Nothing like the satisfaction of fixing your own car, at $140 an hour labor, you’ll save a ton just doing the simple stuff. After all, how hard is swapping out some spark plugs, right!
Spark plugs should be torqued to manufacturers’ specifications. However, it is possible to tighten the plugs satisfactorily without a torque wrench.
Tighten new or reused gasket spark plugs as follows:
- Hand tighten the spark plug until it seats.
- Using a spark plug socket and ratchet, turn new spark plugs (18mm and 14mm thread size) a half turn (180°) clockwise to tighten.
- Tighten used plugs 30°.
Tighten tapered new or reused spark plugs, as follows:
- Hand tighten the spark plug until it seats.
- Using a spark plug socket and ratchet, turn both new and used tapered plugs 22.5° clockwise to tighten.
I’m a mechanic for twenty years and you are absolutely correct to do your own maintenance, it’s not difficult but it is important to get it right. Messing up can cause some serious problems. But by the end of this post, you’ll learn how to tighten your spark plugs without a torque wrench.
The Importance Correctly Torqued Spark Plugs
Spark plugs are right at the business end of your engine. Engine compression, combustion, heating, and cooling cycles can all cause the plugs to come loose if not tightened correctly.
Plugs Too Loose
It’s important they are tight, obviously, as plugs seal off the combustion chamber. When a plug comes loose, cylinder compression is lost and the cylinder inhales oxygen through the loose plug on the downstroke. The excess oxygen inside the cylinder and the lack of sufficient compression cause the engine to misfire and lose power.
A loose plug and a lean cylinder run hotter, and that can cause other problems: Pre-ignition; Piston and cylinder damage; Valve damage; Head gasket damage; Plug electrode damage; Damaged cylinder head threads; Misfiring; Lean codes; Engine light on.
A spark plug may be tightened to spec and still come loose, if that happens suspect cylinder head thread damage, plug thread damage, or incorrect plug length or heat range.
Plugs Too Tight
Over-tightening the plugs causes some hard-to-fix cylinder head damage. Seized plugs, cross-thread, galling. Too much torque can damage the plug too. Over-tightening can easily cause stretching which can split the threads making extraction difficult. Internal spark leaking cracking of the insulator.
There are many different types of plugs as you know. Length, diameter thread type, thread reach, and heat range. But the two basic types that are important when it comes to torque specification are spark plugs with gaskets and spark plugs that are tapered.
Plug With Gasket
Plugs with gaskets are pretty obvious, they’ll have a loose, but attached washer and the thread end. The washer is known as a crush washer and as the plug is tightened, the washer deforms to the shape of the cylinder head, making a perfect gas seal.
The tapered plug is, well, tapered. As the plug doesn’t have a washer to crush, it requires less torque to tighten, make sense?
Tightening the plugs as you know can be successfully executed without a torque wrench. Mechanics have as you know, a calibrated thumb that they use regularly. You generally won’t see a mechanic use a torque wrench unless the job in question is hypersensitive to specs like wheels, large alloy front covers, alloy oil pans, cylinder heads, etc.
Tighten a spark plug comes with experience and over-tightening a few.
All manufacturers will recommend spark plug specs for their cars and in an ideal world with the engine on the bench that’s great. But turn the engine sideways and place it in a shoebox and try to manipulate a torque wrench. In many cases, it isn’t possible and isn’t necessary.
A mechanics answer – tighten by feel, when you do it enough times you develop a sense. But a rule of thumb for the apprentice mechanic goes something like this:
- Screw the gasket (washered) plugs into a cold engine by hand until they seat, then using a ratchet and socket, tighten the plugs a half turn approx.
- For tapered plugs, tighten by hand until they seat, then using a socket and ratchet, tighten a quarter turn approx.
Tightening Using Torque Wrench
Torque specs are there for a reason, so where possible we will respect them. As stated earlier, in some cases getting the torque specs even a little wrong can cause some expensive failures, knowing what you can and can’t torque by feel is important.
A wheel should always be torqued to specification. Check out this post on my torque wrench “1/2 or 3/8 torque wrench”.
Common torque specs for both washer and tapered new and reused as follows:
- New and reused plugs 18mm and 14mm diameter threads 35Nm (26 ft. lbs.)
- New and reused tapered seat 10Nm (7 ft. lbs.)
Spark Plug Fitting Tools
Fitting a spark plug isn’t a difficult job but some engines will be more difficult than others, V6 and 8 can be challenging as real estate is usually tight when the engine is fitted to a sedan.
I improvise with some vacuum hoses attached to the end of the plug, which helps to get into those awkward rear cylinders. I use a magnet or claw to grab the plug from the cylinder, the rubber seal inside the plug socket does the job pretty well when new, but my tools have been around the block if you know what I mean.
If you prefer to torque the spark plugs, you’ll need a 3/8 inch drive torque wrench with torque range in the region of 5 plus ft. lbs. Anyway, here’s a few tools to get the job done as painlessly as possible. You’ll find links to the tools I use, including a 3/8 inch torque wrench here on the Mechanics tools page.
- Plug boot puller
- Plug socket
- Hose pipe
- Plug gap tool
- Di-electric grease
- Torque wrench
- Anti-seize (optional)
Should you lubricate spark plug threads? You should not lubricate spark plug threads, doing so causes over-tightening of the spark plug. Over-tightening the plug causes the plug to stretch and fail. It may also break off and remain partially stuck in the cylinder head.
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.