The nuts & bolts of metric spanner sizes

[cpedw]

I am the proud owner of a set of metric combination spanners and a metric socket set. I've had these for some years; bought without much knowledge but advised by staff of a traditional tool shop. I've often had cause to wonder about the sizes that are missing from these sets, but only recently had the time to do some serious research into what I've got and to wonder why.

I started with Wikipediea (where else) to find a list of preferred metric thread sizes http://en.wikipedia.org/wiki/ISO_metric_screw_thread
Then a table of nut sizes matching the preferred threads http://www.fairburyfastener.com/xdims_metric_nuts.htm

Here's a truncated extract from that table:

Nominal..... Max Width
Size.......across flats (mm)
M3..........5.5
M4..........7
M5..........8
M6..........10
M8..........13
M10.........16
M12.........18
M14.........21
M16.........24


So I can't understand why my shiney spanners include 6,11,12,14,17 and 19 mm but don't have 5.5,7,16 or 18. I'll excuse them 21 and 24mm as exceptional sizes.

The socket set is better but still odd. The smallest is 8mm but it includes 11,12,14,15,17 and 19mm. It lacks 18mm.

Does anyone know why there's such a mismatch between my tools and conventional sizes? I'm accustomed to "suck it and see" spanner sizing followed by adjustable spanner then Mole grips or pliers depending on what's to hand at the time.

Looking forward to an interesting explanation ...

Derek

[marisca]

The 6 fits M3 and also 2 or is it 4BA?
17 does M10
11 does 7/16"AF, 14 does 9/16"AF, 17 does 11/16"AF, 19 does 3/4"AF
12 does those whitworth nuts on Blakes' seacocks.

Can't you find something more serious to worry about?

[wully]

Why didn't they call the bolt that fits an 13 mm spanner an M13 , 8mm and M8 etc?

Did the French invent them or what?

[DaveS]

I think the situation is less uniform than the table suggests. Dismantling car suspension provides some good examples of where a nut requires a spanner size different from that needed for its associated bolt head. I find that a 17mm spanner fits quite a lot of nuts - apart from those that need a 16mm or an 18mm! I'm sure there some system behind this apparent confusion but I don't know what it is.

[claymore]

I think the situation is less uniform than the table suggests. Dismantling car suspension provides some good examples of where a nut requires a spanner size different from that needed for its associated bolt head. I find that a 17mm spanner fits quite a lot of nuts - apart from those that need a 16mm or an 18mm! I'm sure there some system behind this apparent confusion but I don't know what it is.

Love it - does that mean you can get a 17mm spaniard to fit a 19mm knut?