Fight my ignorance when it comes to fasteners. Do reverse-threaded fasteners exist, and why?
The left side pedal on a bicycle is reverse threaded as pedaling forces will keep it tight.
Anything involving something spinning in a counter-clockwise direction will use reverse-threaded fasteners so they don’t come loose. The left bicycle pedal that runner pat mentions is an example.
Here’s a weird example: there used to be a problem with people sneaking into NYC subway tunnels and stealing light bulbs. So they started replacing all the fixtures with reverse-threaded sockets that took reverse-threaded bulbs that could only be ordered in bulk. That successfully deterred people from stealing them, at the cost of significantly more expensive custom light bulbs.
Yes, they exist, generally for one of two purposes - to prevent coming undone due to their rotational situation, or to prevent people from putting together things that shouldn’t be put together. The left pedal on bicycles is an example of the first. With ordinary thread the pedal would work itself loose and turn itself out of the crank arm. With left-handed thread it tightens itself. Gas and propane fittings would be examples of the latter.
The exist all over the place. Mostly in situations where the object is turning in such a way that it would loosen itself if it had right handed threads. For example the hub on my floor buffer has left handed threads.
Follow-up hijack: When did they all decide that right handed threads should be the standard? And why?
I read years ago (can’t find a cite though) that some of the Nazis more secret weapons had anti-tampering devices that consisted of reverse threaded bolts or screws, a bracket, and a glass flask of poison gas. If the screws were turned the wrong way (which would be the right way for normal fasteners of course), the bracket would be bent and shatter the glass flask releasing the poison gas.
Probably because it’s simpler if all fasteners of the same size have the same handedness; you cut the number of fastener types you need to keep in stock in half.
I meant why did they decide right over left. Flip a coin?
I would guess that it’s easier for most right handed people to turn something clockwise then it is to turn it counter-clockwise. Not as big of a deal when you have nuts and bolts with ratchets (a lever) but when you’re trying to put a screw into a piece of wood with just a screwdriver, anything to make it easier is a big help.
Some older Chryslers had left hand thread lug nuts on the left side of the car, also for a similar reason that the rotation of the wheels could theoretically loosen the nuts. More commonly, the spindle nut that holds the wheel hub on the axle are sometimes reverse thread on the left side, as are some wheel bearing adjuster nuts.
I’ve always heard right-handedness is the convention because a right-handed person can put more torque on a right-hand screw than a left-hand one, and something like a wood screw is usually a lot easier going in than coming out.
ETA: Or what Joey P said!
Turnbuckles have to have one side reverse-threaded or they won’t work.
The hot water knob on faucets frequently is reverse threaded. I assume this is because it’s on the left side, so is usually turned with the left hand.
Oxygen gas ‘bottles’ have reverse threads for the regulators.
I once saw a guy struggle with some sort of screw on a Soviet vintage Lada for 20 minutes (to adjust the seat, if you care) before he discovered that the threads were reversed.
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Yes, the purpose being to prevent welding gases from being hooked up wrong. If you reversed the oxygen and acetylene it could be disastrous.
Err… something like a wood screw is usually a lot HARDER going in that coming out. :smack:
Depends how long it’s been in. 
the '84 F-250 I had years ago had left-hand threads on the left rear wheel.
Only the left rear wheel.
International Scouts, at least during a certain period, had left handed lug nuts on one side, so International Harvester did it as well as Chrysler.