# Is a 3/4 wrench and 19mm interchangeable? ?

I’m not sure if this bolt is metric or not. My 3/4 socket has just a little looseness on the head. My 19mm is also just a little loose.

11/16 won’t fit. pretty sure it’s 3/4 SAE

my google calculator says .75in =19.05mm are those two wrenches considered interchangeable?

any other SAE and metrics that are this close to a match?

Perhaps one socket or the other is a cheapie ?

Anyway … if the bolt is any good , it won’t get damaged by this issue, unless its like torqued up to some huge degree ?
Most 19 mm bolts will be left at some low torque.

If you need to put a lot of torque on it, you might be moving to a larger breaker bar system… eg the arm you use for the tyre of a car …where you get a 2nd opinion on the 19mm

Yes 19mm and 3/4 are pretty much interchangeable.
You say they are loose and 11/16 doesn’t fit. Have you tried 18mm? It’s a tad larger than 11/16.
To answer you other question 5/16 and 8mm, 1/2 and 13mm, 7/8 and 22mm, 1" and 25mm are all interchangeable.

A Vise-Grip can replace any of them in an emergency.

1 inch = 25.400mm, so doing the math, 3/4 inch = 19.050mm.

.05mmm is less than two thousandths of an inch - I imagine the tolerances in the manufacture of consumer grade socket sets is on a similar order of magnitude.

This is correct, and you can leave out the “pretty much.” The two sizes are so close that there’s no practical difference. Many American makes use 18mm bolt heads pretty extensively. I’m not sure what “a little loose” means here – all sockets have to have a tad of looseness or they wouldn’t fit on without being hammered. If an 18mm fits, it should be used; if it doesn’t, then it’s got to be 19mm-3/4".

I’ll disagree on the 1/2"=13mm – 1/2" is more like 12.5mm. I’d add 15/16" and 24mm to the list.

Sorta close is 11/16" to 17mm and 7/16" to 11mm. Those inch sizes are a bit bigger than the metric ones, so the fractional wrench can usually be used on the metric bolt heads, but not vice versa – the metrics won’t fit onto the fractional bolt heads.

You may end up stripping the head of the bolt trying this.
When in doubt, use the right tool for the job

Usually you can distinguish SAE bolt heads from metric ones by the markings. Typically SAE have lines, metric have numbers. These relate to the strength of the steel.

And, if the fastener proves stubborn, can make you wish you’d taken the time to find the proper wrench.

Seems pretty unlikely. There’s 0.05mm difference. one twentieth of a millimetre. That’s about half the thickness of one sheet of copier paper.

He may have been referring to the “sorta close” ones I mentioned in post #7. You’re right that there’s no concern whatsoever when it comes to 19mm-3/4".

You got me curious, so I wrote a little Python script to check all sizes in millimeters from 1 to 26 against multiples of thirty-seconds of an inch. All of the following are within 1% of being the same size:

4 mm approximates 5/32 in.
8 mm approximates 5/16 in.
12 mm approximates 15/32 in.
15 mm approximates 19/32 in.
16 mm approximates 5/8 in.
19 mm approximates 3/4 in.
20 mm approximates 25/32 in.
23 mm approximates 29/32 in.
24 mm approximates 15/16 in.

Of those, only 23 mm. and 29/32 in. (differing by 0.08%) are a closer match than 19 mm and 3/4 in. (which differ by 0.26%)

Well 1% on a larger fastening is not likely to be as critical as on a smaller one, despite it being the same ratio.

Materials deform ever so slightly when using torqueing tools, and on small fastenings, that multiples the difference.

I’ve seen spoke keys where the difference of 0.127 to 0.130 to 0.136 is of real significance (3.23mm,3.3mm,3.45) and you will get small variations of these sizes due to different manufacturers of just 0.05mm which is enough to cause a problem, especially when spokes have either cold seized or oxidised in place. Spoke nipples do tend to deform a bit when torqued, so a small variance in the key can be significant.

I finally bought a metric wrench set a couple years ago. Still trying trying to get a feel for the sizes. This was the first time I realized 19mm and 3/4 were the same.

I may eventually have to buy a 18mm. my 10 piece metric wrench set didn’t include that one.

5/16 and 8mm is a difference of .3125-0.3149 = .003 have been the only wrenches I’ve used interchangeably.

Hit or miss on anything else, cheap bolts + cheap wrenches.

No one uses 19 mm.

To the OP - I guess what I’m most (impressed? confused? in awe of?) with is the patience you apparently have for this project.

So:
Working on some project.
Have a bolt you’re not sure of the size of.
Try a couple different size wrenches.
Some are really close but maybe not quite perfect.
Not sure if they’re a good fit.
Get on your computer, post to the SDMB.
Have no idea how long it’s going to take to get an answer.
Go back to your project hours or days later and unscrew the bolt.

You’re certainly eager to do things exactly right, I suppose that’s impressive in its way.

Personally, I can’t see myself doing that - try a couple wrenches, whichever one fits best, crank away - done in a few seconds.