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Bosch Center Electrode Designs

Center electrodes come in a variety of shapes and sizes.

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

Standard Spark Plug 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 center electrode is 2.5mm in diameter, Bosch's iridium series features a 0.6mm 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.


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.

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