The BMC B-series engine is a well-proven and fundamentally robust design that responds well to modification. From mild improvements suited to everyday road use through to more ambitious builds for fast road and competition applications, a structured approach to upgrading, addressing breathing, valve train, and timing, will produce measurable gains in power, torque, and engine refinement.
Stage II Cylinder Heads
The cylinder head is the single most effective area to address. A Stage II head, polished and ported to improve gas flow, fitted with new valves, valve springs, and bronze valve guides alongside hardened exhaust valve seats, transforms the engine’s ability to breathe. The improvement in mid-range torque and top-end power is significant, particularly when combined with other complementary modifications. Stage II heads are available for both chrome bumper in-line feed and rubber bumper offset feed configurations, and can be built on the customer’s own cylinder head or purchased as a new outright assembly.
Camshaft & Valve Train
The camshaft determines the engine’s character, its power band, idle quality, and suitability for road or competition use. Mild road camshafts offer a broader spread of torque and retain a smooth idle, while fast road profiles extend the power band higher into the rev range at the expense of some low-speed tractability. Cam kits are available for all engine types and include valve springs, cam followers, cam lube, and a static timing disc. Cam lube is essential during initial assembly to protect the new cam lobes and followers from scoring before the oil system has fully pressurised.
Roller rocker assemblies replace the standard pressed steel rockers with precision roller-tipped items, reducing friction at the rocker-to-valve interface and allowing more precise valve clearance adjustment. They are available in standard and high-lift ratio options, and in both in-line and offset feed configurations to match the cylinder head type. High-lift rockers must be paired with the correct combination of valves, springs, and camshaft to avoid valve-to-piston contact. A duplex vernier timing gear set allows the camshaft timing to be set in one-degree increments rather than to the nearest tooth, enabling precise optimisation of valve timing for a specific engine build.
The duplex chain also offers greater strength and wear resistance than the simplex chain used on later 18V engines, and is a worthwhile fitment on any engine where the standard timing chain and gears are being renewed. Alloy and chrome rocker covers are available as both a cosmetic and practical upgrade, an alloy cover dissipates heat more effectively than the standard pressed steel item and gives the engine bay a more purposeful appearance. Chrome plated rocker cover nuts and alloy dipsticks complement the look. For a comprehensive approach, the engine modifications above should be considered alongside the fuel system and exhaust upgrades covered in their respective sections.
A Stage II head and performance camshaft will deliver their best results when supported by adequate carburettors, a free-flowing exhaust system, and correct ignition timing. Modified engines should ideally be set up on a rolling road to optimise the fuelling and timing across the full rev range. The MGOC Spares performance range covers Stage II cylinder heads, performance camshaft kits, roller rocker assemblies, vernier timing gears, alloy rocker covers, uprated water pumps and pulleys, and all associated components to support engine upgrades at every level.