Hold on just a minute there, pal. <img src="/images/icons/wink.gif" /> Don't you be trash talking SAE. SAE helped make this country great, and Americans enjoy unsurpassed mechanical technology in terms of disastrous failure modes because of SAE and similar orgs (ASME, NIST, ASHRAE, many many others).If not for the Germans (OK and the French), pretty much the entire world would be copying American institutions on their technology standards. The fact of obsolete British units is a testament to the economic might of the British Empire in full flower, and England being decades earlier than the rest of the world in the industrial revolution.<br /><br />You've got it backwards - America leads the world and our system is one reason why. It isn't a matter of people figuring out how to convert units. It's remedial engineering - units errors by real engineers in critical situations are like doctors amputating the wrong limb or a mechanic putting water in the crankcase.<br /><br />One needs to consider international economics here, realize that the separate systems evolved in response to a long history of economic development.<br /><br />That's the first of three main points I have on this subject. Once upon a time I was able to explain this well (in a lost thread), so I hope to do that again. In fact, let me check my archives . . . darn it, no, oh well, here we go. My profession is Mechanical Engineering, specializing in machine design and design methods.<br /><br />So I'm designing a new piece of space equipment and the directive comes down from on high that this is to be a metric project. During that meeting, us engineers nod dutifully, but back at our desks we exchange knowing glances. What they don't know what hurt them, right? You see, there's metric, and then there's metric. How deep down the rabbit hole do you want to go?<br /><br />If I'm going to bolt some flanges together, I need to choose a bolt size and number of bolts. Based on bolt size, fits and tolerances, the hole sizes <div class="Discussion_UserSignature"> </div>