Why Do Metric Toolsets Skip Sizes?

Factual Questions
1
      • I have noticed this long ago, and realized it again at a hardware store today, pondering buying another set, because the set I had did not cover all the sizes within its range. While S.A.E. sets of sockets and combination wrenches tend to start at a small size and hit every size all the way to the largest size (say, 1/4-inch, every 1/16" up to 1"), metric sizes NEVER do this. They ALWAYS leave a few out. And it’s never the same few, but varies from set-to-set, depending on the manufacturer or product. At least one of 15mm, 16mm and 17mm seem to be commonly missing, and in the US at least–15mm is a very common size used. Another that is frequently gone is at least one of the 9- through about 13mm’s, which are also common. What gives? It doeesn’t seem to matter if the toolsets are el-cheapo’s or expensive ones. Is there some secret law preventing companies from selling complete sets? Does anyone make a cheap set that runs from 8mm up to about 25mm or so, and that doesn’t have any sizes missing?
        ~
2

WAG, that you’re being sold as a ‘metric toolset’ is in reality a partial set. Enough Americans know little enough about the metric system to be content with what they’re sold.

3

My meteric wrenches and sockets are Craftsman and my sets always included every mm size up to 22. I added specific sizes I needed above 22. I don’t recall ever using the 16mm or 18mm but that only applies to water cooled VWs.

4

Way back when I used to drive a Beetle, I remember spending $10 for a 36mm socket that was needed to remove the rear brake drum.

5

Japanese motorcycles and recreational stuff tends to use the 8, 10, 12, 14 17 and 19mm wrench/socket sizes for hex headed bolts. I believe that European motors use the same sizes except that 8mm bolts have a 13mm head instead of a 12mm head like the Japanese use. I think that the European standard carries over for US stuff.

The only time that I have needed a 15mm wrench was on a bicycle (for the crank arms, but it may have been used elsewhere as well.) This was on Shimano equipment, so I assume that it is the standard size. Also, I ran across one off-brand sparkplug that had flats 18mm across. I don’t remember ever needing a 9, 11 or 16mm wrench.

6

The bigger an object is the more gap there must be for a given tightness of fit between the shaft (the nut in the case of a wrench) and the hole (the wrench opening).

For example, for a Class 1 or “loose fit” on a 3/16" wrench the gap can be as large as 0.003. For a 1" wrench the gap can be as much as .009" (Limits of Cylindrical Fits, Engineering Drawing, T. E. French). With an 8-point socket wrench for general use around the home there is no need to have both an 24 mm and a 23 mm because the bigger one will engage the corners of the nut satisfactorily. For a mechanic who might need to really torque something down tight, both sizes would be recommended. For the smaller sizes, since the gap must be less for the same fit both a 7 and an 8 mm, for example, would be needed.

7

I’d say it’s a holdover from the days when one virtually never needed certain sizes. When I first started working on imports (30+ years ago), I NEVER saw any nuts/bolts with a 16 or 18 mm hex. For ages I only saw one 15 mm application (VW steering rack). And most wrench sets excluded those sizes, the notable exceptions being from Snap-On Tools, which tends to include every size. Then the American car manufacturers decided to use metric hardware, and for some reason felt compelled to use 15 and 18 mm hex nuts and bolts (why follow the rest of the world :confused: ).

Nowadays, the Asians tend to use 7, 8, 10, 12, 14, 17, & 19 mm hexes. The Europeans tend to use 6, 7, 8, 10, 13, 17, & 19, with the occasional 11 and 15. The Americans tend to use 7, 8, 10, 13, 15, & 18, with the occasional 16. I’m sure I’ve used my 9 mm wrenches a few (very, very few) times over the years, but I can’t remember the application.