The Spark Plug Page: Size Does Matter

Size does matter, just ask the Society of Automotive Engineers (SAE.) Spark plugs differ in many ways: thread diameter, reach, temperature range, and electrode design, to name just a few. In this, the first of a series of Spark Plug Pages, I want to discuss thread diameter and "Reach." OK, I know your auto service manual gives you all the information you need to buy the right replacement spark plugs for your vehicle but is not it satisfying to know the why behind the how? For a proper match, the spark plug design must match the cylinder head design in thread diameter and in "Reach."

A Word on Tools
As you know by now if you have been reading my articles on auto mechanics published here on Experts123 and elsewhere on the web, I recommend building your tool kit project by project. You will pay more for a quality tool up front, but a quality tool will cost you less in the end because it will last you a lifetime with the proper care. Cheap tools will need to be replaced repeatedly.

Tool You Need to Add to Your Toolbox
1. Now is the time to add a quality set of spark plug sockets to your toolbox. The Wilmar W30936 5 Piece Spark Plug Socket set is a complete spark plug socket set that is very reasonably priced. You could use regular, deep well sockets on spark plugs, but do not do it. You will break or crack the spark plugs porcelain body. Spark plug sockets have a rubber insert that holds the spark plug securely and safely during installation, and during removal.
2. A click-stop adjustable, 3/8-inch drive torque wrench. It is possible to install spark plugs without using a torque wrench but the torque wrench is a tool that you will use almost every day so invest in a good one if you do not already have one.
3. Assorted length 3/8-inch drive extension bars.
4. Spark plug-gapping tool.
5. Spark plug boot pliers.

Spark Plug Thread Diameter
I got my start in auto mechanics when every thread was either United States (US) Standard or SAE and wrenches were sized in inches. The metric system has supplanted the English system of measurement. Today, the SAE has switched to using the metric system exclusively. If you are serious about mastering auto mechanics, either as a hobby or a profession, you need to master the metric system of measurements. There are four standard thread sizes for spark plugs – 10, 12, 14, and 18 mm.

What is Reach?
Reach is the distance the spark plug extends in the cylinder. The threads of a spark plug with the proper reach will end flush with the inside surface of the cylinder head, the combustion chamber’s surface.

The Problems When Too Short
A spark plug with too short a reach will place the spark plug’s electrodes inside the threaded hole in the cylinder head. Shrouded electrodes may misfire causing an intermittent misfire on that cylinder. The exposed threads in the cylinder head will also collect combustion byproducts and cause pre-ignition on that cylinder.

The Problems of Too Long A Reach
A spark plug with too long a reach is more dangerous than one with too short a reach. A spark plug with too long a reach places part of the spark plug’s threaded base inside the combustion chamber, and it is possible for the pistons in some engines to strike the plugs causing serious damage to the piston, connecting rod, crankshaft, and cylinder head. The exposed spark plug threads will over heat and cause pre-ignition. Combustion byproducts will build up the exposed spark plug threads, which will damage the cylinder head threads when the spark plug is removed.

On the next Spark Plug Page we will discuss the importance of using spark plugs with the correct Heat Range. We will take a closer look at what we mean by spark plug heat range.