A fuel injector mounting assembly in an opposed-piston engine allows for mounting of a fuel injector in a cylinder block without significantly deforming the wall of the cylinder into which the injector is configured to deliver fuel. The fuel injector mounting assembly includes a clamping arrangement to clamp the fuel injector to the cylinder block, an elongate tubular sleeve that sheathes a nozzle portion of the fuel injector, and a spanner nut attached to the elongate tubular sleeve. Clamping loads applied to retain the fuel injector in the cylinder block are controlled by the spanner nut.