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Old 01-29-08, 07:36 PM   #1
wrk101
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1975 UO8 Seat Post Size

OK, I would like to purchase a replacement seat post for my UO8 rebuild. My measurement shows the current post is 26mm OD. But doing a search I see others say this bike had a 22 or 24.x seat post size.

I bought this bike "new" (at least it was presented as such) back in 1975, and this is the original steel seat tube. Thoughts/comments appreciated before I possibly waste money on the wrong size post. To make it a little more confusing is that my AO8 (guessing vintage early 70s, I think it is older than the UO8) has a 22mm seat post with a shim. I have had the AO8 for about 20 years, garage sale find way back then. .


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Old 01-29-08, 11:39 PM   #2
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Lots of shops have a graduated post that is inserted into the seat tube to measure the ID of the tube. This is probably the best way to determine what size post you need.

Is there a marking on the current post? Lots of posts have the size engraved in them below the min insertion mark.

Or just measure the current post with a good set of calipers. There are a lot of variations in old U08's so don't just go by what someone else has.

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Old 01-29-08, 11:55 PM   #3
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The various UO-8 variants from that era that I've seen all had 24.0 mm posts... and the shim insert just as you mentioned - with 1 mm thick walls... so 26.0 makes sense. I noticed that the shim on my last such bike did serve the function of spanning around a minor seam or weld inside the seat tube. So, maybe your bike simply had a nice smooth seat tube, had better seamless tubing than some, or was simply already cleaned out.
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Old 01-30-08, 12:17 AM   #4
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I measured mine with a micrometer and it says 26.0 although I don't have seatpost that size to try out. Mine has the same shim/22mm post arrangement.
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Old 01-30-08, 09:17 AM   #5
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I will measure the tube with a micrometer (good idea). Seat post is tight, but I can get it in and out. The internal bore looks clean with no high spots. I don't see any markings on the seat post at all, it looks pretty generic, and is about 6 inches long. Thats probably one reason it fits. I can't imagine a 350mm stem at 26.0 fitting in. Not sure Peugeot had that much quality/consistency on an entry level boomer bike.
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Old 01-30-08, 09:28 AM   #6
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For what it's worth, my 1970(?) UO-8 frame has a thin OEM shim and a steel seatpost with a 25.6mm outer diameter.
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Old 01-30-08, 07:31 PM   #7
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Originally Posted by John E View Post
For what it's worth, my 1970(?) UO-8 frame has a thin OEM shim and a steel seatpost with a 25.6mm outer diameter.
John, is that 25.6mm post + the shim. I seems like several folks have posted various diameter seatposts with shims of varying thinknesses. Looks like Peugeot used whatever seatpost was laying around and shimmed it to fit. I'm going to try 26.0 in mine with no shim since that appears to be the size it measures out to.
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Old 01-30-08, 08:33 PM   #8
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Yes, the post itself is 25.6mm, with which I use a very thin shim. Your 26.0mm idea just might work, although it would require the tubing walls to be just under 1mm thick, based on a French outer diameter of 28.0mm. That sounds about right for Reynolds 531, but feels a bit thin for plain carbon steel.
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Old 01-30-08, 11:10 PM   #9
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Quote:
Originally Posted by John E View Post
Yes, the post itself is 25.6mm, with which I use a very thin shim. Your 26.0mm idea just might work, although it would require the tubing walls to be just under 1mm thick, based on a French outer diameter of 28.0mm. That sounds about right for Reynolds 531, but feels a bit thin for plain carbon steel.
Hmmm...time to bust out the old micrometer again...

Edit: And the answer is: The OD of the seat tube is 28.0mm as expected. The opening at the seat lug is fairly round but the wall edges are not of uniform thickness and there is the slot cut in the back so it's difficult to get a precise measurement but does appear to be 26.0 ID. That could mean a 1.0mm thickness, straight gauge tube which would be reasonable I think even for an old hi-ten steel tube. A fairly heavy tube like Reynolds 531 Super Tourist was 1.0mm at the butt and 0.7mm in the middle being 1.0mm all the way through would be pretty heavy.
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Old 02-08-08, 06:49 PM   #10
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I ended up using a 25.8mm seat post that I got off of ebay. It was plenty tight. According to my micrometer, the one I took out was 25.9, and the hole is 25.85 to 25.90.
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