lambo2124

Ok im a newbie so go easy on me.

I needed a new seat post for my mountain bike so I got a ruler and measured the seat post hole on the bike itself from side to side and supposedly got 31.6mm. So I went on ebay and bought a seat post but it wouldn't fit in.

So now I measured from INSIDE my seat post clamp it self (not the outer) from side to side as shown in this pic :



and I got this:




So now WHAT exactly is my seat post size in MM so I could go ahead and order another one?

dsbrantjr

You need to measure to better precision than you can get with a rule. Seat posts come in 0.2mm increments so you need to measure to about 0.1mm. There are inexpensive digital calipers available from Northern Tool or Harbor Freight which are sufficient for the purpose.

Scooper

You need to measure the inside diameter of the seat tube, not the clamp. The clamp goes over the end of the seat tube and will only give you a range of the outside diameter of the seat tube. Because the wall thickness of the seat tube varies, measuring the outside diameter means virtually nothing.

The best way to measure the I.D. of the seat tube is to use J.A. Stein seatpost sizing rods or similar. Most bike shops can measure this for you, often without charge. Typically, there is a 0.2mm difference between the I.D. of the seat tube and the O.D. of the seatpost to allow for manufacturing tolerances and seat tube warpage when the seat cluster is brazed, although seat tubes are almost always reamed to minimize fitting problems due to tolerances or heat warpage.

Example: the I.D. of seat tubes that take a 27.2mm seatpost is usually 27.4mm.

jsharr

Do you have the old seat post or is it gone? Do you the know who made the bike, what year and model the bike is? If so, you might be able to find archived specs on the manufactures site to help you also.

JanMM

It's gotta be just the right size - too big won't go in at all and too little will slide down no matter what.

FBinNY 

The clamp fits around the outside of the seat tube, and the post fits inside the seat tube. You have to measure the inside diameter of the seat tube, not the clamp that surrounds it.

In any case seatpost fits are very close down to where 0.2mm is a day and night difference, so naked eye, or ruler measurements aren't good enough. You deed a dial or digital caliper, or can let a local bike shop.

Another alternative is to say what brand, model and year your bike is, and odds are someone can give you the correct diameter with decent reliability.