Do Americans use metric system?

smutny · Sep 19, 2025

In keeping with the auto parts subject...

Recently, an engine freeze plug in my CAF unit's old Navy airplane tug cracked and leaked all the coolant out. It's a simple 1950's Chrysler engine. Pulled the old plug, carefully measured it in inches, went to NAPA (now this is an old school NAPA with people that actually still work on their cars) and the young lady behind the counter quickly said, "You need the 41.5mm freeze plug." Apparently metric is the go-to measurement in the car parts world now. Which makes sense with the international supply line that exists today.

BobB · Sep 19, 2025

MIflyer said:
Main use of the metric system by Americans is when we want to shoot someone.

One advantage the SAE system has over metric is that the screw sizes make so much more sense. If you are wondering what the screw is measure the diameter and count the threads per inch; with metric screws that don't work so good.

I keep nothing but SAE wrenches and sockets at the hangar and separate toolbox drawers for SAE and metric at home.

You have to be careful with torque wrenches to make sure you are reading the correct scale for those that have both. Some years ago the USAF went through eradicating all torque wrenches with Metric scales to make sure of that.

When you want to shoot someone, .30-06 is superior to 9mm.

MIflyer · Sep 19, 2025

BobB said:
When you want to shoot someone, .30-06 is superior to 9mm.

Uh, yeah well, rifle rounds generally are better than pistol rounds. .30 cal Carbine is superior to 9MM. By the way I just saw an AI video on Youtube about how US GI's recused a German nurse from rubble. The highly realistic faked B&W "photo" they were using showed the GI's walking through the rubble of a French city, M1 Garands on their shoulders and M1 Carbines in their hands; I don't think that is accurate. GI's tended to accumulate additional gear but both a Garand and a Carbine is a bit much.

smutny said:
Apparently metric is the go-to measurement in the car parts world now.

And I found out about that the hard way. My Ercoupe uses a special washer on the nosewheel, featuring "ears" that bend over to lock it down and a tab on the inner diameter that slides into a slot on the axle to secure it. I put a new wheel bearing on back in 1998 and could not get the axle nut to stay tightened. Finally got down, lay my head,on the ground, and looked at it real close. Turned out the new washer I had used had an inner hole that was too small and a locking tab that was too long. The result was that the washer sat cockeyed on the bearing, the tab pushing it sideways and the inner diameter resting against the bearing on one part and against the edge of the lip of the axle on the other. So you could tighten it up and it would stay tight until you rolled it a couple of feet. Took files and opened up the center hole and filed down the tab. Works great now, Then I found out that the washer is now described in metric terms, and obviously looks like the original ones but does not fit without those modifications.

Engineman · Sep 19, 2025

mikewint said:
Ah yes how well I remember LUCAS, Lord of the Dark

Joe LUCAS, aka Prince of Darkness!

Eng

SaparotRob · Sep 19, 2025

MIflyer said:
.. The highly realistic faked B&W "photo" they were using showed the GI's walking through the rubble of a French city, M1 Garands on their shoulders and M1 Carbines in their hands; I don't think that is accurate. GI's tended to accumulate additional gear but both a Garand and a Carbine is a bit much.

As a kid, I got the distinct impression from Dad that he spent most of the Second World War trying to ditch his Garand for the carbine.

MIflyer · Sep 19, 2025

SaparotRob said:
As a kid, I got the distinct impression from Dad that he spent most of the Second World War trying to ditch his Garand for the carbine.

I own one of each and I can well understand that. The Garand just plain feels heavy. It even feels a lot heavier than the No.4 Mk I Enfield, even though the difference is only something like a half pound. The Enfield is balanced much better as well, and I don't know if that is just a result of not having that Garand gas hardware up under the barrel or if bolt action rifles just have to be balanced better, or else the thing would fall out of your hand when you worked the bolt. But there is no doubt the Garand is the better and more modern weapon. Whether I could hit anything with it, I do not know, but I could fire it while running if needed. The first magazine load I fired from the Enfield I hit a pistol target on the 100 yard range multiple times, which I thought was very impressive, but doing that while running I'd be lucky to hit Planet Earth.

