Why are torque spec's given in a range instead of just one number?

 

Because it is impossible to always exactly hit one number.

 

Thanks Mark.

But I've seen some that have a very wide range. That doesn't make sense to me. I could understand 80-85 lb/ft but not 35-70 lb/ft.

When it is a wide range, what do you set the wrench at? The middle of the range?

 

It also has to do with things like castellated nuts and cotter pins.

The routine goes like this. Torque is let's say 165-185 in lbs. we want the cotter pin to be installed, of course - and - with the nut torqued to specification. The Problem. Well the threads and production tolerances leaves some windage in there. For whatever reason, the cotter pin cannot always be installed at a single value torque setting AND get the cotter pin hole to line up with the nut. So a range is given for more leeway.

The mechanic first sets the wrench to 165 in lbs (the minimum spec) and sees whether the cotter pin can be inserted or not at that torque specification. If it does, great. Install cotter pin, go for a root beer.

But, it might not. If not, now set wrench to 185 in lbs - and continuing in the tightening direction, carefully line up with the (next) castellation with the cotter pin hole in bolt. In no case should the final torque exceed 185 in lbs, (or be less than 165 in lbs.)

In certain instances it's even possible for the cotter pin hole and fastener to not quite line up anywhere along the torque range given.

In this case the proper procedure is to add shims (thin washers) in various combinations or singly, as required. This will change the location of the nut or fastener in relation to the cotter pin hole, while still maintaining specified fastener torque and allow cotter pin to be installed.

Note: you asked

 

Now that makes sense to me. Thanks Tedster!