I've got an old electric impact gun I'd like to measure the torque of, however I'm struggling to come up with any kind of semi-accurate method to do so, without spending lots of time and/or money.

My ideas so far involve threaded rod and a spring stack (suitable spring is the current stumbling block), or a weighted bar (tonight's latest idea, but would need to some work to construct something suitable).

I do have a couple good torque wrenches for comparison.

Anybody got any other suggestions?