I'll throw in my two cents here. A typical single butted Columbus SL or 531 "C" seat tube with an outside diameter of 28.6 mm (1 1/8") and wall thickness of .9/.6 mm will have an inside diameter of 27.4mm at the seattube end. A 27.2 seatpin slides nicely into an unbrazed , raw seattube. Doing the math tells us that the .2 mm difference in diameter amounts to about .008", or about .004" radial clearance. I doubt that one could even enter a 27.4 mm pin into an unbrazed .9/.6 seattube. I always thought the 27.4 size was to suit a .8/.5 seattube, or a handy fix for an overzealously "prepped" .9/.6 tube. As an aside, when 853 became available, Reynolds specified a 27.2 seatpin for a .8/.5 seattube. Presumably the air hardening nature of the 853 wouldn't allow for the light clean up necessary to remove the brazing/ welding distortion even a careful skilled builder would cause. Another aside- anyone ever wonder what sort of seattube Raleigh Internationals used? Didn't they take a 26.6 or something? That would indicate ( with non- metric O.D.'s at least) a .9 or full 1mm wall. Double butted seat tube, maybe? And didn't Cinelli Corsa B's have a similar size?
Best regards,
Wes Gadd
Unionville,CT
spiney norman <nevilmanyeah@yahoo.com> wrote:
> I believe Columbus (?) KL tubing used 27.4 posts
>
> Mark Hoffman
> New Britain, CT
most handbuilt, silver-brazed columbus sl-tubed frames actually need a 27.4 seatpost and the same type of frame with an sp seat tube take a 27.2 because the tube does not ovalize much if at all with the careful, low-temperature construction. brass-brazed, production-type, overcooked frames with sl seat tubes use a 27.2 and sp a 27.0.
many if not most of the ill-fitting seatpost issues with high-end handbuilt columbus sl and sp frames come from the fact that some builders may not have been aware of need for the slightly larger seatpost.
'spiney' norman neville new wavin', ct
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