When machinery is assembled at a factory, all the nuts, bolts, fittings and fasteners are tightened to precise torques by calibrated hand or power wrenches. But farmers and mechanics can’t look up the torque value of every nut and bolt when they’re making field repairs, so it’s a guessing game when to stop cranking on a fastener or connection.
It’s hard to describe "tight enough," but in general:
- When tightening nuts and bolts smaller than ¼" (6 mm, metric), use only your little finger on the wrench. Stop tightening when it feels uncomfortable on that finger.
- For nuts and bolts in the 5⁄16" to 3⁄8" range (8 to 10 mm), center the end of the wrench in the soft part of your palm. Tighten till uncomfortable.
- Fasteners in the ½" to 5⁄8" range (10 to 12 mm) can usually take all the effort an average man can put on an average wrench. An engineer for a bolt manufacturer told me several years ago that tests show an average-sized man, pulling straight toward his chest without bracing against the machine, can pull an average-sized hand wrench to 60 to 100 lb.-ft. of torque.
- Fasteners larger than 5⁄8" (more than 12 mm) usually require all the body weight an average-sized man can exert on a conventional open-end/closed-end wrench.
It just depends. The exact torque value of a fastener depends on whether it is bare metal or plated metal, dry or lubricated, and other variables. But in general:
- 5⁄16" nut/bolt, Grade 5 = 10 lb.-ft.
- 5⁄16" nut/bolt, Grade 8 = 21 lb.-ft.
- 3⁄8" nut/bolt, Grade 5 = 16 lb.-ft.
- 3⁄8"nut/bolt, Grade 8 = 30 lb.-ft.
- ½" nut/bolt, Grade 5 = 38 lb.-ft.
- ½" nut/bolt, Grade 8 = 75 lb.-ft.
- 5⁄8" nut/bolt, Grade 5 = 98 lb.-ft.
- 5⁄8" nut/bolt, Grade 8 = 180 lb.-ft.
Use these guidelines with a simple formula to estimate torque: weight in pounds × length in feet = lb.-ft. of torque. A 150-lb. man resting all his weight on the end of a 2'-long breaker bar produces 300 lb.-ft. of torque at the business end of the breaker bar.
Hydraulic fittings are often overtightened during field repairs. Identify the type of hydraulic fitting and torque it appropriately:
- Before assembling any hydraulic fitting, always lubricate with grease or hydraulic fluid any rubber O-ring used in the fitting.
- Flat-face fittings that have a rubber O-ring in a groove in the end of the fitting should tighten easily until the O-ring makes contact, then become "tight-tight" in less than a half-turn of the tightening nut. In a properly tightened flat-face fitting, most of the seal is between the machined faces of the metal fitting; the O-ring is insurance.
- Straight-thread O-ring fittings use a nut and flat washer to compress an O-ring into a chamfer around the port of the fitting. Install the threaded end of the fitting until hand-tight. If the fitting must be aligned in a specific direction, it can be rotated up to one revolution backwards from the hand-tight stopping point. Tighten the nut against the washer to compress the O-ring and complete the seal.
- Tightening flared hydraulic fittings can be problem-atic. Overtightening can crack the flared portion of the fitting, though it’s hard to disassemble and visually detect the hairline crack. That's because the crack only opens when it is tightened.
Rule of thumb: Once the fitting’s nut is "finger-snug," put a wrench on the stationary fitting and another wrench on the fitting’s nut so you can enclose both of them with one hand, then tighten them until you are uncomfortable with that hand.
That guideline is for generic 3⁄8" and ½" hydraulic fittings—1⁄4" fittings require mere finger pressure on the wrenches for proper torque, while ¾" and 1" fittings favor two hands on the wrenches and at least one good grunt.
- For flare fittings, err toward too loose rather than too tight. You can always re-tighten a leaky flare fitting, but once it’s been overtightened and cracked, it’s necessary to replace the entire fitting.
Cool Tool of the Month
Flare-nut wrenches are box-end wrenches with a gap in their rim so they slip over fuel-injection lines, hoses and other obstacles in-line with the flare nut. From $50 to $170 for a set ranging from 3⁄8" to 7⁄8".