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Posted by Joe Lorenzino [216.197.153.67] on Thursday, March 01, 2012 at 19:54:27 :
In Reply to: Re: Bolt size question posted by Kaegi [24.16.253.154] on Thursday, March 01, 2012 at 19:47:59 :
Glen types faster than me, but I'll post this anyway...
I believe that if you go back far enough, ALL the bolts used to be a different head size than the nuts. The idea was that only 1 set of wrenches would be needed to work on anything. As things became more mass produced, and production optimized (think Henry Ford) things started to change, and most nuts became smaller to match the bolt head: If a tool kit was factory supplied, fewer wrenches were required if head sizes were minimized, so more cost savings.
The 7/16 bolts are not the only "old" ones left, as I see lots of 9/16, 5/8, 7/8 etc that take different wrenches on each end.
We did a project a the shop a while back where I increased the bolt size from 3/8 to 7/16 but needed the small nut size for clearance. Found out that standard 7/16 nuts are 11/16 BUT Nylocks were only available in 5/8.
Lately one of our hardware suppliers has been sending us 3/8 UNC nuts that are 1/2 AF! (across flats) That causes a bunch of confusion in the shop. My guess is they come from offshore, and are cheaper, so that's what they send. Is that an example of "downsizing"?
Thread pitch is another thing that has changed over time. Before they settled on the "unified national" coarse and fine threads, bolt pitches could be pretty much whatever the manufacturer wanted to use. What we call fine thread (UNF) used to be reffered to as SAE as defined by the Society of Automotive Engineers. It's specifications were set down to standardize both sizes and quallity (the marks on the top of the bolt head) of the fasteners to be used in road vehicles.
There are also a lot of other UNS (national special) threads out there that you might run into. What we think of as 1" NF now (14 threads/") used to be the NS size, as standard was 12 TPI. When i started working on antique (pre SAE and UN standards) machinery, I soon learned to keep ALL the fasteners, as many times the thread was no longer in use/available.