The Phantom was Rolls-Royce's replacement for the original Silver Ghost. Like the famed Ghost, the Phantom was constructed both in the United Kingdom and United States, with the US model trailing the UK by one year on introduction and two in replacement. One major improvement over the Silver Ghost was the new pushrod-OHV straight-6 engine. Constructed, as was state of the art at the time, as three groups of two cylinders with detachable heads, the large engine produced excellent power to pull the large heavy car. The engine used a 4¼ in (107.9 mm) bore and long 5½ in (139.7 mm) stroke for a total of 7.7 L (7668 cc/467 in³) of displacement. Aluminum was substituted for cast iron in the cylinder heads in 1928. Semi-elliptical springs suspended the front, while cantilever springs were used in the rear. 4-wheel servo-assisted brakes were also specified, though some initial US models lacked front brakes. Differences between the US and UK models included available wheelbases — both were specified with the same 143½ in (3644.9 mm) base length, but the UK long-wheelbase model was longer at 150½ (3822.7 mm) than the 146½ in (3721.1 mm) American version. Other differences included the transmission, with UK models using a 4-speed and US models using a 3-speed manual transmission, both with a single dry-plate clutch. UK models were built at Rolls' Derby factory, while US Phantoms were built in Springfield, Massachusetts.