A spark plug (10) screwed into the wall (3) of the combustion chamber has a selectable spacer (30) interposed between a stop (14) and a contact face (6) to set the position of the plug (10) so that the ground electrode (12) can be arranged as close as possible to the wall (3), thereby reducing the temperature at the electrode (12) from the conventional level of 900°C, by as much as 100 to 200°C.