Hi Stephen, I can tell you how I checked/callibrated my wrench, but I'm sure others will have alternative methods, my "callibration" was simply to work out what the wrench needed to be set too to get the REAL torque setting I wanted, these figures are shown below.
I guess some wrenches will have an internal adjustment of some sort that means you can adjust it so the wrenches reads correctly. I'm pretty sure my wrench can be taken apart to get at the callibration adjustments, but as I'm intending to return it to the shop I'm not going to do that.
I checked my wrench by clamping the 1/2 inch drive in a vice so the bar was horizantal and the weight of the bar was resting on the ratchet, with the ratchet set in the "tighten" position.
I decided to use "check point" 1.378 (420mm) from the middle of the 1/2 inch drive as a point to add test weights. This was very close to very end of the wrench.
With no test weights on the wrench the (upward) force needed to lift the "check point" just off the ratchet was 1.54 lbs. I've called this the "zero" weight in the results below.
I then made up a selection of test weights and weighed them. I used bottles of water, but anything will do as long as you can hang it from the "checkpoint". I have some scales which are accurate to 5kg so I made up a few test weights at less than that (I've converted the weights I used from kg to lb in the results below).
I then hung the the weights on the "check point" using a piece of string and adjusted the wrench torque setting so it was nearly, but not quite "clicking" to indicate the torque setting had been reached. With my wrench the extra little "push" down to get it to click was much less than 1 lb so having the wrench "nearly' clicking was accurate enough. My wrench was quite good at "repeatedly" giving the same reading for a given weight. Shame it consistently gave a wrong reading!
For the "torsion bar" type wrenches which do not "click" but just give a reading I think the same method can be used to check those.
I then used a calculator to work out the "real" torque on the wrench, for each test weight and wrench reading.
The results were as follows:
Zero Weight Test Weight (lbs) Total Test Calcuated Torque ft lb Observed/actual Wrench
(lbs) Weight (Total weight x1.378) Setting ft lb
1.54 16.54 18.08 24.91 50
1.54 23.15 24.69 34.02 60
1.54 30.87 32.41 44.66 70
So I know to get my head bolts at 34 lb ft I set the wrench to 60 ft lb, and for the (billet) con rods to get 45 ft lbs I set the wrench to just over 70 ft lb.
Once I'm done using the wrench (I parted the crankcases last night to re-do the con rods) I intend to return it to Repco as it has a lifetime warranty, and in my opinion the inaccuracy is way outside of what it should be, even though I've owned it a couple of years I've hardly used it. I'm not expecting them to replace it but it's worth a go.
A quicker/easier method to test the wrench is to set it the other way up in the vice, set it to various settings starting (say) at 25 ft lb and use bathroom scales to push the end of the wrench "up" and then use the same calculation as above, I also used this method and obtained pretty much the sames results, but note bathroom scales may be quite inaccurate around the 20 to 40 lb mark (especially if the wife has adjusted them).
For those with metric wrenches, which usually measure in kg meters, the conversion for that is:
1 kg meter = 7.233 ft lb (so 70 ft lb = 9.67 kg m)
If it helps understanding ... if one has a 1 ft long spanner on a bolt, and exert a force on it of 30 lbs, you have done the bolt up to 30 ft lbs, if the spanner is 2 feet long the same force of 30lbs will do the bolt up to 60 ft lbs eg double the tightness, the same principle applies to torque measured in kg meters.
