Read a spark plug reference chart by finding the make and model of your spark plug and then reading all the other spark plugs on the same row of the chart. All spark plugs on each row of the chart are compatible with one another, and are interchangeable in most engines. However, because different manufacturers use different materials and different equipment when making spark plugs, slight variations sometimes exist.
Spark plugs have four main differentiating characteristics: reach size, hex nut size, seat shape and heat tolerance. Some spark plugs also have a rubber gasket that seals the plug into the engine block, while others do not.
Reach size measures how far a spark plug protrudes into the engine block. Hex nut size is the size socket required for installing the spark plug. Seat shape is the shape of the spark plug's base. Spark plugs can have flat, conical or tapered seats.
Heat capacity is one of a spark plug's key characteristics. Manufacturers often produce several spark plugs with identical reaches, hex nut sizes and seat shapes but different heat capacities. Heat capacity measures how hot or cold a spark plug gets when producing a spark. Typically, high performance engines for racing and heavy industrial applications require hotter spark plugs, while compact cars, motorcycles and small engine equipment only require relatively cool spark plugs.