Tech Tip #56: Spark Plug 101

Dr. Diesel
Written by Dr. Diesel


Great tips for removing and installing your spark plugs.

While most of our customers now run diesel engines, a significant number still operate spark-ignited gas engines typically Ford Industrial, Continental, Hercules or Wisconsin. These engines are often found in lift trucks, as auxiliary engines in older sailboats, and even power generation plants running on digester gas produced in landfills. This Tech Tip, one of a number we publish, deals with the care and maintenance of the lowly spark plug. Old timers may chuckle as we go over familiar ground, but read on.

Removing Plugs

First, clean out the dirt around the base of the plug with compressed air to avoid it falling into the combustion chamber. Remove the old plug carefully from a cool head with a steady continuous pressure. Be careful not to use too much force which could result in stripping of threads and/or breaking the top of the plug. Once out, inspect the plug for the condition of the insulation and the color of the deposits. Wet, oil-soaked plugs are as bad as dark, soot-laden plugs. If the present plugs are going to be re-used, carefully clean the threads and apply some anti-seize.

Installing the Plugs

Use a wire type spark plug gap measuring tool. Check your owner’s manual for the recommended gap and then carefully thread in the new plugs by hand. Tighten with a torque wrench to the specs given below.

Torque Recommendations
Cylinder Head
Spark Plug Thread Size Cast Iron Aluminum
Squared shouldered, 8-12 8-12
Gasket Type
10 mm 10-18 10-18
12 mm 26-30 18-22
14 mm 32-38 28-34
18 mm
or a one quarter turn after finger tight
Tapered Seat 7-15 7-15
14 mm
18 mm 15-20 15-20

or one-sixteenth of a turn after finger tight.

Once clean, re-gapped plugs are back in, check your wires. Ignition wires will deteriorate after a couple of years of exposure to heat, oil, grease, and vibration. New premium wires are an inexpensive way to upgrade your system. We recommend ACCELL ignition wires. They’re a favorite of performance people and premium wires will go a long way to enhancing engine performance, and cost pennies more than wires from Walmart.

Another upgrade worth considering is to convert from a point-style distributor to a solid-state distributor. See Foley  Tech Tip #52  or Tech Tip 198 for a discussion of our solid-state conversions. Finally, many customers have reported success with ACCELL Super Coils and electronic spark enhancers.

We hope this tech tip has been helpful.  We publish this series of tech tips because we believe that tech support matters and we want to advance the knowledge base in our marine and industrial engine industry.


Dr. Diesel
Written by Dr. Diesel
Knowledge is power. Power to build ties to engine users. To build a relationship with our customers, we share with you our 105 years of knowledge in many ways. We have this special section called “Ask Dr. Diesel™” where you can pose questions about engines, transmissions, industrial hand clutches, exhaust scrubbers, etc.