Fuel gas (G) is supplied via a pipe to a pre-combustion chamber of an internal combustion engine. The pre-combustion chamber is formed inside a chamber body which is received in a cavity of the engine body, while the pipe is received in a passageway of the engine body which communicates with the cavity. A seal which may be made from an elastomer comprises a wall defining an interior space opening through the wall at first, second and third openings. A first portion of the wall defining the first and second openings is arranged in the cavity so that the chamber body can be inserted through the openings into the interior space of the seal, while a second portion of the wall comprising the third opening is received in the passageway so that the pipe can be inserted into the interior space of the seal via the third opening. The pipe is sealed in fluid communication with the pre-combustion chamber via an inlet in the chamber body by sealing regions of the seal.