John Barry wrote:
> Presuming that you had a frame that you knew was
> either made of 531P, or 753 (spec'd with the same tube
> wall thicknesses), would there be a way to tell which
> it was?
The tubes may have a stamp that could possibly tell the tale. Certainly if a stamping is visible and it says 531, then the question is answered. The stamp is within a couple inches of one end of the tube - the end with the shorter butt. This can be either end of a top tube or down tube, but will always be at the bottom on a typical, single-butted seat tube. Look for lettering running in a circle around the circumference.
Lots of factors can prevent the stamping from helping though, including:
1: Stampings too faint to see through paint - or even with the paint off - quite common.
2: Stampings may have been cut off when the tube was shortened
3: Newer tubes (starting late 80s maybe?) got some sort of electronic marking instead of a stamping - those markings were surface only and would be utterly hidden by paint -- even if they weren't already removed by sandblasting or acid-etching, which they usually are.
4: The stamping, even if visible, might be inconclusive. Often they just said "REYNOLDS BUTTED" - and nothing else. "REYNOLDS BUTTED" occurs more often on 753 tubes in my experience. 531 tubes are more likely to say "REYNOLDS 531 BUTTED 21/24" (the numbers at the end will vary - that's the tube thickness in BWG units) Still, lack of "531" doesn't necessarily mean it's 753.
If there's no conclusive, visible stamping, then there is no non-destructive way to tell for sure, that I know of. 753 tubes are just 531 tubes that have an extra heat-treatment to make them stronger, so not even a chemical analysis will help: They are the same alloy.
If you really needed to tell for some reason, a thin strip of the frame tubing could be cut out and subjected to a hardness test, usually Rockwell or Brinell. You'd want to choose a part of the frame away from the heat-affected zones, so as to be in the as-delivered heat treat state. Those hardness tests can't be done on a tube that's still attached to a bike frame, that I know of, so the frame would be ruined.
A tensile strength test would also tell the tale, but those are harder to do (and are also destructive). A hardness test would be the easiest way to answer this question. Hardness correlates very well with strength, so once you had a Rockwell number, you could deduce the strength, and then compare that to the claimed strengths of 531 and 753 in the Reynolds catalog -- adjusting those catalog numbers down a bit of course, because they lie...
Mark Bulgier
Seattle WA USA