MiTasol · Sep 19, 2025

MIflyer said:
!!!!???!!! Must be nice to have a LASER INTERFEROMETER handy to measure such teeny tiny things! And you do not have to count a whole inch worth of threads. A quarter inch or so will do fine.

Saw where some guy said the US will never ever go metric. He said all those little screws used to secure switchplates in US homes do not have a metric equivalent.

I had one odd thing occur. I was replacing the voltage regulator on my Toyota and overtorqued a mounting screw. The regulator was secured using some rubber grommets and thus the screw did not get tighter and tighter but just broke. So I removed the remainder of the screw and went to a large, very old fashioned hardware store and told them I needed another. The guy measured it and told me it was 1/4-28. I was very surprised, since I figured it had to be metric. So I bought a 1/4-28 screw and took it home, where I found it did not fit the car. Turns out that the original metric screw got stretched out by the overtorquing to essentially the same as a 1/4-28.

The good thing with metric threads is that even with poor eyesight you can see the size of most metric threads. If it looks about 1mm it is 1mm because there are no "odd" measurements like 1 1/16 mm per threads. Even with the smaller threads if it looks about 3 threads per mm it is 3 threads per mm - because there are no 3 1/4 threads per mm.

MiTasol · Sep 19, 2025

MIflyer said:
I keep nothing but SAE wrenches and sockets at the hangar and separate toolbox drawers for SAE and metric at home.

You lucky blighter. I have to have both of those plus British spanners and they really knew how to screw things up.

BSF nuts are smaller than BSW nuts. Fortunately the BSF spanners fit one size smaller BSW nuts. Then they have the BA theads which is their equivalent of the US gauge series threads. No quality tool manufacturer makes BSF/W or BA spanners any more so I have to make sure I never damage them. In a jam I can often use metric spanners on BSF/W hardware and Aus (and a couple of other stupid countries) refuse to use the ISO heads on metric hardware so in Aus we must have a complete metric set rather than just the ISO sizes.

As an example ISO use 19mm heads/nuts but Aus uses 18mm heads/nuts and 18mm is a fairly good fit on BSF/W hardware. Not perfect but...

MiTasol · Sep 19, 2025

MIflyer said:
I own one of each and I can well understand that. The Garand just plain feels heavy. It even feels a lot heavier than the No.4 Mk I Enfield, even though the difference is only something like a half pound. The Enfield is balanced much better as well, and I don't know if that is just a result of not having that Garand gas hardware up under the barrel or if bolt action rifles just have to be balanced better, or else the thing would fall out of your hand when you worked the bolt. But there is no doubt the Garand is the better and more modern weapon. Whether I could hit anything with it, I do not know, but I could fire it while running if needed. The first magazine load I fired from the Enfield I hit a pistol target on the 100 yard range multiple times, which I thought was very impressive, but doing that while running I'd be lucky to hit Planet Earth.

The earlier Enfields were very poorly balanced and felt like they weighed way more than the later ones when in fact the difference was almost nothing.

swampyankee · Sep 19, 2025

MiTasol said:
What is so difficult in measuring the thread OD (same measurement as with SAE) and then the distance between each thread in mm? You do know that on all common (4mm dia and above from memory) they do not use anything but full or half treads per mm so it is not like you have to count 4.3mm per thread.

It is easier than counting the number of threads in an inch - especially on fine threads like 4-40, 8-32, 10-32 and 10-24. And metric does not switch from gauge diameters to mm diameters like SAE does.

Thread gauges are not that expensive.

SaparotRob · Sep 19, 2025

MiTasol said:
You lucky blighter. I have to have both of those plus British spanners and they really knew how to screw things up.

BSF nuts are smaller than BSW nuts. Fortunately the BSF spanners fit one size smaller BSW nuts. Then they have the BA theads which is their equivalent of the US gauge series threads. No quality tool manufacturer makes BSF/W or BA spanners any more so I have to make sure I never damage them. In a jam I can often use metric spanners on BSF/W hardware and Aus (and a couple of other stupid countries) refuse to use the ISO heads on metric hardware so in Aus we must have a complete metric set rather than just the ISO sizes.

