25Nm vs 28Nm - How accurate should a torque wrench be?

Question

I am soon doing a coolant change on my car, including draining the coolant from the block using the drain plug, and I have heard of a few people apparently snapping these off due to over torquing them, so I was going to invest in a torque wrench.

My manual says the torque should be 25Nm for the drain plug, and I am looking at a torque wrench with a range of 28Nm to 220Nm, as it is the best value and has a very large range.

Will this extra 3Nm make a difference? I'm not sure how to quantify it but I'm guessing that is virtually no difference at all.

Also, is it wise to use a torque wrench like this with such a large range, or do people tend to have a low torque wrench and a high torque wrench?

Thanks!

Answer 1

Well, 2 things:

• yes, it is important to torque fasteners to the required spec, and I do have 2 torque wrenches (one high-range, one low-range)
• 25 nM is "hand tight" in my book. Do NOT use a tool with a long handle on this, if using a socket and a ratchet I would either grab the ratchet by the pivot (not the handle), or use an extension without the ratchet (think like a screwdriver)

Or you can set your torque wrench to 28nM, I don't know how accurate it is at the low end of it's range, but it isn't necessary for this application. Torque specs are mostly important when holding something that either has some weight or that will sustain some force while driving the vehicle, like brake calipers, suspension components, wheels or your water pump.

Answer 2

I don't think that is the right wrench. I think you'll find that for most of the jobs you'll do on a car a 3/8" (I know, I know) wrench with a range of about 10-150 Nm would be a better choice. It will cover more of the range that you'll actually encounter and it will be less cumbersome to handle.

As to your original question, you'd be over torquing the plug by 10%, that seems like a lot to me – especially since that is on top of any errors in setting the wrench and the wrench itself.

You might also consider a beam, rather than clicker, style torque wrench. They are usually less expensive and are less likely to go out of calibration.

That said, a torque wrench is a good and useful thing, but you probably don't need one for this job. If you've never broken some bolts, I'd suggest that you buy a few in a few different sizes, strengths, and materials and tighten them until they break. It will help you develop a feel for how much torque different bolts can take.