A long-stroke engine, though closer to square than BMC contemporaries, with a cast iron block and cylinder head using Weslake patents, its overhead valves were operated by pushrods.

The twin-cam Riley and OHC Wolseley engines were expensive to make, sold in low volumes and both had had reliability problems, especially with overheating valves under sustained high loads.

This should, theoretically, have provided the C-Series with superior 'breathing' and efficiency than the smaller engines since it still used the same highly-effective heart-shaped Combustion chamber design by Harry Weslake.

[citation needed] Other signs of the Morris origins of the design were the crankcase being cast with strengthening ribs (a feature not introduced to the A-Series until 1980) and the big end bearings being split diagonally rather than horizontally.

The C-Series also had a hydraulic tensioning unit for its timing chain and had the oil pump and distributor driven via separate sets of skew-cut gears on the camshaft.

With demand for the Austin-Healey 3000 still strong and development of what would become the Austin 3-Litre underway BMC carried out a thorough redesign of the C-Series with the aim of improving the power output and reducing both the engine's weight and its overall size.

The engine was reworked to carry seven main bearings (each reduced in width compared to those in the four-bearing version), steel ancillary castings were swapped for aluminium alloy ones where possible.

Similarly, the new cylinder head had the Weslake combustion chambers located symmetrically and centralised on each bore, instead of being offset to the exhaust side as on the original engine.

BMC aimed for a 30 per cent weight reduction at 175 lb (79 kg) and a maximum reliable power output in competition tune of around 200 bhp (149 kW; 203 PS).

The head had a higher compression ratio, larger valves (from the Ford Essex V6), revised porting and redesigned exhaust system as well as using triple twin-choke Weber carburetors.

These specialised versions of the C-Series made an easy and reliable 200 bhp (149 kW; 203 PS) with a weight loss similar that hoped for in the redesign process.