As an example ISO use 19mm heads/nuts but Aus uses 18mm heads/nuts and 18mm is a fairly good fit on BSF/W hardware. Not perfect but...

That's screwy. The whole thing is nuts.

MIflyer · Sep 19, 2025

Then there are the Indian made metric and SAE box end and open end wrenches, in which they have solved the problem of making the two different standards by making them about halfway between the two nearest sizes and then stamping them as whatever they need to claim. I think I have managed to find all of those I had and throw them away.

MIflyer · Sep 19, 2025

swampyankee said:
Thread gauges are not that expensive.

And invariably cannot be located when you need them.

MiTasol · Sep 19, 2025

and invariably ten minutes after you cannot find them you go back to where you thought they were and they stand up and wave "gotcha" at y ou.

MiTasol · Sep 19, 2025

I have just come in for a coffee from working on a piece of Aus made tooling that has another form of stupidity on it. The bolts are 1/4 UNC (what the Yanks call SAE C) and full threaded and the nuts are the typical 1/4 UNC for a 7/16 tool but the heads are 3/8. And there is a tonne of room around the bolt heads so there is absolutely no reason to use a special bolt. I a,m replacing them with standard 1/4 UNC bolts with shanks.

StackedWings · Sep 20, 2025

swampyankee said:
The US does use the metric system, as all units' legal definition have been in terms of the metric system since 1893. See https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication1038.pdf

Agreed, but the U.S. also uses the Imperial system. Just consider how we named and retained the names of two of our most profoundly admired weapon calibers, the 45 ACP is never in polite society called a 11.43mm, like wise our venerable 50 Cal is never referred to as a 12.7mm. (At least among US aircraft aficionados, again, polite society). Both of these Imperial measurements came about way after your mention of the link to Special Publication #1038 which is dated May 2006. And I interpret the Mendenhall Order of 1893 and the even earlier Metric Act of 1866 (both U.S.) as allowing but not requiring the metric system. So this puts us back to the original purpose of this Forum, which is to ask which do you prefer. As a known curmudgeon pushing 80 I don't mind running against the wind and voting…..Imperial.

StackedWings · Sep 20, 2025

DerAdlerIstGelandet said:
Put men on the moon in a spacecraft whose guidance computers was making calculations using the metric system…

Probably true, I'll take your word for it. Just goes to show that a hybrid system may be the best in the world. Just saying….What was the ceiling height of a recent aircraft flight? As in how many thousands of Feet? Fired your 11.43mm pistol lately? (Said nobody, that would correctly be 45 Cal ACP). How about the plethora of 50 Calibers that decorate so many of our aircraft, or do you prefer COMBLOC 12.7mm? As I just said elsewhere, we're right back to the original purpose of this Forum, asking which system do you prefer? As a known curmudgeon pushing 80 and being the perfect poster child of that old dog that can't learn new tricks I still vote for Imperial. As a young GI in West Germany I was amazed at, and always adhered to that sign: Nicht Sprechen Mit Der Wagonfuhrer!

StackedWings · Sep 20, 2025

GrauGeist said:
View attachment 847615

I love it! Just shared this with my mechanic buddies.

SaparotRob · Sep 20, 2025

StackedWings said:
Agreed, but the U.S. also uses the Imperial system. Just consider how we named and retained the names of two of our most profoundly admired weapon calibers, the 45 ACP is never in polite society called a 11.43mm, like wise our venerable 50 Cal is never referred to as a 12.7mm. (At least among US aircraft aficionados, again, polite society). Both of these Imperial measurements came about way after your mention of the link to Special Publication #1038 which is dated May 2006. And I interpret the Mendenhall Order of 1893 and the even earlier Metric Act of 1866 (both U.S.) as allowing but not requiring the metric system. So this puts us back to the original purpose of this Forum, which is to ask which do you prefer. As a known curmudgeon pushing 80 I don't mind running against the wind and voting…..Imperial.

DerAdlerIstGelandet · Sep 21, 2025

As someone who has used both systems my entire life, I'll take metric any day. Its a simpler and better system. Just my humble opinion of course..:

Do Americans use metric system?

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