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I just broke two 3/8 in. extensions trying to crack an underdrive pulley loose (specs say the pulley is at 130 ft-lbs, but it feels much higher than that).

This begs the question: are there standard tolerances for tools of a given thickness (a maximum amount of torque for 1/2 in and 3/8 in tools)? If so, what are they?

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    I would imagine it varies among tool manufacturers and the material used. Interested in hearing what the community says and if there is an actual standard. – DucatiKiller Mar 16 '16 at 22:32
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    Agreeing with @DucatiKiller - You can know right off that the 1/2" drive is going to be more substantial. It's what we used for most of our repairs on large military trucks while I was in the Army. The problem with your question is it depends on the manufacturer. The best thing you can do is get a tool which has an unconditional guarantee with easy free replacement. If something breaks (usually at the most inopportune time), at least you have recourse and can get a replacement. A good 1/2" drive will stand up to a lot of abuse. – Pᴀᴜʟsᴛᴇʀ2 Mar 16 '16 at 23:27
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    For what it's worth my 1/2 drive torque wrench goes up to 150 ft/lbs, my 3/8 drive goes to 750 inch/lbs (62.5 ft/lbs) – mikes Mar 17 '16 at 0:30
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    Agreed that it varies. About 8 years ago I had a Harbor Freight 1/2" breaker bar. I was trying to get a axle nut off (180ft/lb) and sheered off the head. I bought a new one from Harbor freight (newer style) and have installed/removed axle nuts at least a dozen times without issue. – rpmerf Mar 17 '16 at 11:51
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    Just because it was torqued to 130 to put on doesn't mean that's all it will take to remove it. Corrosion and other factors will see to that. Is impact an option? Heat? Penetrant? – JPhi1618 Mar 17 '16 at 13:12
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Good question, while we probably can't just ring up a tool manufacturer and ask, we can derive it with some accuracy:

Maximum socket size I can find for 1/2" is 36mm, that translates to an M24 fixing: http://stainlessautomotivefastenings.co.uk/pdfs/HEXAGONHEADIDENTIFIER.pdf

Standard torque figures for an M24 using a 12.9 grade fixing (strongest grade) is 926 lb ft http://electronicfilters.tpub.com/TM-10-4330-237-13P/css/TM-10-4330-237-13P_105.htm

So I think it would reasonable to say that, even with a tiny factor of safety, a 1/2 drive extension should be able to handle 1000 lb ft

Applying the same logic to 3/8", that would be a 22mm (M14) fixing, which equates to 177 lb ft. So 130 lb ft plus seizure could well exceed the limit for 3/8" hardware.

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I can't answer for 1/2" but I just bought a 1/2" to 3/8" coupler from Facom and stamped on the side it says "maxi 202 Nm", which equates to about 150 ft lbs. I think 150 ft lbs is about the most a 3/8 drive can safely withstand.

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I tested this out accidentally while doing an axle nut. 150 ft-lbs will break a 3/8" drive usually and about 300 ft-lbs will break a 1/2" drive. These are on regular breaker bars that are partially hollow and have the little ball in the space of the head to hold the socket.

A SOLID 1/2" heat treated impact rated drive anvil will go up to about 1200 ft-lbs.

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A VERY GOOD 1/2" extension might get close to 500ft-lbs on an impact, but if you put 250 lbs on a 2 ft pull handle, it'll twist it off EVERY TIME

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Many tools are made to Fed Spec GGG-W-641E which you can still find on the internet as of July 2019. This spec gives test-torque requirements for 1/4, 3/8, 1/2, 3/4 and 1.0 inch drives for ratchets, universals, hinged breaker bars, extensions, etc.

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If you assume a reasonable quality tool steel that can handle 100K psi, a 1/4" drive can take about 30 ft-lb, 3/8" about 100 ft-lbs, 1/2" about 230 ft-lbs, and 3/4" about 800 ft-lbs. Really good steel will bump this up, while special tools like a wobble extension may cut it to half.

  • Hi MetalsGeek, welcome to the site - good answer, but do you have any sources/references for the numbers? – Nick C Jul 30 '18 at 8:23
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Failure of the 3/8 extension is very likely a poor tool or maybe a one time defect. I have put 200 ft/lb on Craftsman extensions ( with "cheaters", I carry a 12" cheater in my tool box.). You should have a hardness about HRc 30 /35 ( Rockwell hardness) for a good tool. I have split a Craftsman 3/8 drive socket with an impact wrench- there is a limit to what load you can put on them.

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