Hey so I got my reynolds 531 bike built up, on the sticker it says 531 butted tube, fork stays. Now how do I know if this is double butted or single butted. Did reynolds make 531 in plain guage, single butted and double butted. Or just double butted?

On a butted tubeset, the main triangle has double-butted tubes on the top and down tubes, but the seat tube is typically single-butted: the top of the seat tube is not butted, but as thin as the center portion of the tube. Example: A Reynolds 531 seat tube that's 28.6 mm in diameter takes a seat post 27.2 mm in diameter. The difference is a wall thickness of 0.7 mm, which is the thinner of the wall thicknesses (the thicker being 0.9 mm?).

Often, the quick-and-fairly-reliable way to tell if a frame is made of butted tubes is to measure the seat post hole, or the seat post; if the post is, in fact, not undersized. If measuring the hole, I take several measurements at different locations, because it's fairly common to find the top of the seat post has been ovalized somewhat, or a lot, if too small a post was fitted/clamped.

The other way to tell is to weigh the frame. I have several frames the same size, and one is plain gauge steel. It weighs from 500 to 600 g more than the ones constructed with butted tubes. That's not so much really, in terms of total bike weight, but it's about a fifth of the bike frame weight, so hard to miss.

Reynolds made all varieties of tubing: plain gauge, single-butted, and double-butted.

My 1972 Raleigh Super Course was built out of Reynolds 531 plain gauge tubing (not double butted). The models above the Super Course in that year (Gran Sport and above) were all double butted. I suspect that the 531 butted designation would actually specify plain gauge. What kind of frame do you have?

I use a 27.2 mm seatpost in my bike.

[Double] butted, which means, as noted earlier in this thread, what you might call "single butted" seat tube, and butts at each end of the top tube and the downtube.

Well, I don't think you can really go by seat post diameter. My Raleigh has a 26.4mm seatpost and the frame is plain gauge, as I've said.

You don't see to many Reynolds 531 DB that are not 27.2.

Great info!

Just a small correction. The 531 seattube is .9/.6. Not .9/.7. The OD is 28.6, subtract 1.2, (.6 x 2). The ID is 27.4. The correctly sized seatpost is .2 smaller. That's how you end up with 27.2.

For decades there was no gauge hint to Reynolds 531 on the transfer.
Later they use professional, and SL among other names to help indicate thickness but you still needed the "decoder ring" to know.

Add in that there were for a time more gauge options for the metric tubes and it all gets quite complicated.
Schwinn got in trouble in the 70's for the provided Reynolds 531 "double butted" transfer.
The solution was a unique transfer that helped clear up the "legal" claim. It can be identified by the Text all being of proper alignment and two green stars on either side of the 531 text.

As stated, seat tubes are single butted in the vintage era. (From time to time a frame got bade with an inverted seat tube, butted region to the top), Ooops.

Only some of the time.
Considering the metric options of .3/1.0, .5/1.0, .7/1.0 and you get lots of options with a 28.0 OD tube, 26.0 OD for the top tube.

+1 I recently had the time and inclination to measure the ID of one of my 531 ST with Starrett inside gauges (aka "snap gauges") and that's what I found: 27.4mm ID on a 28.6mm OD tube. Measurement taken about 30mm below the seat lug.

Makes sense as 27.2 post in a 27.2 tube would drag all the time IF no manufacturing tolerances got in the way. A wee bit of 'play' would work much better.

Aside from the seat tube, there is no possible reason for having "single-butted" tubing on a frame; not for weight, strength, or cost. The tubes are either butted at both ends (occasionally to different gauges, where those would be called TRIPLE-butted), or not at all, which would be called STRAIGHT gauge. Butting only 1 end, and leaving the other end thin would cause incredible weakness at that end.

I think the OP is unclear on the subject, and is just picking up that theoretically, triple-butting costs more than double-butting, ergo, there must also be a non-existent "single-butting". I would suggest a quick Wikipedia search on the terms "swaging" and "butting" of tubes.

Seat tubes and steer tubes are single-butted because they need to hold a post or stem at one end. Fork blades are "taper gauge" (a form of single butting) so the wall thickness remains constant after the blade is tapered. Head tubes are plain gauge. So the only truly "double-butted" tubes in a double-butted tube set are the top tube and down tube.

The point was that you need to figure in the .2 slop to make the post fit well. The post I replied to didn't consider that.

Tange #4 and Tange Mangaloy 2001S were both single butted tubesets, in that all three main tubes were single butted. There may have been more but these are two that immediately come to mind.

Edit: A bicycle with Tange #102 just surfaced in another thread. It's another tubeset with single butted main tubes.

While this is a good rule of thumb, there were double butted seat tubes. Tange used double seat tubes on most of their double butted tubesets (#1, #2, #3, Mangaloy 2001, 900, 1000, Infinity), Ishiwata used a double butted seat tube on 017 and True Temper SP also had a double butted seat tube.