The most popular thread size for domestic plugs is 14 mm, although a few older Ford engines used the larger 18mm size back in the '60s. Thread reach does vary, but the two most common flavors are the short 0.460-inch and the longer 0.750-inch versions. The longer thread reach is generally used most often with aluminum heads, while the shorter versions are used with iron heads.
The Cold War Heats Up
Big horsepower means more heat, so when it comes to choosing a spark-plug heat range, the best advice is also the simplest-put spark plugs in the engine that will live. This goes for both race and street engines. Spark plugs are especially sensitive to heat, and as a result, designers offer spark plugs in several heat ranges to be able to handle the wide range of cylinder temperatures. A mild street engine idling around town prefers hot plugs to keep the center electrode at the optimum temperature to burn off deposits that otherwise lead to misfires and lost efficiency. An 850hp drag-race motor that spins to 9,000 rpm will require much colder plugs capable of quickly transferring all that additional heat away from the center electrode.
As you might have guessed, a strong street engine might actually need two different sets of plugs, depending on how the engine is being used. Those hot plugs for around-town cruising might well need to be one or perhaps two steps colder for those high-rpm, Saturday-night blasts at the strip if you're using nitrous or cranking up the boost on a supercharger.
How To Read Spark Plugs
Here is where the science of spark plugs diverges into more of an art form. We will begin to outline all the details of spark-plug reading, and even though there is much left to interpretation, there are a few simple techniques that can help you get closer to understanding what your plugs are telling you.
With unleaded gasoline, and especially the new ethanol-blended fuels like E85, the signs present on spark plugs will take some close scrutiny. Begin by finding a good spark-plug viewer. ZEX and others sell these inexpensive tools. The most expensive ones we've seen are medical-grade tools that let you see right down into the bottom of the plug porcelain, but they cost $150. It's also best to tune with new plugs that have perhaps two or three passes on them to help you read the results.
Think of spark plugs as windows into the combustion process. The end of the porcelain closest to the center electrode is used to read the idle mixture. The center portion of the porcelain will indicate part-throttle air/fuel (A/F) mixtures, while the bottom of the porcelain will indicate the wide-open-throttle (WOT) air/fuel ratio. Obviously, a black, sooty plug is way rich and a bone-white porcelain is probably way lean. The best way to judge WOT mixtures is to run the engine at WOT and then cut the ignition clean at the end of the track and safely shift into Neutral. Coast to a safe stop on the return road, pull a spark plug out, and use the viewer to closely inspect the plug.
 Note the shallow ceramic insulator...  Note the shallow ceramic insulator on the plug on the left. This is a very cold Autolite race plug. The greater length of ceramic with the plug on the right creates a longer heat path, which makes this a much hotter plug. |
 The spark-plug companies don't...  The spark-plug companies don't make heat ranges easy to remember. Companies like AC, ACCEL, Autolite, Bosch, and Champion use lower numbers for colder plugs, while NGK and Denso use higher numbers for colder heat ranges. |
 Too much ignition timing can...  Too much ignition timing can cause detonation. This is most easily read on a spark plug's ceramic insulator as black speckles. |
 A light tan ring near the...  A light tan ring near the bottom of the porcelain (arrow) is an indication of A/F ratio with unleaded fuel. This is a very subtle trend, so you should also use other measuring devices, like a wide-band oxygen sensor and/or quarter-mile trap speed numbers as reinforcing evidence. |