A person doesn't get very far on vehicles these days without metric tools. When I had my tool business, inch sized tools were on the wane excepting in the heavy truck, equipment & agriculture equipment fields. Not that people weren't buying some but the bias was in the metric direction for both new tool aquisition and replacement.
Not disputing j-d's posts at all but simply making an observation. In the larger sizes the sizing becomes so close to identical that it doesn't matter so many of we old timers are light in the metrics above one inch while the young guys are often the other way.
For the home hobby type wrenching, it might be worth while to emphasize j-d's excellent point regarding the ability of 13/16" to make a tight fit onto 21 MM as this technique can be very useful in both directions in dealing with rusted or partially rounded heads. One can often tap a smaller sized socket into place to deal with a damaged head.
Another trick which should be part of everyone's tool kit is to use a tin of valve lapping, sometimes called "valve grinding compound" which is carbide grit in a water or grease carrier. Go out to your local auto supply store and buy a tin! Also buy some blue Loctite as is at least as useful.
The valve lapping compound is a tremendous help in dealing with damaged screw heads, rounded bolt/capscrew heads, especially good when one has to use an open end wrench. The additional grip is equal to spreading sand onto an icy sidewalk. Seriously! Get some as a tin will last forever in normal use.
Have a tight screw in a plastic light housing or dash panel? Push hard enough to make the Phillips screwdriver grip and you will likely break the plastic. Dip the screwdriver into lapping compound and no problem to get enough grip without hard push.
How about all the different "Phillips type" screw heads? Add to that the likely that you don't own professional quality screw drivers. Use lapping compound. Done!
Brilliant when used on power screw driver or cordless drill bits.
Loctite (blue) is usually misunderstood as something which makes things very difficult to remove but on new threads only adds about 10% to the break away torque (the effort needed to start the screw turning). On used threads, especially when there might be some corrosion, the Loctite prevents corrosives from entering the threads and so often makes for easier removal.
HIH
