3/8 or 1/2in Torque Wrench? Range, Accuracy and When to Calibrate

Not sure which torque wrench to buy, or whether yours still reads true? Pick the range from your torque figures, check the stated tolerance, recalibrate yearly and correct for added length.

PartsForMachines lists over 170 torque wrenches and torque tools from Sealey, Chicago Pneumatic, Sykes Pickavant, Draper and Norbar. They run from 1/4in micrometer wrenches to 1in split and cordless models, plus angle and torque gauges, torque sticks and multipliers.

3/8in or 1/2in: Which Drive Size and Range Do I Need?

Choose the range first, from the torque figures in your handbook, and the drive size follows. Accuracy is normally specified from 20% to 100% of the scale, so pick a wrench where your usual figures sit above the bottom fifth.

A 1/2in wrench suits wheel nuts and suspension parts: the STW102 1/2in micrometer torque wrench covers 40–210Nm. Small engine fasteners need a smaller wrench, such as the 20–100Nm STW702 on 3/8in drive, so many owners end up with two. Our 1/2in hand wrenches stop at 340Nm. If a job calls for around 450lb.ft (about 610Nm), step up to a 3/4in wrench such as the STW907 (160–800Nm).

How Often Should a Torque Wrench Be Calibrated?

Every 12 months or 5,000 cycles, whichever comes first, is the usual default, and sooner after an overload or other misuse. Heavy daily use can justify checking every six months. Even with occasional DIY use, a wrench that is several years old is worth having checked before safety-critical work.

Many new wrenches come with a certificate showing how they tested. The 80134000 push-through torque wrench (1/2in, 60–340Nm) has a traceable calibration certificate, and Sealey’s calibrated micrometer wrenches, such as the STW704, carry an individually numbered test certificate. After that, a calibration service can test, adjust and re-certify your wrench.

Is ±4% Accurate Enough, and Does Accuracy Drop at the Bottom of the Scale?

For a click wrench, yes: ±4% is the tolerance ISO 6789 sets for click wrenches above 10Nm, with ±6% at 10Nm and below. A wrench individually tested to ±4% with a certificate, such as the STW102, meets that standard, so judge any wrench by its stated tolerance rather than its badge.

The weak spot is the bottom of the scale. The ProTronic 200 (130519) electronic wrench is rated ±2% from 20–100% of capacity but ±4% from 5–19%. Check how a digital rating is worded: the STW310 digital angle torque wrench (30–340Nm) is rated ±2% of its stated capacity (about ±7Nm), not of the reading.

Do Extensions, Swivels or Crowfoot Adapters Change the Torque?

Anything that adds length to the wrench, such as a crowfoot, changes the torque at the nut, so correct the setting. Multiply the torque you need by the wrench’s normal length, then divide by the extended length. For extension bars, swivels and universal joints, check the wrench’s instructions before relying on the reading.

Where a socket won’t fit, such as on an engine bay pipe union, a spanner end fitting is often the better choice. A handle that takes end fittings, such as the 80091000 torque wrench handle (8–60Nm), gives you that option.

What Should I Check Before I Buy a Torque Wrench?

• the torque figures you work to, sitting in the upper part of the range rather than the bottom fifth
• a drive size to match your sockets, and a second wrench if your figures run from small fasteners to wheel nuts
• the type (click, push-through, breakback or digital) and whether it works in both directions
• the stated accuracy and the standard behind it, such as ±4% to ISO 6789
• the certificate supplied, and whether the wrench can be serviced and recalibrated

Browse the torque wrench range, or contact the PartsForMachines team by WhatsApp, telephone or email with the torque figures and drive size you need.

Product options, delivery options and returns terms are shown on each product page. A 30-day return policy applies to eligible items.

Frequently Asked Questions

Should I wind a click torque wrench down after use?

Yes. Wind it back to the lowest marked setting, but not below, to take the pressure off the spring. Then clean it and store it somewhere dry, in its case if it has one.

Can I check my torque wrench’s calibration myself?

Only roughly. Comparing it with a wrench you trust, or with a digital adaptor such as the STW291 digital torque adaptor (±2%, 20–200Nm), shows whether it is far out. For adjustment and a new certificate, use a calibration service.

Can I tighten wheel nuts with an impact gun instead?

Not on its own. A torque stick such as the VS2247 torque stick limits an impact wrench to 135Nm and helps prevent over-torqued wheel nuts. Each stick is colour-coded to one rating, so match it to your handbook figure.

Click or digital: which is better for wheel nuts and suspension?

Both do the job. A click wrench gives a distinct click once the preset torque is reached. A digital wrench such as the STW310 shows the reading, signals with lights, vibration and a buzzer, and measures angle too, so it suits angle work.

What batteries do digital torque wrenches take?

It varies by model, so keep spares in the case. The STW310 runs on four AAA batteries, supplied, and the STW291 adaptor uses two CR2032 cells and switches itself off to save them.

Do I need a reversible torque wrench for left-hand threads?

Yes, you need one that torques in both directions. Some click wrenches, such as the Draper Expert 17738, work clockwise only, while push-through designs and flip-reverse wrenches such as the STW704 torque both ways.

How do I angle-tighten bolts without a digital angle wrench?

Use an angle gauge with your torque wrench: tighten to the torque figure first, then turn through the angle in the manual. The 34571 angle torque gauge fits any 1/2in torque wrench and has an adjustable reaction arm.

Can an old torque wrench that reads out of spec be fixed?

Possibly, so ask a calibration service for a quote to adjust it. Full after-sales service and calibration is available for Motorq wrenches, for example. Compare the quote with a new wrench that comes with its own certificate.

How do I reach very high torques, such as HGV wheel nuts?

Use a 3/4in or 1in torque wrench, or a torque multiplier with your 1/2in wrench. The 80060000 multiplier gears up 5:1 to 1300Nm, and the AK629 takes up to 300Nm in and gives up to 1000Nm out.

Can I lock the torque setting so it doesn’t move?

Yes, on many wrenches. The STW201 locks the adjustment once the figure is selected, and push-through wrenches such as the Speedline range have a pull/push lock that prevents accidental changes to the set torque.