# screw thread direction **URL:** \ **Category:** Factual Questions\ **Created:** [September 6, 2009, 4:18am UTC](https://boards.straightdope.com/t/screw-thread-direction/509036 "2009-09-06T04:18:14Z")\ **Posts on this page:** 18\ **Page:** 2
**Author:** ![Uncertain](https://avatars.discourse-cdn.com/v4/letter/u/6a8cbe/32.png) [@Uncertain](https://boards.straightdope.com/u/Uncertain)\ **Post date:** [September 6, 2009, 4:56pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/21 "2009-09-06T16:56:05Z")
> [@Mk\_VII](#): > > Whitworth developed the first thread standard, before that people cut threads to their own private standard or individually, as anyone who tries to restore an old steam engine soon discovers. Yeah, that really threw me off my first time. ---
**Author:** ![Uncertain](https://avatars.discourse-cdn.com/v4/letter/u/6a8cbe/32.png) [@Uncertain](https://boards.straightdope.com/u/Uncertain)\ **Post date:** [September 6, 2009, 5:08pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/22 "2009-09-06T17:08:41Z")
Another place you’ll find left-handed screws is on turnbuckles. They need to have one left-handed end and one right-handed end to work. ---
**Author:** ![fortytwo](https://avatars.discourse-cdn.com/v4/letter/f/f9ae1b/32.png) [@fortytwo](https://boards.straightdope.com/u/fortytwo)\ **Post date:** [September 6, 2009, 5:10pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/23 "2009-09-06T17:10:42Z")
> [@Xema](#): > > I’ve done a fair amount of recharging aircraft oxygen systems, but not yet encountered a left-hand thread. It is true that essentially all gas bottles have a thread unique to that gas type. I’ve charged up plenty of aircraft O2 systems myself and I can assure you that some airlines do have LH thread connections, BA is one such airline. ---
**Author:** ![Randy\_Seltzer](https://avatars.discourse-cdn.com/v4/letter/r/ee7513/32.png) [@Randy\_Seltzer](https://boards.straightdope.com/u/Randy_Seltzer)\ **Post date:** [September 6, 2009, 5:24pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/24 "2009-09-06T17:24:15Z")
Table saws and panel saws often (always?) have a reverse-threaded spindle to attach the circular blade. ---
**Author:** ![justrob](https://avatars.discourse-cdn.com/v4/letter/j/a9adbd/32.png) [@justrob](https://boards.straightdope.com/u/justrob)\ **Post date:** [September 6, 2009, 5:50pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/25 "2009-09-06T17:50:31Z")
Bench grinders have a right handed thread for the right wheel and left handed thread for the left wheel. If you don’t as soon as you bear down on the left wheel the nut will come off. ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [September 6, 2009, 9:04pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/26 "2009-09-06T21:04:04Z")
LH lug nuts on one side - Old International Scouts, too. ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [September 6, 2009, 9:22pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/27 "2009-09-06T21:22:16Z")
The standard toilet handle is affixed by reverse turning: you turn the bolt to the left to tighten. It is designed that way because you are actually turning it to the right to tighten if you were to tighten it from inside the tank. Right to tighten is the same as clockwise, but what is clockwise from one side is counter-clockwise from the other. ---
**Author:** ![Lobsang](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lobsang/32/4067_2.png) [@Lobsang](https://boards.straightdope.com/u/Lobsang)\ **Post date:** [September 6, 2009, 9:38pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/28 "2009-09-06T21:38:56Z")
When I saw that title I thought “GQ is a strange place for a thread about internet discussion digression apathy” ---
**Author:** ![Uncertain](https://avatars.discourse-cdn.com/v4/letter/u/6a8cbe/32.png) [@Uncertain](https://boards.straightdope.com/u/Uncertain)\ **Post date:** [September 6, 2009, 10:07pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/29 "2009-09-06T22:07:03Z")
> [@barbitu8](#): > > The standard toilet handle is affixed by reverse turning: you turn the bolt to the left to tighten. It is designed that way because you are actually turning it to the right to tighten if you were to tighten it from inside the tank. But the threads are standard right-handed threads, right? It’s not “designed that way”, it’s a consequence of using conventional threads. It’s just like what happens when you tighten or loosen a nut on a surface facing away from you, e.g., when working on a car. But perhaps I misunderstand. ---
**Author:** ![cplif](https://avatars.discourse-cdn.com/v4/letter/c/48db29/32.png) [@cplif](https://boards.straightdope.com/u/cplif)\ **Post date:** [September 7, 2009, 8:32am UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/30 "2009-09-07T08:32:36Z")
Threaded plumbing can be right or left (for assembly purposes), cast iron or aluminum radiators are assembled with RL nipples (you screw in one direction to tighten both sides), different gas connectors have different directions, threads. I haven’t checked ISO standards, but left threads are still in use in many places, saw blades (as mentioned) drill chucks, etc etc, anything that exerts rotation can be fastened by left thread, it’s cheaper (not better) than other methods. I don’t know offhand if there is a way to check for torq direction on a screw head ---
