Snowbum's BMW Motorcycle Repair & Information Website

Snowbum's response (edited & expanded for this article):

It is due, probably, to the system of measurement the wheel guys used.  I'd like to make this fairly simple. Unfortunately it is not simple.  If it was METRIC SYSTEM for real electrical wire, there would be NO problem, ....THAT IS simple.  A direct link to the metric/American wire system method is here: https://bmwmotorcycletech.info/wires&codes.htm.  UNfortunately, this is not electric wire you are talking about.  It may be called wire gauge, but it is not electrical wire gauge.

There used to be many metric and American and other standards for all sorts of things.  Many "standards", either never became very popular for long periods of time; or, are either obsolete, and/or forgotten ....or misused, ....or whatever else.

Some things use a combination of metric and inch sizes/measurements.   A common instance are spark plugs (14 mm size, but length in inches, for Airheads, 3/4").  Another common instances is wheels.  Wheel rim sizes are not metric, but in inches.  Thus, at times, there are multiple systems of specifications for something and I do NOT mean metric converted to inches or vice-versa.

This sort of thing also involved drills, threads, sheet material, etc.  Piping for plumbing was an example.  Slowly things consolidated, but are still rather messy at times.  I'll try to explain your 9 ga spokes.  Note that there are British and American versions of spoke specifications, and they may be combined, but it all should be clear by the end of this article (?).

From not all that long after America's founding, there were numerous methods/charts, etc., for various measurements of things.  As the Industrial Revolution proceeded, things got more standardized in some areas, yet more complicated in other areas.   So-called "wire gauge" could refer to electric wires or to taps and drill bits (MADE from thick wire! ...but you can think of it as ROD-WIRE!), or to sheet and plate steel or iron.  RE-read that last sentence!!   The British measured things in a form of inch measurements.   All this is very simplified here.

By 1900, give or take a few decades, each major manufacturing country, if standardized much, had many specifications, which finally got consolidated, more or less into three as it is NOW.   There are variations on the three ...a whole lot of variations.  There are at least two thread forms in very popular use in the metric system, ISO being the commonest now.  I won't get into all those systems, just some basics in this article.

YOUR wire (spoke) is 4.01 mm by your measurement. That is 0.1579 inches. Assuming that your measuring is accurate, you are possibly measuring the basic spoke size plus any PLATING on that spoke, if there is plating.   That is a MAYBE. Certainly spokes can vary, and there is NO specification for tolerances, any "commonly accepted tolerance" is not to be had; or, hard to find.

For what we typically think of as 'wire', 9 gauge is 0.114".  For drills and taps, etc., that are made from what we might think of as thick wire rod, 9 gauge is 0.196 inch.

But, what about your 4.01 mm (0.1579")?
There are TWO probable answers.  I will describe them one at a time:

(1)  9 gauge sheet and plate is 0.156", and that is the standard they used for the spokes.   ...naw, don't ask me why....because I don't know why a sheet and plate standard is used for spokes.  No idea at all. OAK might have known, with his rolling mill background, but I failed to ask him before he died. Perhaps Buchannan's would know.

(2)  The other and, in my opinion stronger and most likely possibility ....is that the spoke is BSC, British Standard Cycle, mostly obsolete, but still in use. There is a size that is 0.156" in diameter. The threads for this standard are 60 degrees, NOT the more common 55 degrees.

NOTE carefully! ...many seemingly knowledgeable websites on the internet will not have accurate charts for BSC, which is also known as BScy. Charts may mix up another standard, the CEI, which stands for Cycle Engineers Institute, and even call them the same. Thus, it is entirely possible that an Internet search could show that there is no 32 pitch threads in your size, but only 40 turns-per-inch threads. Those charts are WRONG, simply picking up prior WRONG work and duplicating it.

Here is an accurate chart.  Note the second item from the top, note the 0.1563 size, and note the 32 Tpi.

Below is a listing for motorcycle and car wheel spokes  .....notice TWO things that are a bit strange!  FIRSTLY, as the gauge goes down in numerical number, the size INcreases. This goes right along with American/SAE electric wire sizes. The second thing is that your spoke of so-called gauge 9 is shown as gauge 8.  That confusing issue is because spokes are sometimes made in DUAL diameters.  This is not all that uncommon, when made of Stainless Steel.  So:  You CAN buy spokes that might be sold as: 8/9 gauge.

If your spokes were 32 threads per inch, you likely have BSC specification spokes.  You could also measure the threads angle.

Of course, the folks that gave you the bill showing 9ga might have made an error!

Spoke Gauges are as follows:
12 = 2.60mm (.104” dia)
10 = 3.20mm (.124” dia)
9 = 3.60mm (.142” dia)
8 = 4.00mm (.156” dia)
7 = 4.45mm (.171” dia)
6 = 4.87mm (.192” dia)
5 = 5.15mm (.203” dia)
4 = 5.70mm (.224” dia)