In line with the E-type range the Jaguar Mk X received the new 4.2 XK engine for the 1964 season. The 4.2 Mk X also received the vastly improved, all synchromesh manual gearbox which dispensed with the old "Moss Box", a new automatic box was also available.
As with the 3.8 litre Mk X the 4.2 came with a substantial four door saloon body that could easily seat 6 people. A divided limousine version was also available although not many were built.
In 1966 the Mk X 4.2 became the 420G to bring it into line with other model names in the Jaguar saloon range and continued to be built in this form until 1970.