The 100- 6 was introduced for the 1956 model year, the biggest change being the installation of a new Morris Motors six-cylinder engine. Although the 2639cc six-cylinder engine promised a 12 per cent rise in power output, it actually delivered less torque – 142 lbf.ft at 2400rpm – against the four pot’s 150 lbf.ft at 2000rpm. Designated the BN4 series, the 100- 6’s cabin was redesigned to include two-plus-two seating in the rear. Other changes included a bonnet that was now hinged at the scuttle, while restyled bumpers and headlamp rims ensured the 100- 6 looked externally different from the previous model.
Installing two small leatherclad rear seats resulted in the two six-volt batteries located under the rear deck being ditched in favour of a single 12volt battery relocated to the right hand side of the boot. Coker’s intricately designed folding/sliding windscreen now gave way to a fixed alloy framed screen and removable Perspex side screens replaced the previous hinged items.
The 100- 4’s gearbox and rear axle (a higher ratio for nonoverdrive cars) had been carried over to the BD4 and in a bid to increase power larger SU 1¾in H6 carburettors and a new 12-port cylinder head complete with separate external inlet manifold replaced the original six-cylinder engine’s Siamese ported head. Power output went up to 117bhp at 4750 rpm and torque was increased to a very useful 149lb/ft at 3000rpm and in the autumn of 1957, a small batch of BN4’s were fitted with all-round disc brakes. The BN6 two-seat roadster 100- 6 broke cover in March 1958 and this was produced alongside the two-plus-two BN4 until 1959.