Rick's Free Auto Repair Advice

Spark Plug torque

Proper Spark Plug Torque is critical

Now that car makers are using aluminum alloys in their cylinder heads, tightening spark plugs to the proper spark plug torque is critical.

What happens if you over-tighten a spark plug?

Overtightening a spark plug in an aluminum head can cause the spark plug shell to distort or stretch, which breaks the gas seal between the metal and porcelain components. This gas leak can cause  pre-ignition or detonation resulting in engine damage.

Worse yet is that the expansion and contraction can cause an overtightened spark plug to rip the threads right out of the cylinder head. That’ll cost you at least $500 to $1, 000 to repair. The shop will have to drill a new opening, tap it with new threads and install an insert.

What happens if you under-tighten a spark plug?

Under tightening a spark plug prevents proper seating in the cylinder head. That reduces heat dissipation, resulting in overheating. Spark plug overheating can cause misfires and rapid plug wear.  can cause the spark plug to loosen. Under-torquing is often caused by a failure to clean debris from around the spark plug before removal. During removal, the dirt can collect on the seat, preventing full contact during installation.

Under-torquing can also cause a spark plug to blow out of the cylinder head.

Use a torque wrench to get the most accurate spark plug torque

Trust me, you can’t guess about spark plug torque. It’s just too important to trust to your “gut.” With costly engine damage in the mix, there’s no excuse for not using a torque wrench. You’re already saving a fortune by do the work yourself, why risk it by not using the proper tools?

Which spark plug torque wrench to buy?

Beam style torque wrenches are the least expensive and they’re accurate throughout their rated range. But they’re hard to read if your spark plugs are located on a rear bank. Buy a beam style if you’re installing spark plugs on the top of an engine or on a small engine and want to save money.

Click style torque wrenches are the second most affordable. However, a click style torque wrench is most accurate in the middle range of its rated torque. Since some spark plug torque recommendations are as low as 7-ft/lbs, it’ll be hard to find a 3/8” drive torque wrench where 7-ft/lbs is in the middle range. That means you’ll have to buy an inch/pound torque wrench or a ¼” drive wrench with a ¼” to 3/8” drive adapter.

How to convert inch pound readings to foot pounds of torque

If the recommended torque is in ft/lbs and you’re using an inch/lb torque wrench, multiply the foot/lb recommendation by 12. So a spark plug torque recommendation of 8-ft/lbs converts to 96-inch/lbs.

If the recommended torque is in inch/lbs and you want to convert to ft/lbs, divide by 12. So a spark plug torque that’s stated as 120-inch/lbs converts to 10-ft/lbs.

If you don’t have a spark plug torque specification

The spark plug thread size, the type of seat/gasket and the cylinder head construction determine how much you should tighten the spark plug. If you don’t have a torque spec or a torque wrench, use this chart

If your spark plug has a metal gasket use these recommendations if your engine has a CAST IRON cylinder head

10MM thread size 8-12 ft/lb. or tighten to hand tight and then turn 1/4 Turn
12MM thread size 10-18 ft/lb. or tighten to hand tight and then turn 1/4 Turn
14MM thread size 26-30 ft/lb or tighten to hand tight and then turn ½ to 5/8 Turn
18MM thread size 32-38 ft/lb or tighten to hand tight and then turn ½ to 5/8 Turn
7/8” thread size 50-55 ft/lb or or tighten to hand tight and then turn ½ to 5/8 Turn

If your spark plug has a metal gasket use these recommendations if your engine has an ALUMINUM cylinder head

10MM thread size 8-12 ft/lb. or tighten to hand tight and then turn 1/4 Turn
12MM thread size 10-18 ft/lb. or tighten to hand tight and then turn 1/4 Turn
14MM thread size 18-22 ft/lb or tighten to hand tight and then turn 3/8 to 1/2 Turn
18MM thread size 28-34 ft/lb or tighten to hand tight and then turn 3/8 to 1/2 Turn

If your spark plug has a tapered seat (no gasket) use these recommendations if your engine has a CAST IRON cylinder head

14MM thread size 7-15 ft/lb or tighten to hand tight and then turn 1/16 Turn
18MM thread size 15-20 ft/lb or tighten to hand tight and then turn 1/6 Turn

If your spark plug has a tapered seat (no gasket) use these recommendations if your engine has a ALUMINUM cylinder head

14MM thread size 7-15 ft/lb or tighten to hand tight and then turn 1/16 Turn
18MM thread size 15-20 ft/lb or tighten to hand tight and then turn 1/6 Turn

A word about anti-seize on spark plug threads

When plantinum and irridium spark plugs first came out, shops and DIYers had a bear of time removing the spark plugs when it was time to replace them. That’s because they sat in the cylinder head for 60,000 to 100,000 miles and corroded to the aluminum alloys. So the recommendation for replacing spark plugs was to apply a light coating of anti-seize to the threads of the new plugs. But the anti-seize acted as a lubricant and shops and DIYers didn’t adjust the torque settings to compensate for the lubrication. So the spark plugs were actually over-tightened by 10-15% and that caused engine damage.

Spark plug manufacturers responded to the problem by applying a nickel coating to their spark plug threads which allows them to be easily removed, even after 120,000 miles. In most cases, car makers and spark plug manufacturers NO LONGER recommend using ANY anti-seize on the threads of new spark plugs. DON’T apply anti-seize to any spark plug threads unless the shop manual explicitly direct you to do that.

There is an exception however. The nickel coating is a one-time use. If you install the spark plug and remove it for any reason and then want to reinstall it, you must apply anti-seize to the threads AND THEN reduce the torque by 10-15% to compensate for the lubrication.

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat



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