A Spitfire owner sent me this interesting technique:
"Take your wrench and bathroom scale to the garage. Have your Mrs. join you.
Put a wrap of tape around the handle of the torque wrench exactly two feet
from the center of its square drive. Put the bathroom scale next to your
vise, set the wrench to the torque value you need for the head gasket, then
clamp the wrench's square drive in your vice. Stand on the scale and have
your wife write down the reading (your weight). Now (while standing on the
scale) slowly, steadily pull up on the wrench (ON THE TAPE MARK) until the
wrench clicks. Have your wife watch the bathroom scale and record the
scale's reading when the wrench clicks. Repeat this a half dozen times and
average the readings. Subtract your initial weight reading from the average.
And finally, multiply that value by two (2) since you were pulling on the
wrench at the 2 foot mark. Compare that to the value you set on the torque
wrench and you'll have a good idea how accurate (or inaccurate) your wrench
is. Armed with that you can set your wrench accordingly."
He had heard a version of this on CarTalk apparently.
I tried this on my relatively new wrench and it showed it was reading
consistently low (ie I would be inadvertently be under-tightening).
Apparently, storing the wrench under load knocks it out of calibration.
Interestingly enough though, my old wrench (made in Taiwan but very similar
to the ones at HF) was spot on (+/- 0.5 ft-lb) at 10, 20, 30,
40, 50. After 50, things get a bit difficult - which reminds me - a word to
the wise - please wear safety glasses and don't use a light weight vise:
http://i12.photobucket.com/albums/a241/AdrianJones34/Vice.jpg
Cheers!
Adrian
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