A rocker shaft assembly to be mounted to a rocker shaft mount (62, 64) of an engine, comprising: - a rocker shaft body being substantially cylindrical; - a first shaft face and a second shaft face provided on either end of the rocker shaft body; - a mid-shaft flat portion provided on the circumference of the rocker shaft body approximately half-way along the length of the rocker shaft body, the mid-shaft flat portion being a squared surface around the otherwise circular cross-section of the rocker shaft body; - two end-shaft flat portions provided on the circumference of the rocker shaft body adjacent the first shaft face and the second shaft face; the two end-shaft flat portions being squared surfaces around the otherwise circular cross-section of the rocker shaft; wherein the end-shaft flat portions and the mid-shaft flat portion share a common plane; - an arcuately shaped mid-shaft shim comprising a body extending around greater than half of the circumference of the rocker shaft and the dimensions of the arcuate shape of the mid-shaft shim are adapted such that the mid-shaft shim clasps the rocker shaft, the body of the mid-shaft shim having an interior surface portion contacting the rocker shaft and an exterior surface portion adapted to contact a rocker shaft mount (62, 64), wherein at least one of said interior surface portion of the mid-shaft shim and said exterior surface portion of the mid-shaft shim is composed of a copper alloy, the mid-shaft shim being mounted circumferentially opposite the mid-shaft flat portion; - two arcuately shaped end-shaft shims each comprising a body extending around greater than half of the circumference of the rocker shaft and the dimensions of the arcuate shape of the respective end-shaft shim are adapted such that the respective end-shaft shim clasps the rocker shaft, the body of each end-shaft shim having an interior surface portion contacting the rocker shaft and an exterior surface portion adapted to contact a rocker shaft mount (62, 64) in use, wherein at least one of said interior surface portion and said exterior surface portion of the end-shaft shim is composed of a copper alloy, the end-shaft shims being mounted circumferentially opposite the respective end-shaft flat portions. - a rocker shaft body being substantially cylindrical; - a first shaft face and a second shaft face provided on either end of the rocker shaft body; - a mid-shaft flat portion provided on the circumference of the rocker shaft body approximately half-way along the length of the rocker shaft body, the mid-shaft flat portion being a squared surface around the otherwise circular cross-section of the rocker shaft body; - two end-shaft flat portions provided on the circumference of the rocker shaft body adjacent the first shaft face and the second shaft face; the two end-shaft flat portions being squared surfaces around the otherwise circular cross-section of the rocker shaft; - an arcuately shaped mid-shaft shim comprising a body extending around greater than half of the circumference of the rocker shaft and the dimensions of the arcuate shape of the mid-shaft shim are adapted such that the mid-shaft shim clasps the rocker shaft, the body of the mid-shaft shim having an interior surface portion contacting the rocker shaft and an exterior surface portion adapted to contact a rocker shaft mount (62, 64), wherein at least one of said interior surface portion of the mid-shaft shim and said exterior surface portion of the mid-shaft shim is composed of a copper alloy, the mid-shaft shim being mounted circumferentially opposite the mid-shaft flat portion; - two arcuately shaped end-shaft shims each comprising a body extending around greater than half of the circumference of the rocker shaft and the dimensions of the arcuate shape of the respective end-shaft shim are adapted such that the respective end-shaft shim clasps the rocker shaft, the body of each end-shaft shim having an interior surface portion contacting the rocker shaft and an exterior surface portion adapted to contact a rocker shaft mount (62, 64) in use, wherein at least one of said interior surface portion and said exterior surface portion of the end-shaft shim is composed of a copper alloy, the end-shaft shims being mounted circumferentially opposite the respective end-shaft flat portions.