This post isn’t necessarily about the SBDU, but I thought I’d write it as I get asked regularly to confirm seat pin sizes, or to try and decipher what an SBDU frame might be made from based on the seat pin and tube size. After collecting several SB frames over the years, I’ve seen lots of different seat pin sizes used in Reynolds frames. I have seat pins ranging from 26.6 to 27.4 – and 753 frames with 4 different size pins. That demonstrates just how varied their tube gauges were. So here is a little post that lists some of the most common seat pin sizes and the associated Reynolds tube gauge (wall thickness).
Although you will hear the term ‘Double Butted’ used when people talk about steel frames, the seat tube of a frame is typically ‘Single Butted’, with the ‘butt’, or thicker and stronger end of the tube fitted into the bottom bracket shell. For a 531c frame, the seat tube is single butted and measures 0.8 mm/0.5 mm – the end that fits into the BB is 0.8 mm thick and the end of the tube that accepts the seat pin is 0.5 mm thick. It can also be represented in the Standard Wire Gauge (SWG) notation of 21/24.
I’ve seen Reynolds give wall thickness dimensions to varying degrees of accuracy in their documentation, so 0.5 mm has also been given as 0.55 and 0.56. If you look at SWG conversion charts, it is also given as 0.559; but for the sake of simplicity, I’m sticking with 0.5 mm.
So how does tube diameter and gauge relate to seat pin size?
A well known example is an Imperial size 1 1/8″ (28.6 mm) outer diameter seat tube, with an internal single butt of 0.8/0.5 such as Reynolds 531c – this seat tube requires a seat pin size of 27.2 mm.
But I’ve always scratched my head thinking about how that actually works out using basic maths!
The seat tube outer diameter “28.6” minus twice the wall thickness “(2 x 0.5)” equals “27.6 mm” and not “27.2 mm”
The result is out by 0.4 mm
Another well known example is the original 753 which is a metric size 28 mm outer diameter seat tube with an internal single butt of 0.7/0.3 – this seat tube requires a 27.0 mm seat pin.
But again, the basic maths doesn’t add up.
The seat tube outer diameter “28.0” minus twice the wall thickness “(2 x 0.3)” equals “27.4 mm” and not “27.0 mm”
Again, the result is out by 0.4 mm
0.4 seems to be a constant. So here is a list I’ve made using the constant (0.4 mm) applied to a range of tube diameters and gauges…
I wouldn’t ever say that this size guide is always going to be 100% accurate, and it shouldn’t be used on it’s own to determine frame tubing. This is because tubes and seat pins have +/- tolerances, seat lugs get mis-shaped and tubes get reamed! Frame weight and frame features also play an important part in helping to determine what an SBDU frame may be built from.
But hopefully, as a guide, it may be useful.