One inch equals 25.4mm. The math is easy with a handheld calculator. If you start with inches, say 0.020", multiply by 25.4 to get the metric equivalent in millimeters, 0.020"=0.508mm. If you are starting with a metric value, say 1.35mm, divide by 25.4 to obtain equivalent inches, using a calculator. 1.35mm = 0.053"

Click the link to see an online conversion site that has been helpful for me.

Here is a link that might be useful: Convertible

1MM = .03937"
Call it .040"
So a plug gap of .75MM would be .030" etc.

Well, Thanks, fellers! Very helpful! But----clear as mud to me!
I guess my use of U.S. measurements over the years has removed any chance of changing to metric now!
But, i'll keep the link you sent, and convey it to my friend, who is as much "in the dark" about metric measuring. And, i'll continue on using my US tools to reset spark plug gaps, and points! And, the cardboard strip for coil gaps!

Pocket calculators are cheap, reliable, and powerful. But like any tool, you have to use it a while before it becomes comfortable second nature. I use mine to calculate miles per gallon on trips, and a few other routine chores. It seems easy to me, because I can't remember what it was like to learn how to use the thing, back in the 1960's. But I can remember learning how to use a computer, back around 1988, and that wasn't easy at all!

rusty if buy new feller guage it have mm on guage I think it was 25 years ago they start putting mm on guage blades.

Bit Rusty is an old feeler!

Sounds like the pot callin' the kettle "black"!
Bill, i did auto body work most of my working years, about 60 years or so, and we didn't get into the metrics, until we had to buy new sockets and hand tools to work on the newer vehicles. I didn't do engine repairs, or any of that kind of mechanical work when i was employed, just the removal of body parts, etc., and when they went to metrics, i had to buy a bunch of metric sets of hand tools, such as sockets and wrenches, etc.
And, thats why i'm having trouble with metric measurements for spark plug gaps, point gaps, and the like.
At 84 yoa, it is mind boggling! I accessed a web site that somebody here gave me, but its all greek to me. I'm going to let my friend look at it, so he'll understand why i told him to "Eyeball it" and don't worry!
I can download some facts from one site, so if my friend wants it, i'll do it. But, when i tried to understand it---my mind did a flop, so i gave it up!
Too old to change now, i am!

It's really not that hard-
1MM = .040"
.75 MM = 75/100MM = 3/4MM = .030"
OR .040" times .75 = .030"

.8MM = .040" times .8 = .032".

.6MM = .040" times .6 = .024".