Abstract: In an internal combustion engine, the use of natural gas containing hydrocarbon or hydrogen may result in backfires into the intake manifold. Thus, a flame arrestor is disclosed for insertion into intake runners of the intake manifold. The flame arrestor may comprise at least two substrates, separated by an internal air gap of 3-9 millimeters (e.g., substantially 4 millimeters). The substrates may each comprise a metallic mesh with channels. The channel of one substrate may be unaligned with the channels of the other substrate. The internal air gap and/or unaligned channels of the disclosed flame arrestor effectively quench backfires into the intake runners.