Friday, September 05, 2008

Learning to Calibrate Torque Wrenches

Today I got my second Craftsman brand torque wrench which like my first, I got through Ebay. I bought a 20-150 ft/lb 1/2 inch drive model to complement my first torque wrench, a 25-250 in/lb 1/2 inch drive model.

I'd been thinking I'd take them to Sears and they'd calibrate them for me so I would know that when I set a torque setting, it's accurate and I don't mistakenly overtighten or undertighten a fastener on my motorcycles as I service them.

Well, let me tell you, the Sears people I called on the phone were "uncertain" to put it kindly. The basic gist was: I think we send it somewhere and it'll come back someday, not sure what the cost will be though.

With that resounding assurance, I bagged that idea!

A few minutes of googling found me this LINK.

I followed the instructions, which even for a math-challenged guy like me were simple enough. I actually repeated the tests twice to verify my calculations and its good that I did because I missed a step on the second wrench!

I marked up the cases in which the wrenches are stored. So, now I know that for my inch/lbs wrench, I must multiply the desired torque setting by 1.25 and set that value on the wrench. For my foot/lbs wrench, I must multiply the desired torque setting by 1.13 and set that on the wrench to deliver an accurate torque on the fastener in question.

In other words, I've been undertightening my fasteners! This Maybe explains some slight seepage I saw on a couple of drain plugs after the last services I did on Brigitta. So, when's the last time you had your torque wrenches checked?

Update: 15SEP08: Turns out, where I am presently contracting for work, UAL's Flight Training Center, they've a Torque Wrench Testing device near the tool shop! I took my wrenches there today and checked both of them. They were reported, near as I can figure it out, as accurate! I am of course inclined to believe this certified testing device vice the crude methods I used to derive the mathematical values above. Oh well.


HondaRacer said...

Mathematically that might be a way to check the calibration but for me to know that my wrenches are 100% accurate and avoid the math is well worth the $30-$40 bucks to have it checked and adjusted. Especially when you are talking about motorcycles or engines. My calibration certificate states that the accuracy is within .25%, I know that type of accuracy is not possible with doing it that way. I have heard of to many horror stories about over tightening, under tightening and wheel offs that for the piece of mind it is worth it.

Charlie6 said...


totally agree with you, I still want to find a local source where I know that they'll actually check them there, not send them somewheres and who knows when they'll return.

you wouldn't be in the denver metro area would ya? : )

HondaRacer said...

I actually send mine to a company called Team Torque, luckily for me they are only a couple hundred miles away but I have always been impressed with them and every time I have sent a tool into them they have got it back to me quick, usually only a week or a little more with shipping times. With you being in Denver it should be about the same amount of time, maybe another day or so. They are located somewhere in North Dakota, forget where off the top of my head. Their website is

What I meant by my calibration certificate was the certificate I get back with my tools that has a disclaimer on the bottom showing the accuracy of their equipment.

Hope this helps……

Jack Riepe said...

Dear Charlie6:

I do not profess to be the kind of guy who does his own work. There are two reasons why I don't.
a) I suck at it.
b) I know lots of people who are good at it. I'd rather trade them money for expertise.

But I like to have the tools at hand to do a job in my garage. I have two torque wrenches that I got at Harbor Freight. It never occurred to me that they could be off. Shouldn't a tool like this come certified?

By the way, I liked the way your bike was pictured in the last BMW MOA ON (September). They ran a shot of mine, in a group, last February, in a story I wrote about a ride to Centralia. It was my 1986 K75 with the Sprint Fairing, that got squashed by a mini-van in June.

Fondest regards,
Jack Riepe
Twisted Roads
Polically Correct Cigar Smoking For Social Terrorists

HondaRacer said...


To be quite honest most of the tools you buy are not certified unless they come with a certificate stating they are. Even if they do come with a certificate they still should be calibrated every year or so.
They can become off if you do not set them down to the lowest setting or if they are dropped or even from extreme hot or cold. I had done my own mechanic projects on the side for years and had no clue about any of this until I started working at a repair shop and the owner made us have ours calibrated.
It is honestly scary that most shops don't even do this. I always check my tires whenever I take something into somewhere.