While there were some significant exterior alterations, the big change was under the bonnet where there was a result of more than two years technical collaboration between BMC and Rolls-Royce. The aluminium Rolls-Royce FB60 engine was a short-stroke version of the B series engine: 4, 6 and 8 cylinder units of which more than 30,000 had already been produced. The 6-cylinder engine weighed only . Its cubic capacity was . Over-square: bore was , stroke ; with a 7.8:1 compression ratio its output was @4,800 rpm. Twin SU carburettors were fitted. Both block and head were aluminium, tappets were hydraulic self-adjusting operating on overhead inlet and side exhaust valves. The counterbalanced crankshaft ran in seven bearings.