Exactly right. A 3/4" socket is 19.05 mm. The difference of 0.05 mm, about 0.002", will never be noticed with normal tolerancing. It won't do any damage. Whenever a size is close like this, the next Customary size on either side will always be a VERY poor fit. 13/16" = 20.64 mm and neither a 20 mm or 21 mm will do the job properly. 8, 19, 27 mm are examples of extremely close fits. 11 and 16 mm are examples of fair fits. Worse than that will likely do damage.
--- On Wed, 8/24/11, Martin Vlietstra <[email protected]> wrote: From: Martin Vlietstra <[email protected]> Subject: [USMA:51011] Re: US should weigh up one vital change To: "U.S. Metric Association" <[email protected]> Date: Wednesday, August 24, 2011, 12:49 PM The customary sized wrenches probably work because they are “about” the right size, but because they are not quite the right size, they wreak havoc, rounding the edges of the bolt heads. Tell them that and they will get a shock. I am hazarding a guess that the customary-sized wrenches are often cheaper than the metric ones – if so, well you get what you pay for, From: [email protected] [mailto:[email protected]] On Behalf Of Michael Payne Sent: 24 August 2011 17:42 To: U.S. Metric Association Cc: U.S. Metric Association Subject: [USMA:51009] Re: US should weigh up one vital change I wrote to Mr Chen thanking him for bringing this up and telling him how much I agree. He's correct, we could sell a lot more to other countries if we changed. I've recently talked to 2 different people who said "No way" when I told them all vehicles made since 1975 are metric only. Why do my inch wrenches work they asked? Luck! This appears to be a very common misconception, that probably wastes a lot of time, and money considering that all probably buy both inch and metric wrenches, all of which are probably made in China. Doubling the amount of money sent to China on Wrenches along! Mike Payne On 23/08/2011, at 20:14 , John M. Steele wrote: Interesting editorial suggesting that the US adopt the metric system. It is written by a Chinese editor, living in the US on foreign assignment. http://www.chinadaily.com.cn/cndy/2011-08/23/content_13167725.htm
