Squarebody Lug Nut Torque by Weight Class (C10, C20, C30)

The numbers, by truck

Lug nut torque on 1973-1987 Squarebody trucks varies by weight class because the studs and wheels themselves differ:

These are factory specs for original-equipment steel wheels with original taper-seat lug nuts. Aftermarket alloy wheels often spec a different torque value — read the wheel manufacturer's documentation.

Bolt pattern identification

The two patterns used across the Squarebody platform:

• 6x5.5" (also called 6 on 5.5): half-ton trucks (C10 / K10). Six lugs in a circle 5.5 inches in diameter.
• 8x6.5" (also called 8 on 6.5): 3/4-ton and 1-ton trucks (C20, C30, K20, K30). Eight lugs in a circle 6.5 inches in diameter.

The 6-lug 5.5" pattern is identical to GM passenger-car patterns of the era and to many other 1970s-1980s pickups. Wheels interchange across a wide range of GM vehicles. The 8-lug 6.5" pattern is shared with HD GM trucks, Suburbans of similar weight class, and some Olds and Cadillac applications. Wheels interchange more narrowly.

Why HD trucks use higher torque

The studs on a C20 or C30 are physically larger than on a C10. The factory used 1/2-13 thread on C10 lugs and 9/16-18 thread on C20/C30 lugs. The larger studs can take (and need) higher clamp load to seat the wheel properly under load — these were trucks designed to haul or tow at GVWR every day, and wheel walk under load is a real risk if torque is wrong.

If you're swapping wheels from a C10 to a C20 (different stud pattern, won't interchange) or vice versa, you can't just use one truck's torque on the other — the studs are different sizes and the thread pitch differs. Apply the spec for the truck you're driving, not the truck the wheels came from.

Dually rear wheels — special case

C30 and K30 dual-rear-wheel trucks (those with two wheels on each rear hub) have unique torque considerations:

• Inner and outer rear wheels each torque to 140 ft-lb (sometimes 150 ft-lb depending on year and stud configuration).
• Tighten the inner wheel first, then the outer. Both wheels tighten against the same studs.
• Re-torque after 50 miles of new dually wheel installation. Heat-cycle compression on dual wheels is significant and lug nuts work loose more readily than on single-wheel applications.

If you don't re-torque a dually rear wheel after 50 miles, you risk losing a wheel on the highway — this is a documented failure mode and a major-injury risk. Take the re-torque seriously.

Pattern for tightening

For 6-lug wheels: star pattern, three passes (40 → 70 → 90 ft-lb on a C10). Imagine the lugs as positions 1-6 around a clock face — tighten in this order: 1 → 4 → 2 → 5 → 3 → 6.

For 8-lug wheels: star pattern, three passes (50 → 90 → 120 ft-lb on a C20). Imagine the lugs as positions 1-8 — tighten in this order: 1 → 5 → 3 → 7 → 2 → 6 → 4 → 8.

When to deviate

Use the wheel manufacturer's spec instead if you have:

• Aftermarket alloy or steel wheels with a different seat type or stud size — most aftermarket wheel manufacturers spec their own torque.
• Wheel spacers or hub-centric adapters — thread engagement changes; verify the spacer manufacturer's recommendation.
• Aftermarket studs (ARP, Moroso, etc.) — follow the stud manufacturer's instructions, including their recommended lubricant if any.

Common mistakes

1. Lubricating threads. All factory specs are for dry threads. Oil or anti-seize over-clamps the stud by 20-30%, leading to stretched studs and cracked hubs.
2. Mixing up C10 and C20 torque values. A C10 torqued to 120 ft-lb (the C20 spec) can stretch the smaller half-ton studs. A C20 torqued to 90 ft-lb (the C10 spec) can lose wheels under load.
3. Skipping the multi-pass star sequence. Single-pass tightening on 6+ lugs distorts the wheel against the hub.
4. Forgetting the dually re-torque at 50 miles. Wheel walk on a dually is catastrophic.
5. Not verifying the truck's actual weight class. A "C20" badge doesn't always mean the truck has C20 underpinnings — emblems were swapped, plates fade. If you're working on a truck whose history isn't documented, pull a wheel and measure the stud size before applying any torque spec.

A reminder on safety

These are research-derived values, not factory shop manual data for your specific truck. Always verify against the actual factory service manual for your specific year and weight class. Lug nut torque is a wheel-detachment failure mode if applied wrong — under-torque allows the wheel to walk loose and eventually leave the truck; over-torque stretches the stud and can crack the hub. The HD trucks in particular were engineered to take real load, and the factory specs reflect that.