Happy New Year!
Craig, several others have discussed this in the current thread but here goes again...
Seat tube lugs are heated to braze both the top tube and the seat tubes. Later they are reheated to attach the seat stays. It's fairly easy to overheat the lug causing it to warp out of round, or it could have been somewhat out of round to begin with! Additionally seat tubes frequently bulge inside the seat lug.
The fix for the problem is simple, drive a proper diameter steel pin into the top of the seat tube to remove any bulges and round out the seat tube. Afterward the seat tube should be reamed to the proper size.
This frequently wasn't/isn't done! It's easier to use an under size seat post than properly fix the problem!
I've seen pictures of finished brazed frames having the head and seat tubes cut to size on special machines or old lathes.
Here's a VAR Mondiale machine that could be used for all of the final operations.
http://www.sheldonbrown.com/
A year ago I bought an all Campy 1972 Gitane Super Corsa. When I went to adjust the seatpost height something didn't look right. I pulled the Campy seatpost and it was 25.8mm diameter instead of the correct 26.4mm size for the most common metric tube Reynolds 531 frames.
The inside of the seat tube was so bulged and the lug so out of round that the 25.8mm seatpost was all that would fit. It appeared to have come from the factory that way.
Most of the bikes or frames I've ever worked on looked as if the only reaming done to the seat tube was to remove excess paint during final assembly!
I suspect that a lot of bike manufacturers had several sizes of seatposts on hand and used whatever fit the seat tube!
I have a collection of 1/2" drive sockets that I use to remove bulges and round out seat lugs when necessary. I made a simple slide hammer out of 1/2" all thread rod and some nuts and large flat washers to drive the sockets in and slam hammer them back out. Afterwards I ream the seat tube to size with an adjustable reamer and then smooth the ID with a brake cylinder hone and in a hand drill.
AS far as the 2mm wall thickness on the Bates seat tube, that doesn't sound normal for Reynolds 531 tubing? Gas pipe bikes didn't usually have tubing that thick? But who knows?
During the 1970s Reynolds 531 Straight Gauge seat tubes came standard in the following wall thicknesses:
20 gauge (0.91mm or 0.036") 21 gauge (0.81mm or 0.032") 22 gauge (0.71mm or 0.028")
Ishiwata, Tange and some others used straight gauge seat tubes with 1.0mm wall thickness and carbon steel gas pipe tubing ranged from 1.2mm to ~2.0mm.
Concerning 26.2mm seatposts there are several other possibilities to consider: the seat tube may have been installed upside down with the thicker butted end in the seat lug, or... They used a double butted down tube instead of a seat tube! It happened!
Chas. Colerich Oakland, CA USA
cmontgomery15@cox.net wrote:
> When this thread started I went out to check the seatpost sizes on my bikes and noticed something. My '38 Bates has 531 Plain Guage with a 26.2 seatpost as does my '70 Jack Taylor. When I took out the posts to check their sizes I noticed that the wall thickness of the Bates is about 2mm whereas the wall thickness on the Taylor is almost paper thin. Difference in manufacturing Plain Guage or is it something Norman did to the tubes? And why?
>
> Craig Montgomery
> Tucson