circumference, convert to diameter, and subtract 2 x the seat tube diameter (d=c/pi-2xseatpost thickness Note: do division then subtract 2 times thickness of the wall of the seat tube). Problem is that a tape measure is not precise and measurement of the seat tube thickness can vary at the top of the seat post as we see here due to the welding of the stays.
b. Use of calipers: We'll measure at 2 points that are right angles. One just off to the left of the binder slot and one just to the right. We'll do this several times and take the biggest number. We'll then us a post 1-2mm smaller. Our problem here is that the brazing or welding around the binder slot has all kinds of distortion and warping going on. And this miss shaping is greatest at the binder section, just where the caliper jaws contact the ID. One really needs to get a deeper check before really knowing what's the best post size.
c. Seatpost Diameter Gauge: A better way to measure the seatpost diameter provided you've cleaned the seat tube and straighten the seat tube slot. The Seatpost Sizing Gauge tool has various seat tube marking. The size at 25.4 mm, as an example, is right in the middle of the part marked 25.4. toward the bottom of the marking it's 25.3 and toward the top it's 25.5 mm. Let the sizing gauge drop gently into the seat tube. That's gently and don't push it down. At least in my opinion, this will give the best measurement for the post although there's lots of debate. I have one of these tools since I do bicycle restoration work. The cost of the sizing tool will run about $30 but your local bike shop should have the tool for this purpose and is worth having them do this rather than getting the wrong size post.