Originally Posted by
3alarmer
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...from prior experience, I would have guessed 26.6, but there are several things going on here. One is that posts really only came in designated intervals of .2 mm from SR in the 70's. At least that's all I've ever seen. The second is that often they can be off by .1mm. So sometimes you can fit several seat posts in a frame and discover that one seems to fit better than the others. Lastly, the final finishing work on a frame includes reaming the seat lug and the seat tube, to get a nice, even, cylindrical fit for whatever post is fitted to the bike. Sometimes, the manufacturing process distorts the seat lug enough off round, that reaming the thing (and the top 8 or 10 inches of the seat tube) a little oversized is the easiest and simplest fix.
So your bike might have left the manufacturer with a slightly larger post.
I use a lot of LaPrade posts in my own French bicycles here, because they are relatively common in the used parts stream, in sizes that fit French metric tubing, like 26.4 and 26.6. There are plenty of fluted ones around, and many of them use a single bolt from underneath setup. Personally, I think the ones with side plates that clamp onto the saddle rails are a little more solid, once installed, but they all work pretty well.
I've never found online charts of seat post sizes of particular help in all of this. By the time a bicycle gets to me from the 1970's, it has had many adventures, some of which can affect seat post fitting.
That's some interesting information there. Of course, it's a bit naive of me to assume the seat posts would be all manufactured with total precision. The reason why I decided to ask people for the measurements of their seatposts is primarily so I can make an "educated guess" as I couldn't find official information anywhere. Looking at the seat post today, I think the top of the seat tube slot / cut out goes a bit wider than the bottom (will have to measure that later). That would mean the seat post itself might be a bit too wide. Attached the photo (red bike).
I have SR CT-P6 with side plates type of saddle rails clamp on my other bicycle. Or something similar, it's not branded, but they sometimes used stickers instead of etching. I didn't have to adjust it even once last year with the saddle and bicycle in daily use. I sort of like this type for aesthetic reasons as well, the look is more unique. I'm wondering if any Italian brand used similar design.
