I never thought I would find myself defending the metric system in this area. here goes:

JLP will actually get told to cut 6mm off the board and it will be close enough to 1/4" and be fine. chances are good that any decent 1/4" plywood you get even in the USA will actually be 6mm. the metric system is excellent for rapid and easy conversion between units and makes international trade easier.

the problems with metric are actually more closley related to the size of the units themselves. a milimeter is too small to measure most human scale things nicely, and the next common size up is meters which are good for sports sized things, but not much else. a kilometer is fine for longer measurements because people dont measure much distance by counting paces anymore. even if they did, the mile has changed and is no longer 1000 paces. the next common size below a millimeter (micron or micrometer) is too small for most machine work.
remember, the us customary units were never set up to be used together, they were set up for the industries and groups that used them. your shoes are still measured in barleycorns because it works and doesnt need to interchange with other industries. you probably never use hogsheads or butts, because they were trade units of the brewing industry. same for chains and links which only really mattered to surveyors who couldnt care less how mariners used fathoms.

it gets better though, because the inch is actually a metric defined unit. 1 inch is defined as 25.4 mm. to put it differently, the way one determines an inch is actually an inch is to measure out 25.4 mm

P.S. the base 10 theory is mostly bunk, much of the world used base 12, base 20 or base 60 for much of human history.

I am soon to be 59 and ever since I was in elementary school I have been hearing we will all be going to the metric system. It hasn't happened yet and I suppose it never will. I really don't care all that much except for when I work on a new mower and it has all metric bolts in the engine and then it has all American in the frame and everything else. If we would have changed in 1970 none of would even be thinking about it now.

_________________
2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Many years ago, I think it was a Pinto, when they first came out. Working in a gas station, I
needed to do something on it, but the bolts were metric. So I went and bought some basic metric wrenches, and sockets. A couple of weeks later, got another one of the same model car, for the same thing. Got out my metric wrench, and it wouldn't fit. The bolt was American. Tell me that made sense.

_________________
Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

The normal system is the only thing left that separates us from the animals!

I had a '76 Pinto. Only the engine was metric.

_________________
Official Cookie and Mater Tormentor.

Amen.

The only real practical disadvantage I've found to the Metric system is that when motor fuel is sold by the litre rather than the gallon, any given price increase looks a lot smaller, which widens the door for profiteering: price goes from $1.31 to $1.39 per litre…eh…does't look like such a big change. But that's the equivalent of going from $3.56 to $3.75 per gallon—a much sharper pain in the wallet.

Other than that, I have to really reach to come up with any real advantages to the U.S. Customary system (not counting my being more familiar with the U.S. system because that's what I grew up with). The human-scale issue mentioned by NEVjr isn't practically an issue, just psychologically. If you need something bigger than a centimetre and smaller than a metre, the decimetre (1 dm = 0.1 m = 10 cm, just shy of 4 inches) serves just fine. It's not commonly used because in metric countries people grow up using cm and m for everyday measurements. Likewise, the decilitre (1 dl = 0.1 l = 100 ml, a little under half a cup) works fine if you gotta-just-gotta have something bigger than a ml/cc and smaller than a litre. Both a mile and a kilometre are bigger-than-human-scale, so no advantage either way there. Fuel consumption in litres/100 km has been argued as more useful than fuel economy in miles/gallon—I don't perceive much advantage either way. Fahrenheit degrees are smaller than Celsius degrees, so there's more precision in Fahrenheit temperatures, but only if we're not allowed to use a decimal point, and we are, so no practical advantage either way there. One big, central advantage of the Metric Systm is to figure out the next step up or down in size of a tool, all you need is to be able to count. You don't have to memorise or calculate 5/16, 21/64, 11/32, 23/64, 3/8, 25/64, 13/32, 27/64, 7/16, etc.

