On Tue, 3 Dec 2019 13:32:49 -0800 (PST), TimR
wrote:

On Tuesday, December 3, 2019 at 2:39:46 PM UTC-5, Ralph Mowery wrote:

Does a bolt really have much more strength in size 21 than a size 20 in
metric ? I would think that as wrenches/bolts reach around size 12
metric they would go up in steps of 3 to 4 to gain enough to make them
worth while to make.

I'm with you. A lot of machines could use one size bolt versus 4 or 5. Some would be stronger than necessary but it would make repair a lot simpler.
A toyota car uses 4, 6, 8, 10 and 12 mm thread sizes with 8, 10, 14
and 14mm heads for almost everything, with the occaisional larger
bolts where required. You would not want to use a 10mm bolt where most
of the 4 and 6 mm bolts are used, I can assure you!!!!!