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Seatpost size concerns

Old 01-12-22, 03:44 PM
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Seatpost size concerns

As I was musing in another topic, on my 1974 Zeus track bike project, “My stash of seat posts, one of which was surely on the bike, includes a couple far too small and one so tight it might be wrong too.” Well, sometimes I really hate measuring diameters with a micrometer because I get varying results; that said, the post seems really close to 27.2mm, the seat tube opening I get varying reads of 27.0 to 27.2-ish. The frame is “supposed to be” Reynolds 531, isn’t that always to be 27.2mm? I figure it might be smart to get a “generic” micro-adjusting post instead of a straight post and a clamp, kinda want to get the size correct the first time.
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Old 01-12-22, 06:28 PM
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You want something like this, right?

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Old 01-12-22, 06:38 PM
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Here is one by Ice Toolz. I have a chain whip and something else from Ice Toolz, and they work just fine.

https://www.ebay.com/itm/IceToolz-E3...-/202101458951
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Old 01-12-22, 07:07 PM
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Originally Posted by tiger1964 View Post
... the post seems really close to 27.2mm, the seat tube opening I get varying reads of 27.0 to 27.2-ish.
If you aren't getting a consistent read with a micrometer, perhaps the opening has been ovalized?

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Old 01-13-22, 06:38 AM
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I'm surpirsed to hear 27.0-27.2mm in a 1974 Zeus, as they were were typically manufactured with metric standard tubing during this period, at least on the road models. Regardless, during this period Reynolds offered their standard 531 single butted seat tube in two different gauges and the plain guage seat tube in three different gauges. In both cases, the heaviest gauge seat tube was intended for a 27.0mm post, so that could be the correct size. It was common to spec the heavier gauge tubes on larger frames. It would also be appropriate for track frames intended for sprinting, where stiffness is a desired characteristic. This may also be the reason for the larger diameter, imperial standard tubing.
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Old 01-13-22, 08:54 AM
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One more data point: an "Anpi"-branded fake Campy seat post (made in Spain) that I have in my parts stash is labeled 27 (with a paper label).


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Old 01-13-22, 10:55 AM
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My early '70s Zeus with metric 531 has a seatpost of 26.6mm.
It is not a track bike.
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Old 01-13-22, 12:51 PM
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Originally Posted by Drillium Dude View Post
If you aren't getting a consistent read with a micrometer, perhaps the opening has been ovalized?
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"?

Originally Posted by T-Mar View Post
I'm surpirsed to hear 27.0-27.2mm in a 1974 Zeus, as they were were typically manufactured with metric standard tubing during this period, at least on the road models. Regardless, during this period Reynolds offered their standard 531 single butted seat tube in two different gauges and the plain guage seat tube in three different gauges. In both cases, the heaviest gauge seat tube was intended for a 27.0mm post, so that could be the correct size. It was common to spec the heavier gauge tubes on larger frames. It would also be appropriate for track frames intended for sprinting, where stiffness is a desired characteristic. This may also be the reason for the larger diameter, imperial standard tubing.
I am leaning towards thinking I should be looking at 27.0, but again that's conjecture.

Originally Posted by Wildwood View Post
My early '70s Zeus with metric 531 has a seatpost of 26.6mm. It is not a track bike.
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.
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Old 01-13-22, 02:38 PM
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My Zeus post in very good condition ( which is NOT on the bike ) measures 26.4 and pinches the seat tube lug closure (ears?). I believe this frame to be from 1972 - approx.
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Old 01-14-22, 08:19 AM
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Originally Posted by tiger1964 View Post
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.

Last edited by T-Mar; 01-14-22 at 08:22 AM.
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Old 01-14-22, 10:03 AM
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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.
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Old 01-14-22, 01:05 PM
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Originally Posted by tiger1964 View Post
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.
OK, it appears that it is built with metric standard tubing like other Zeus of the era. The nominal outside diameter is almost certainly 28.0mm. With an inner diameter of 27.2mm, that would mean the wall thickness is only 0.4mm! That's very thin. Reynolds didn't have anything in that range until they introduced 753. It appears that someone used a reamer or similar tool at some time, to enlarge the inner diameter, possibly to accept a certain seat post. I've seen cases of overly aggressive factory reaming, but not to this extent.
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Old 01-14-22, 01:24 PM
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Originally Posted by T-Mar View Post
It appears that someone used a reamer or similar tool at some time, to enlarge the inner diameter, possibly to accept a certain seat post. I've seen cases of overly aggressive factory reaming, but not to this extent.
If so, it was me a long time ago.

So, 27.0 post?
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Old 01-14-22, 01:38 PM
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Favorite tool for measuring seat tube inner diameter

I like to use a telescoping gage, placed an inch and a half or more into the seat tube, to measure the inner diameter.

Then read the measurement with a vernier caliper or micrometer.

I'll take 2-3 different location readings in case the tube has been compressed or ovalized.

Remember too that a lot of vintage seat tubes can benefit from a good honing to clear accumulated oxidation and debris.
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