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Bosch Center Electrode Designs

Center Electrode Location on a Spark Plug

Center electrodes come in a variety of shapes and sizes.

Standard spark plugs typically feature a traditional 2.5mm center electrode. Center electrode variations include fine wire, recessed, and taper cut.


Standard Spark Plug Center Electrode

Fine Wire Center Electrode

Originally designed to improve starting and reduce fouling on two-stroke engines, a fine wire electrode was found to improve performance in four-stroke engines as well.

A smaller electrode requires less voltage to jump the gap, resulting in fewer misfires, which translates to increased fuel economy and horsepower. A smaller electrode also reduces flame quenching.

Reducing the electrode size on a standard nickel plug would result in a drastically shortened life span, so the smaller electrodes require exotic metals such as platinum or iridium to maintain (and at times surpass) the longevity of a traditional spark plug.

While a traditional electrode is 2.5mm in diameter, Bosch's iridium series features a 0.6mm center electrode.

Recessed Center Electrode

Retracted or recessed center electrodes are designed to place the spark out of the mainstream air/fuel flow. This makes it difficult to initiate a good flame front, but is necessary when valve or piston clearance is insufficient for conventional plugs, or when boost pressures and/or fuel type can cause excessive combustion chamber temperatures.

Taper Cut Center Electrode

A taper cut center electrode serves a similar function to a necked down, fine wire and v-cut center electrode. All trimmed designs have the same purpose: to reduce quenching and shadowing by reducing the surface area between the electrodes which could hinder the growth of the flame nucleus.