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Old 03-03-07, 01:47 AM   #1
sac02
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Machinists? Framebuilders? Seatpost insertion problem

Cross posting this, thinking maybe someone here has run into this problem on an older bike, or perhaps specifically on a UO-8.

OK I'll do my best to describe this. I am replacing the steel seatpost on my Peugeot UO-8 conversion with an alloy post. The seat tube is 25.4mm (so is the alloy post) but the original steel post is something like 22mm with a shim.

The problem is that there is a raised seam of metal on the inner diameter of the seat tube - like a "ridge" running down the back of the inside of the seat tube . (It's a cheap old bike and I'm assuming the tubing was rolled and then welded along this seam rather than the tube having been extruded or drawn.)

Anyways, with the original 22mm+shim setup, this seam of metal was a non-issue. But now that I'm trying to fit a 25.4mm post in a 25.4mm tube, this seam is creating interference, and I can't get the post in the seattube more than about an 1.25inches.

What, in your professional or non-professional opinion would be the best way to remove a raised seam of metal on the inner diameter of a 1inch tube? I would like at least 3 or 4 inches of seatpost insertion. My dremel can only reach about 1-1.5inches. Would a "pencil" extension/adapter for the dremel be the best plan? A round file?

Thanks for the help,

Mac
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Old 03-03-07, 02:05 AM   #2
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jmac?
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Old 03-03-07, 02:10 AM   #3
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Quote:
Originally Posted by sac02
Cross posting this, thinking maybe someone here has run into this problem on an older bike, or perhaps specifically on a UO-8.

OK I'll do my best to describe this. I am replacing the steel seatpost on my Peugeot UO-8 conversion with an alloy post. The seat tube is 25.4mm (so is the alloy post) but the original steel post is something like 22mm with a shim.

The problem is that there is a raised seam of metal on the inner diameter of the seat tube - like a "ridge" running down the back of the inside of the seat tube . (It's a cheap old bike and I'm assuming the tubing was rolled and then welded along this seam rather than the tube having been extruded or drawn.)

Anyways, with the original 22mm+shim setup, this seam of metal was a non-issue. But now that I'm trying to fit a 25.4mm post in a 25.4mm tube, this seam is creating interference, and I can't get the post in the seattube more than about an 1.25inches.

What, in your professional or non-professional opinion would be the best way to remove a raised seam of metal on the inner diameter of a 1inch tube? I would like at least 3 or 4 inches of seatpost insertion. My dremel can only reach about 1-1.5inches. Would a "pencil" extension/adapter for the dremel be the best plan? A round file?

Thanks for the help,

Mac
Assuming the seatpost is thick enough, machine a slight groove in the seat post like a flute on a rifle barrel. Use a ball end mill. Any home machinist or actual machine shop could do this in about 1/2 hour including setup. Or you could stick tape on either side of the seat post creating a boarder and use your dremel tool with a stone and grind out a groove in the post yourself and finish with your round file and sandpaper. You will also have to have the post lathe turned down first to the diameter of the i.d. of the seat tube. If you grind out the weld the seat tube may crack. A hone won't work since the raised weld would force the hone to grind the tube out of round. A reamer might work but without a pilot on the end of it, it will cut to one side also. I'd get a thick walled seat post and turn it down and machine a groove for the weld. You may have to machine one from straight bar stock and use a separate seat clamp rather than an intregal seatpost clamp. In order to get a thick enough wall after all the machining. Any machine shop can do this. But you may look into someone with a home lathe and mill who is handy. There are quite a few retired machinists out there who like to tinker for some extra cash.
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Old 03-03-07, 06:15 AM   #4
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How about using a 25.0mm alloy post with a pop-can shim that allows for that raised seam? Bikepartsusa.com has a Laprade style alloy post in that size: http://www.bikepartsusa.com/product_...&p=01%2D127543

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Old 03-03-07, 07:59 AM   #5
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Helllloooooo... the shim is there for a reason. To compensate for the crappy tubing. Put the shim back in and get an alloy post that fits insife the shim... or get a thick walled post & turn it down to fit.
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Old 03-03-07, 09:13 AM   #6
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UO-8 Rebuild

I had no problem putting a 25.4 Kalloy seatpost on my UO8. However I'd try to hone out the seattube if you can't get it to fit.
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Old 03-03-07, 09:20 AM   #7
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A good LBS or frame builder should be able to ream the seat tube enough to remove most of not all of that seam I'd think. My Gazelle's seat tube was a very very firm fit for its 27.2 post till I took it to a local frame builder. 15mins later with the park reamer and the exact same post slid smooth.
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Old 03-03-07, 07:14 PM   #8
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Posts and reaming seat tubes

The problem, as I stated earlier, is that, if you have a weld seam your reamer will not cut straight without a expandable pilot of some sort. If you use a standard reamer, it will cut to one side of the tube....the weld being harder than the tube and the weld seam being raised, it will force the cutter to the opposite side and cut the tube all goofy. On a non seamed tube, you can easily ream out for a tight fitting post. I have found alot of posts that were oversized and what should be done is, the post should be turned round and to the proper O.D. on a lathe. Alot of posts are also out of round too!
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Old 03-03-07, 07:23 PM   #9
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I've heard of using a file for this problem, just file down the ridge for the first few inches inside the seat tube. Seems to me that would be the easiest solution if you're going to use a non-shimmed post-
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