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Old 01-14-22, 10:03 AM
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Bikes: Drysdale/Gitane/Zeus/Masi/Falcon/Palo Alto

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Now that's a bunch of detail I can sink my teeth into! Thanks. BTW, maybe there's very little on earth that's 100% round; I just tried the outside of the seat tube at several points and various "rotations" and as I turn the micrometer, the readings change a wee bit. Anyway, it seems to be 28.2mm but that is including the thickness of the new powder coating. The micrometer is digital and has a "zero correct" feature.

No "pinch/taper" to the seat tube slot at all.

Checking again, inside of seat tube does average about 27.2, and the diameter of the seat post is 27.2; hmm, is this the problem due to "nominal clearance between a seat post and seat tube is typically 0.2mm diametral "? Again, the years dim the memories and I was 18 years old, working in a bike shop and assembling this bike from a bare frame and mostly grabbing stuff from the parts bins, so I may have succumbed to "get a bigger mallet" so I may just be looking at a post never intended to fit.

Originally Posted by T-Mar View Post
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.
Larry:1958 Drysdale, 1961 Gitane Gran Sport, 1974 Zeus track, 1988 Masi Gran Corsa, 1974 Falcon, 1980 Palo Alto. Susan: 1976 Windsor Profesional.
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