match for the inlet ports. These engines therefore make excellent bases for some modification, achieving approx. SAE net and increased torque merely by "flowing" the head. Published horsepower ratings of the various versions of the 4.2-litre are complicated by the switch from gross to net horsepower systems during its lifetime. For example, in 1965 the 4.2-litre with triple SU HD8 carburetors fitted to the US-specification E-Type was rated at @ 5400 rpm SAE gross and a maximum torque of @ 4000 rpm whereas by 1969, now with twin Zenith-Stromberg carburetors, this had fallen to SAE gross. The difference indicates the effect of the revised carburetion and other changes made to reduce exhaust emissions from US specification engines. Jeff Daniels' reference states that the similar twin Zenith-Stromberg version fitted to a US specification XJ6 was rated by Jaguar at using the DIN net horsepower system, the difference being largely due to the effect of the gross versus net horsepower rating systems. The 1987 Jaguar XJ6 was the last Jaguar car fitted with the XK6 engine. In 1992 the last Daimler DS420 Limousine was produced. Like all other DS420 limousines, it was equipped with a 4.2-litre XK6 engine. The 4.2 variant was also used in some military vehicles with relatively few modifications. The XK engine has the rare accolade of having been in production and service in cars spanning six consecutive decades. All of the 4.2-litre XK engines used the straight port head. After the arrival of the ribbed cam covers in 1967 the heads normally remained unpainted, which some sources describe as "silver". The 4.2-litre XK6 was used in the following cars: Jaguar Mark X 4.2 - 1964-66, "straight port" cylinder head, painted gold, 3 × SU HD8 carburettors Jaguar 420 - 1966-68, 2 × SU HD8 carburettors Daimler Sovereign - 1966-69, 2 × SU HD8 carburettors Jaguar 420G - 1966-70, painted gold, 3 × SU HD8 carburettors Jaguar E-Type Series 1 4.2 - 1964-68, painted gold, 3 × SU HD8 carburettors (from 1967 US specification E-Types used 2 × Zenith-Stromberg 175 CDSE carburettors for reduced emissions) Jaguar E-Type Series 2 - 1968-70, 3 × SU HD8 carburettors (US specification E-Types used 2 × Zenith-Stromberg 175 CDSE carburettors for reduced emissions) Jaguar XJ6 4.2 Series 1 - 1968-73, 2 × SU HD8 carburettors (North American specification XJ6 used 2 × Zenith-Stromberg 175 CDSE carburettors for reduced emissions) Daimler Sovereign 4.2 'XJ6'