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Seat Tube Lug - how to re-round it?

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Seat Tube Lug - how to re-round it?

Old 09-21-18, 03:17 PM
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mrv 
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Seat Tube Lug - how to re-round it?

Got a circa '76 Schwinn Super LeTour 12.2. When I picked up the frame I did notice the paint was chipped / cracked below the seat binder slot, below the seat binder bolt. The frame looked OK from the outside, just chipped paint.
At home my calipers are telling me the seat tube lug is no longer round: 26.6mm across the frame (left - right); 26.2~26.3mm front - to - back. It looks like over the past 40 year the seat binder bolts was over-tightened. .... surprise.....
- Can I make this hole round again? And how?
I've purchased a 26.4mm post. I think with a great deal of enthusiasm I could work the post into the seat tube. Is there a better way? Smaller seat post? Some cool tool I don't have that I need? A 1in drift?
https://www.homedepot.com/p/Klein-To...-100647957-_-N
I can add a photo.
Thanks.
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Old 09-22-18, 02:01 PM
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machined steel mandrel, Tapered?
(that looks a bit too steep a taper, but you can try it.. )


or if a standard ID like 25.4 ie one inch , I hammer in a steel tube seatpost..
[found a short one for BMX bikes] for a larger ID, use some shims
from like soda cans to get close to the needed ID.. wrapped around it.
Of course you are measuring things , right?
Leave a way you can hammer it back out.. via a slap hammer bolt thru the post ?


when close, maybe get a seat post 0.2 or so mm larger,
and use an adjustable hand reamer to shave 0.1mm of wall thickness out until this new seatpost fits
Use Cutting oil on the blades, and take your time.

I have done the reaming, shop tool ..
My 27.0 mm ID RB1 frame seat tube now, precisely, takes a 27.2mm seat post..





....

Last edited by fietsbob; 09-22-18 at 02:09 PM.
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Old 09-24-18, 10:01 PM
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I prefer a slightly smaller steel post to do the rounding work. Step up to a slightly bigger size as it will fit. Tapered shafts with so larger a taper only work on the fat interface of the tapered shaft. Not the whole depth of the distorted lug. Reaming is a next to last step, removal of wall is to be avoided at all costs. This type of repair is one where less is more.

But I wonder if the distorted lug is holding the current post well already. Wishing for a perfect post/lug/tube fit is great but if the post doesn't slip then why fix something that isn't a functional issue? If you're just leaning that's cool, the frame is a pretty cheap one to mess with. Andy
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Old 09-25-18, 05:58 AM
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Thanks @fiestbob & @Andrew R Stewart

Originally Posted by Andrew R Stewart View Post
I prefer a slightly smaller steel post to do the rounding work. Step up to a slightly bigger size as it will fit. Tapered shafts with so larger a taper only work on the fat interface of the tapered shaft. Not the whole depth of the distorted lug. Reaming is a next to last step, removal of wall is to be avoided at all costs. This type of repair is one where less is more.

But I wonder if the distorted lug is holding the current post well already. Wishing for a perfect post/lug/tube fit is great but if the post doesn't slip then why fix something that isn't a functional issue? If you're just leaning that's cool, the frame is a pretty cheap one to mess with. Andy
I got the frame with no post, so I purchased a new Kalloy UNO at 26.4mm. I am pretty sure I can force it into the frame, but then I'll end up with zig-zag patterns all over the tube, something I want to avoid. If I can open things up a bit (0.2mm on the diameter) to get the post to go smoothly, that's what I'm after. (roger that on the tapered tools ~ I was looking at those adjustable chucking mandrels or some kind of expanding arbor. $20~$30..... not sure. My machining experience was long ago...)

I don't want to remove any wall thickness. In this case, less is more: less work = more safer.
I'm going to check out a tool shop here at work to see if they have anything I can gently move some metal with. I don't have a steel seat post available, and as you mentioned, it's not an expensive frame. I want to ride it and beat on it, not turn it into a show piece. (If I had the cash for an All City Nature Boy I would have gone that way for a steel single speed.....)

Last edited by mrv; 09-25-18 at 06:06 AM.
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Old 09-25-18, 06:27 PM
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Andrew R - I took your advice about the BMX seat post. All I had on hand was black pipe. My MITUTOYOs said the black pipe was between 26.6~26.9mm
So I filed the square edges into a slight taper and used a couple feet of Emory clothe to polish things a bit.
I tapped the pipe in an inch or so, removed, rotated 90 degrees, tapped in again. I didn't seem to split any steel, and the 26.4mm fits smoothly, all 350mm of it.

I'm calling it good until I get my handle bar and I can take it for a spin.
Thanks for the help, Gents!



1 Seat binder bulge. Saw this before purchasing, but no cracks. No change after “repair”
2 Bike with seat!
3 26.4mm seat post installed
4 Black pipe filed and abraded



Last edited by mrv; 09-25-18 at 06:32 PM.
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