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The repeatability of the operation of internal combustion engines is a key indicator of the regularity of the combustion process, significantly affecting the possibility of increasing the engine\'s thermal efficiency in the context of a lean mixture. One of the important directions for improving the stability of the engine operation of reciprocating units is the modification of the ignition system, including the design of spark plugs. This article presents an experimental study on the effect of using spark plugs with a variety of ground electrode geometry on the stability of the combustion process in a single-cylinder port fuel injection AVL 5804 gas engine fueled by methane. The tests were carried out at a constant 1500 rpm engine speed, a variable excess air ratio (λ) from 1.0 to 1.4, and a variable ignition coil charging time of 1.5 to 5.0 ms. For each point, 100 consecutive engine cycles were recorded, evaluating the repeatability of engine operation based on the coefficients of variation of indicated mean effective pressure (IMEP), cylinder peak pressure (Pmax) and the angle of 50% of heat released (AI50). The results indicate that using a “J” type spark plug from modern gasoline direct injection (SIDI) engines makes it possible to obtain stable combustion process parameters over the entire analyzed range of λ variation and coil charging times. This plug type is characterized by significantly less sensitivity to changes in engine operating conditions compared to a plug with a flat ground electrode. On the other hand, a spark plug with a flat ground electrode shows greater sensitivity to changes in the analyzed parameters. However, it allows a clear improvement in combustion stability for moderately lean mixtures (λ = 1.0-1.3) and at the leanest mixtures, especially at maximum ignition coil charging times. The study confirms the significant influence of spark plug design on the repeatability of engine cycles while pointing out directions for further modifications of ignition systems for reciprocating engines fueled with gaseous fuels.