DIY Torque Wrench recalibration (with another one)

Problem: My old cheap Draper torque wrench was last used 7 years ago, and in that time I left it set to 40NM. Stoopid! Now I have no idea if it’s reading correctly and don’t want to risk snapping the studs on my head.

Solution: I borrowed a good (Digital Snap-On… oh my…) Torque Wrench from a mechanic mate to test mine against. Here’s the method that worked for me:

Step 1: I set the Draper to 50NM. It’s a bit of a rubbish adjuster so had to make a best guess.

Step 2: The Draper is 1/2″ drive and Snap-On is 3/4″. I have more 3/4″ sockets than 1/2″, so I looked for one that would fit the square end of the Draper as though it’s a bolt or nut. Turned out to be a 15mm 12-point one:

Might be a bit hard to imagine what I mean about using the 3/4″ socket on the 1/2″ drive, so here’s a pic:

Step 3: Now I set both wrenches to ‘ratchet’ anti-clockwise, i.e. The way it usually is to tighten stuff! I also set the Snap-On to 60NM so it would NOT beep/vibrate when the Draper clicked (hopefully) but would save the max torque measurement. I then placed the wrenches parallel to each other, offset slightly, and squeezed the two handles together until the Draper clicked. ‘Gripping’ the two levers together like this allows for great control. As soon as the Draper clicked I slack off, and the Snap-On’s display stays on the highest torque reading. In my case it read 55NM, so my Draper was over-reading by 5NM!

Step 4: To adjust the Draper a small brass cover screw had to be removed:
… and under this is a hex/Allen head adjuster. Turning clockwise reduces the reading, anti-clockwise increases it. I found a 180° clockwise turn made the Draper read between about 49.5 to 50.5NM. Seemed about as accurate as I could get:
The end… done, I’m happy enough with that now. I tried several other settings and the draper is now reading about 1% over / under.