An internal combustion engine including: at least one combustion chamber; an engine body component, the engine body component defining a wall bounding the combustion chamber; an ignition system; a cooling system for cooling the engine body component in use; and at least one pre-combustion chamber assembly; the pre-combustion chamber assembly including: a pre-combustion chamber body, a pre-combustion chamber defined within the pre-combustion chamber body, a first electrode, and a second electrode; the pre-combustion chamber body including: a thermally conductive body portion, and a nozzle portion, the nozzle portion including at least one flame outlet; the pre-combustion chamber being arranged in fluid communication with the combustion chamber of the engine via the at least one flame outlet; the first and second electrodes (30, 40) being connectable to the ignition system of the engine to produce a spark within the pre-combustion chamber between the first and second electrodes when energised by the ignition system; the thermally conductive body portion of the pre-combustion chamber body being arranged within said engine body component of the engine, outside the combustion chamber, to transfer heat to the engine body component in use; wherein a first electrode aperture extends through the thermally conductive body portion of the pre-combustion chamber body, the first electrode aperture having an inner end opening into the pre-combustion chamber and an outer end (26") opening outside the pre-combustion chamber, the first electrode being electrically insulated from the pre-combustion chamber body and mounted or mountable to extend in use through the first electrode aperture into the pre-combustion chamber; and wherein a second electrode aperture extends through the thermally conductive body portion of the pre-combustion chamber body, the second electrode aperture having an inner end opening into the pre-combustion chamber and an outer end (27") opening outside the pre-combustion chamber; the second electrode being arranged in the second electrode aperture to extend along an adjustment axis from the inner end of the second electrode aperture towards the first electrode; the second electrode being positionally adjustable along the adjustment axis relative to the first electrode and relative to the pre-combustion chamber body; characterized in that the engine body component is configured to provide access for a tool to be inserted through the engine body component from outside the engine to adjust the second electrode in the second electrode aperture. at least one combustion chamber; an engine body component, the engine body component defining a wall bounding the combustion chamber; an ignition system; a cooling system for cooling the engine body component in use; and at least one pre-combustion chamber assembly; the pre-combustion chamber assembly including: a pre-combustion chamber body, a pre-combustion chamber defined within the pre-combustion chamber body, a first electrode, and a second electrode; a pre-combustion chamber body, a pre-combustion chamber defined within the pre-combustion chamber body, a first electrode, and a second electrode; the pre-combustion chamber body including: a thermally conductive body portion, and a nozzle portion, the nozzle portion including at least one flame outlet; a thermally conductive body portion, and a nozzle portion, the nozzle portion including at least one flame outlet; the pre-combustion chamber being arranged in fluid communication with the combustion chamber of the engine via the at least one flame outlet; the first and second electrodes (30, 40) being connectable to the ignition system of the engine to produce a spark within the pre-combustion chamber between the first and second electrodes when energised by the ignition system; the thermally conductive body portion of the pre-combustion chamber body being arranged within said engine body component of the engine, outside the combustion chamber, to transfer heat to the engine body component in use; wherein a first electrode aperture extends through the thermally conductive body portion of the pre-combustion chamber body, the first electrode aperture having an inner end opening into the pre-combustion chamber and an outer end (26") opening outside the pre-combustion chamber, the first electrode being electrically insulated from the pre-combustion chamber body and mounted or mountable to extend in use through the first electrode aperture into the pre-combustion chamber; and wherein a second electrode aperture extends through the thermally conductive body portion of the pre-combustion chamber body, the second electrode aperture having an inner end opening into the pre-combustion chamber and an outer end (27") opening outside the pre-combustion chamber; the second electrode being arranged in the second electrode aperture to extend along an adjustment axis from the inner end of the second electrode aperture towards the first electrode; the second electrode being positionally adjustable along the adjustment axis relative to the first electrode and relative to the pre-combustion chamber body; characterized in that the engine body component is configured to provide access for a tool to be inserted through the engine body component from outside the engine to adjust the second electrode in the second electrode aperture.