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How in the world do i figure out my seatpost size (83' course)

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How in the world do i figure out my seatpost size (83' course)

Old 07-03-22, 05:58 PM
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Raymundoj94
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How in the world do i figure out my seatpost size (83' course)

I have a french made 1983 peugeot course with 181 tubing. Does anyone either know the seatpost size, or tell me how to find out? Everyone says calipers arnt accurate enough as well as those tapered post sizing tube things. I apologize in advanced if this has been covered. Thanks
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Old 07-03-22, 06:00 PM
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P!N20
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Old 07-03-22, 06:02 PM
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Raymundoj94
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Wow thanks for the quick reply. But iv heard these arent accurate. I dont wanna spend 25 bucks on one if all they do is provide a starting point
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Old 07-03-22, 06:07 PM
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T-Mar
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1983 should still be a metric seat tube (i.e, 28mm nominal outer diamter).
Vitus 181 nominal seat tube thickness at the post opening was 0.7mm
Therefore, nominal inner diameter is 26.6mm[ I.e. 28 - (2 x 0.7)]
Subtract an additonal 0.2mm for post to seat tube clearance and the theoretical seat post size is 26.4mm.
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Old 07-03-22, 06:17 PM
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Originally Posted by Raymundoj94 View Post
But iv heard these arent accurate.
In my experience they're accurate enough. If you seatpost ID falls in between sizes, you're probably going to struggle finding a seatpost that fits without some modification.
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Old 07-03-22, 06:17 PM
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https://www.harborfreight.com/6-in-c...xoCcCoQAvD_BwE

(just one possible store/spot to find a cheap one, and it measures all sorts of things bicycle and non-bicycle related...)
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Old 07-03-22, 06:38 PM
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Sizing post over the digital caliper in my experience BUT BUY BOTH.
The digital caliper comes in handy in so many situations you will wonder how you survived without it.
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Old 07-03-22, 09:36 PM
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I took an '84 gitane made out of 181 to the lbs this week to get this checked - 26.4
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Old 07-03-22, 10:34 PM
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26.4 is too large, unless i have to try pushing it in more, which iv neevver had to do on a bike before i saw a restore build of an 81 that had 26.4 thats why i bought it. Guess ill just get the tools. Thanks for everyones help
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Old 07-03-22, 10:45 PM
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Originally Posted by Raymundoj94 View Post
26.4 is too large, unless i have to try pushing it in more, which iv neevver had to do on a bike before i saw a restore build of an 81 that had 26.4 thats why i bought it. Guess ill just get the tools. Thanks for everyones help
...could be the ears are pinched in too far. You can use a hone, sandpaper or steel wool contraption to clean up the seat tube and let you take a good measurement and look...make sure the slot is parallel.
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Old 07-04-22, 04:35 AM
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Digital calipers are something every bike mechanic should have. Do yourself a favor and don't buy the plastic ("composite") ones - spend the extra few dollars and get a good set of stainless steel ones. Then, to your question, just measure the seatpost ID.

Adoric Digital Caliper, Electronic Digital Caliper Stainless Steel Body with Large LCD Screen | 0-6 Inches | Inch/Millimeter Conversion
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Old 07-04-22, 11:23 AM
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It's not uncommon to see frames with seat tubes that appear to require a smaller than typical seat post. There are a number of factors that may be in play. The seat tube could have burrs, scale or some other foreign material. It may have become distorted during frame manufacture or be otherwise out of spec. A previous owner may have installed an undersized post. As suggested by jdawginsc, I'd be checking for a slightly pinched cinch slot and opening up the mouth of the tube, before looking at a smaller post.
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