Originally Posted by

**tiger1964**
Or perhaps "user error"?

Now, it's not much out, a tenth or two from min to max. Follow up question - what is the difference, if any, between set tube diameter and seta post diameter. Zero? "Interference fit"?

I am leaning towards thinking I should be looking at 27.0, but again that's conjecture.

Hmm. Wild card. again, " what is the difference, if any, between set tube diameter and seta post diameter." If it's somewhere about .5mm clearance, that 26.6 starts to make sense.

Seat posts should be a clearance fit, even when the seat post diameter is at it's maximium tolerance limit and the inner diameter of the seat tube is at it's minimum tolerance limit. However, the seat tube can sometimes get distorted to the point where there is interference.

The nominal clearance between a seat post and seat tube is typically 0.2mm diametral / 0.1mm radial. As an example lets take looks at the common Columbus SL/SLX seat tube. In their imperial standard version these have a 1-1/8" (28.6mm) nominal outer diameter and a 0.6mm nominal wall thickness at the seat tube end. This produces a nominal inner diameter of 27.4mm. Applying 0.2mm diametral clearance, results in a 27.2mm seat post, which is what is typically fitted to these frames.

My preferred method to judge the whether a seat post size is correct, is look at the cinch slot in the back of the seat post. With the binder bolt properly tightened, the slot will be narrower at the top that at the bottom. With a nominal 0.2mm clearance , this difference will be 0.628mm (0.2 x pi), Of course, due the manufacturing tolerances this can vary. It's typically 0.3 - 0.9mm. If it's more than 0.9mm, the post is too small.

Assuming the inner diameter of your seat tube is truly 27.0mm, I'd be using a 26.8mm post.

I'm still perplexed by a seat tube inner diameter that is more representative of an imperial standard seat tube, for a time period when Zeus typically used metric tubing. Please measure the outer diameter of the seat tube to detemine if it is imperial (28.6mm) or metric (28.0mm). The measurement should also tell us if there is a calibration issue with your calipers.