Water-jacket o-rings for the H74 and M18 are now available at Microcosm.
H74
microcosm-engine.com H74 uses a specification with 15.2X1.8 - H74 uses a specification with 15.2 inner holes and 1.8mm line diameter.
M18
microcosm-engine.com M18 uses a specification with 15.5 x1.5 - M18 uses a specification with 15.5 inner holes and 1.6mm line diameter.