**Author:** ![rowrrbazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rowrrbazzle/32/414_2.png) [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)\ **Post date:** [September 7, 2009, 9:03pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/31 "2009-09-07T21:03:20Z")
NY subways used to have light bulbs with left-handed threads (and fixtures to match) so people wouldn’t steal them. ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [September 7, 2009, 9:06pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/32 "2009-09-07T21:06:14Z")
> [@rowrrbazzle](#): > > NY subways used to have light bulbs with left-handed threads (and fixtures to match) so people wouldn’t steal them. They still do. A lot of those old fixtures are being replaced by fluorescents, though. Buying wrong-threaded replacement bulbs for a century has become expensive. ---
**Author:** ![Gbro](https://avatars.discourse-cdn.com/v4/letter/g/c6cbf5/32.png) [@Gbro](https://boards.straightdope.com/u/Gbro)\ **Post date:** [September 7, 2009, 9:24pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/33 "2009-09-07T21:24:36Z")
> [@Uncertain](#): > > But the threads are standard right-handed threads, right? It’s not “designed that way”, it’s a consequence of using conventional threads. It’s just like what happens when you tighten or loosen a nut on a surface facing away from you, e.g., when working on a car. But perhaps I misunderstand. I believe CPLIF was correct, > [@](#): > > The standard toilet handle is affixed by reverse turning: you turn the bolt to the left to tighten. Its the handle nut, I was handcuffed by that one also:(. ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [September 8, 2009, 7:16am UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/34 "2009-09-08T07:16:03Z")
[QUOTE=lobotomyboy63] Right, I think propane for the gas grill and probably indoor nat gas connections have these. [/QUOTE] The old and all but gone propane fittings were left-handed. The “quick connect” or QC-1 current style is right-hand and such a large thread that you’re unlikely to connect anything else by mistake. Everything else in the propane (or natural gas) plumbing is normal RH pipe thread. Sizing is another issue entirely - there are at least three fitting sizes in common use, but they’re all RH thread. To maintain backward compatibility with old but still perfectly safe and usable propane-fired equipment, the old-style left-hand threads are still available on your barbecue cylinders. Just to be weird, the little “throwaway” propane cylinders have RH threads. ---
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [September 8, 2009, 6:09pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/35 "2009-09-08T18:09:54Z")
I thought the OP was going to be in praise of hijacking threads. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [September 8, 2009, 6:30pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/36 "2009-09-08T18:30:47Z")
And, of course, the MIT IHTFP award is a screw with a left-handed thread. Hot Water faucets often have left-handed turning (which used to require a left-handed thread, but my current faucet doesn’t) In the second of Harry Harrison’s _Deathworld_ novels the native builders of the steam-powered cars used left-handed threads as a sort of booby-trap to prevent reverse-engineering of their devices. (Trying to undo the screws as if they were right-handed would release a stream of sulfuric acid) ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [September 8, 2009, 6:31pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/37 "2009-09-08T18:31:57Z")
> [@gotpasswords](#): > > The “quick connect” or QC-1 current style is right-hand and such a large thread that you’re unlikely to connect anything else by mistake. It is also an acme thread form, so a triangular thread form will interfere. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [September 8, 2009, 8:20pm UTC](https://boards.straightdope.com/t/screw-thread-direction/509036/38 "2009-09-08T20:20:56Z")
> [@CalMeacham](#): > > In the second of Harry Harrison’s _Deathworld_ novels the native builders of the steam-powered cars used left-handed threads as a sort of booby-trap to prevent reverse-engineering of their devices. (Trying to undo the screws as if they were right-handed would release a stream of sulfuric acid) I guess that’s incentive for the builders to make sure their torque wrench is accurate! (Yeah, Fred you’re on the assembly line today. Remember, tighten anything too far and your new name is Scarface) [Previous page](https://boards.straightdope.com/t/screw-thread-direction/509036.md?page=1)