Torque by thread diameter, seat type and head material — plus the turn-angle method for when you cannot fit a torque wrench.
Short answer: a 14 mm gasket-seat spark plug in an aluminium head takes 18–22 ft·lbs (25–30 N·m). A 14 mm taper-seat plug takes 11–18 ft·lbs (15–25 N·m) — taper seats always take less, because the cone does the sealing instead of a crushing gasket.
Gasket-seat plugs have a flat washer that crushes against the head. Aluminium heads take the lower end of each range.
| Thread size | Cast iron head | Aluminium head | N·m (aluminium) |
|---|---|---|---|
| 10 mm | 7–11 ft·lbs | 7–9 ft·lbs | 10–12 |
| 12 mm | 11–18 ft·lbs | 11–15 ft·lbs | 15–20 |
| 14 mm | 18–26 ft·lbs | 18–22 ft·lbs | 25–30 |
| 18 mm | 26–33 ft·lbs | 26–30 ft·lbs | 35–40 |
Taper-seat plugs have no gasket — a machined cone seats directly in the head. They need noticeably less torque, and over-tightening one can crack the head seat.
| Thread size | Cast iron head | Aluminium head | N·m (aluminium) |
|---|---|---|---|
| 14 mm | 11–18 ft·lbs | 7–15 ft·lbs | 10–20 |
| 18 mm | 15–22 ft·lbs | 15–20 ft·lbs | 20–27 |
Plug manufacturers publish an angle method for when a torque wrench will not fit down a deep plug well. Seat the plug finger-tight — until the gasket or taper just contacts the head — then rotate by the angle below.
New gasket-seat plug, 14 mm or 18 mm: half a turn (180°) past finger-tight.
New gasket-seat plug, 10 mm or 12 mm: one third of a turn (120°) past finger-tight.
Re-used gasket-seat plug: one twelfth of a turn (30°) — the gasket has already crushed once.
Any taper-seat plug: one sixteenth of a turn (about 22°) past finger-tight. Barely more than snug.
Only work on a cold engine. Aluminium heads expand more than steel plugs. Pulling plugs from a hot head is the classic way to bring the threads out with them.
Start every plug by hand. Use a length of hose over the terminal as a torque-limiting starter — if it cross-threads, the hose slips instead of the plug cutting new threads.
Anti-seize is usually wrong. Modern plugs have plated threads that do not need it, and manufacturers' torque figures assume dry threads. If you use it anyway, cut torque by about 20% or you will over-clamp the joint.
Do not over-torque to stop a misfire. A plug that will not seal is a damaged gasket, a damaged seat, or the wrong reach — none of which are fixed with a longer bar.
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For plugs specified in inch-pounds
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It depends on thread diameter and seat type. A 14 mm gasket-seat plug in an aluminium head takes 18–22 ft·lbs (25–30 N·m); a 14 mm taper-seat plug takes 7–15 ft·lbs (10–20 N·m). 10 mm plugs take 7–9 ft·lbs and 18 mm plugs 26–30 ft·lbs.
Use the turn-angle method. Seat the plug finger-tight, then turn a new gasket-seat 14 mm plug half a turn further, a new 10–12 mm plug a third of a turn, a re-used plug one twelfth of a turn, and any taper-seat plug only one sixteenth of a turn.
Generally no. Modern plugs have plated threads designed to go in dry, and published torque figures assume dry threads. If you do use anti-seize, reduce torque by roughly 20% — otherwise the same wrench setting produces far more clamp load and can pull the head threads.
You can strip the aluminium head threads, crack the plug's ceramic insulator, or distort the plug body enough to close the gap. Taper-seat plugs can crack the seat in the head, which cannot be repaired without machining.
Wait until it is cold. An aluminium head expands around the steel plug threads when hot, and removing plugs from a hot head is the most common cause of pulled threads.