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Hello all!

I'm new to the forum so please forgive me if I am posting this question in the wrong place.


I have a general question about torque tolerance. For the moment, I'm not speaking of any particular bolt or nut or whatever you're torquing :wink:. But I imagine all car specifications for torque values should have (or even publish) the allowable tolerance or acceptable range the torque can be set to and it will be effective and safe. As an example, on the car I have, 1997 Boxster, the oil drain plug is stated at 39 ft./lbs. Great! That's probably the nominal value (middle of the range), but what is the range?


Digging a little deeper, I'd imagine that just about every single bolt or screw has its own allowable tolerance and there is no one size fits all, but I want to stay general just for a moment longer...assuming published torque settings have a tolerance, generally speaking are we talking about a couple % either side of nominal or are torque setting tolerances measured even tighter, say a quarter of a % high and low of nominal?


To continue the previous example in a bit more detail, let's say the oil drain plug wants 39 ft./lbs. +/- 3.9 ft./lbs. In this case the tolerance would be 10% of the nominal value. I'm just making up numbers now to tease out the point.


Here's the deal...if I set my crappy torque wrench that has a published accuracy range of +/-4%, and I set it to 39 ft./lbs, and I use it with good technique I will hear the "click" as low as....well wait a second...we have to first understand what +/-4% actually means. 4% of what???


So here's a little more detail.

If I have 3/8" TW with a published range of 10-80ft./lbs., the actual range of the tool is 70. The accuracy is +/-4% of 70. This yields an accuracy of 2.8 ft/lbs. So going back to my previous example when I heard the "click", the actual torque applied to the oil plug was as low as 36.2 ft/lbs. and as high as 41.8 ft/lbs. Considering, again from my made up numbers from above, the oil plug has an allowable tolerance of 3.9 ft/lbs. the true range of acceptability is 35.1-42.9 ft/lbs. If this were all true, my crappy torque wrench is not so crappy after all as I hit the tolerance pretty well; I'm right in there.


So first, is my logic correct?

Second, what is a good general rule of thumb for specified torque tolerance, 1%? 10%? Yes I know some things are more sensitive than others, but rule of thumb here...

Third, are the torque settings that are published, published with a tolerance? If not, what is the rule of thumb for those?


Ultimately, this conversion, and understanding the math and data can effectively help you save a TON of money on buying your next torque wrench. Do you need to pay for 2% accuracy or is 4% just fine?


Thoughts please!

Happy Holidays to all!!







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First off, I like the way your mind works, reminds me of me. I wonder about the same kinda stuff. 


I hope this discussion can remain true to your intent, which is to be in a general, "rule of thumb" arena. Many great minds on this forum. I'll step back and let the more informed chime in. 

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  1. Buy the best torque wrenches you can afford, and stop looking at them as an expense, but rather consider them an investment in maintaining your vehicles.  Better equipment typically gives better accuracy as well and maintains the accuracy over time.  Literally, "you gets what you pays for" when it comes to tools.
  2. Every fastener in your car has a torque spec range, which can be found in the OEM service manuals for the vehicle.  Again, consider the manuals to be an investment in correctly maintaining your vehicle.  Using most spec ranges mid values as your torque targets would keep you in good stead with either a +/- 2% or 4% tool.
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