The main practical disadvantage of the Metric System is that it's alien to people who didn't grow up with it and so don't have it as a "first language". I've lived in Canada for almost 20 years, and Metric is still a second language to me. I speak it pretty well; I know the various conversions off by heart (454 grams is a pound, a litre is about a quart, 100 km/h is just over 60 mph, 20°c = 68°F, 350°F = 180°C, 1 teaspoon = 5 ml, etc) but it's still not a direct flight between what I'm seeing/reading/hearing and what I understand, or what I have in mind and what I'm saying—it still makes that stop for Customary unit translation. That's really the biggest hurdle to conversion, is that it's a nuisance for those who grew up with the Customary system.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

I recall seeing a lot of 90 kph speed limit signage being installed on I-95 in the Southern states in 1975 due to the US metrification act. ( 56 mph, close enough for the national speed limit). At some point the US halted any further signage and later removed them. I forget the total boondoggle cost but it was not a small sum. Maybe Canada or somebody got a real deal on the signage? I don't know if it had been started on any other Interstates or not. Of course the US market speedometers have been double scale since then. Fasteners were supposed to go all metric on US vehicles at some point, '80-'82? Several 80's Fords that I unfortunately bought new had a mix of SAE and Metric, but mostly metric. I worked in the wine and spirits business from early '80 to '98. In early '80 the last of the "fifth", quart and real half gallons were nearly gone. So far as I know, only the booze business was permanently mandated into Metric in the US. Many, many years of explaining equivalent in fluid ounces. Many years of irate customers. The fifth drinkers got cheated out of about an ounce. The pint guys about 3 oz. The half gal. guys in spirits, over 4 oz. The half gal. wine guys lost almost 13 oz. and the industry started referring to that as the euro "magnum". Which wasn't a metric measurement anyhow. The Federal Proof gallonage spirits tax is still levied by a US gallon. The beer industry had it easy, still 12oz, 16 oz and quarts. They just printed labels with the metric measurement of those same sizes.

Like Dan mentioned, I've become familiar with quick conversions of most metric conversions when necessary. With the exception of Celsius. I know 20 C or 30 C but have to take a WAG at anything else. Not an issue with US weather forecasts. Having owned a few Japanese make cars and a few German and Japanese bikes, I can think in mm or cm. I do think they should have gone all the intended way with switching over once they started it. Or never have started it at all. 225 or 3.7?

_________________
1982 Dodge D150 ( Ram)
225 with A833OD. 2bbl. Non power brakes

Jay Leno Garage presented a '57 Imperial this week. Original and unrestored.

The rear styling treatment of that '60 wagon was very well done

It was worse than that...……

They had bolts that were metric threads and SAE heads......…...…..

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

And there's 3 Teaspoons in a Tablespoon

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

and to throw another wrench in the works, I learned metric all through school except shop class where we were allowd to use whatever units we prefered. we all went quickly to inches because that is what material was sold in.

Inches are more useful in construction. when you purchase 48" wide sheets you can easily have 6" or 12" or 16" or 24" spacing for studs and it works out fine. with metric, you start having some mental backflips that the average drywaller is not capable of. modern building prints are generally all in millimeters. a switch will be 1255 mm off of finished floor for example. this gets wierd in conference rooms when the nearest wall for a floor box or light fixture might be 5000mm away. then you get 2'x4' ceiling tiles speced as 609.6mm by 1219mm and other silly things like that. I suspect europe has ways around this, but they have very different building techniques.

my other complaint with metric and cars is the lack of standardization. we all know what is in a 7 wrench set for customary units, but in metric, who knows? they go up by .5mm in small sizes, but no one agrees where that ends. larger sizes go up by 1mm until they dont and go up by 2mm, but is it at 20mm? is it at 21mm? i have a set that skips 16mm, 18mm, 20mm, and so on, and another that goes up to 22, then skips 23, 25, 27, etc. Even the same size threads dont use the same head sizes. a 9/16 fits a 3/8" threaded fastener almost always, but what fits a 10mm thread? it could be 14, 16, or 17 depending on the manufacturer.