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I have been told not to use extension bar when torqing to spec because it allegedly compromises the accuracy thereof. However, reality is such that some bolts are so inconveniently set that it is impossible to reach them without an extension bar. My question is, in such cases, how much does extension bar reduce torque wrench accuracy, if any?

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There is no escaping the fact using an extension will reduce the accuracy of your torque. In this case, what you have to do is mitigate the inaccuracy as much as possible. The way you do this is by ensuring the head of the torque wrench pivots at the point directly in line with the fastener you are trying to torque. To do this, place your hand on the head of the wrench and prevent it from moving except to provide the twisting motion as would be normal. If you were to look straight down at the head of the torque wrench, the socket should be directly under it. Unless you are using some sort of automated torquing device, you will never have 100% accuracy no matter how you are applying torque. There will always be variance between fasteners. You just want to make sure you do the best you can to ensure the torque applied is as uniform as possible.

The largest inaccuracy while using an extension comes from if the extension causes an angle to form between the head of the torque wrench and the head of the fastener. A secondary inaccuracy comes from the twisting force which occurs on the extension itself. You can't negate it, so sometimes you just have to deal with it.

Something to remember about torque force applied to a fastener is that it isn't all about the amount of force, but about equal force between the fasteners. This is so the force applied on whatever you are fastening is applied equally. As an example, when torquing a head, you want the force applied equally across the head so you minimize the chance the head will warp. Obviously, you want the force applied to be tight enough so it won't have issues, but hopefully it makes sense. When applying torque to the fasteners, it really isn't the torque which is needed, but rather you want a certain amount of stretch on the fasteners. This stretch provides the clamping force which holds the object in place. The torque we apply is the way we judge the amount of stretch.

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Using any extension requires attention to details when torque is applied. As long as the extension is directly inline/perpendicular along the fastener axis and a hand placed to counteract the applied torque trying to move the torque wrench away from being directly in line with the fastener, all torque is xmitted to the fastener. Not holding the torque wrench at the point of rotation will create vector forces reducing torque values as its distributed away from the fastener. Using an extension is a two handed procedure; one on the handle applying force, the other hand preventing the torque wrench from moving away from having the extension move sideways. This may not be well known when unfamiliar with torque wrench use. enter image description here

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  • Having to use an extension implies that one likely lacks the space to hold the fastener with the other hand?
    – findwindow
    Dec 7, 2022 at 16:26
  • The image shown shows clearance for a hand to brace the wrench head against dispersing torque in more than one vector. Even if a hand cannot used, any brace will do as long as the wrench head remains directly inline with extension to fastener. Any sideways forces created while torqueing is unwanted and reduces torque.
    – F Dryer
    Dec 8, 2022 at 17:13

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