The timing-gear assembly, the chain, gears, and tensioning arrangements, is responsible for keeping the camshaft turning in precise time with the crankshaft, with the valve timing varying by a few thousandths of an inch from the specification meaning the difference between a smoothly-running engine and one that struggles. The standard timing-gear specification fitted to classic-MG engines during production is reliable but offers limited adjustability and limited tuning headroom, while performance timing-gear kits allow the cam timing to be adjusted precisely and the chain durability to be improved for sustained high-output use. A complete chain and sprocket set should be renewed during any engine rebuild where the timing cover is removed, as a worn chain rattles from the front of the engine, retards valve timing, and causes reduced power and rough idle, while a stretched chain allows the valve timing to drift progressively from its set position, eroding performance gradually and invisibly until renewed.
Duplex Timing Chains
The original engines used a single-row simplex timing chain on some applications and a duplex double-row chain on others, the simplex chain being adequate for standard-specification use but the limiting component on heavily-modified engines where chain stretch under sustained load causes timing drift and eventually chain failure. The 1275cc A-Series uses a duplex chain running between crankshaft and camshaft sprockets with no separate tensioner, while the 1500 engine uses a single-row chain with a spring-loaded tensioner, chain stretch being a known characteristic of the 1500 as the chain elongates and timing retards progressively, the tensioner compensating for moderate stretch but requiring renewal of both chain and tensioner once at its limit, which should be inspected at every major service. Duplex timing-chain conversions for engines originally fitted with a simplex chain double the strength and durability of the chain for the same load profile, the duplex chain running on appropriately wider sprockets at both crankshaft and camshaft requiring the matched sprockets as well as the chain itself, the conversion typically done during a major engine rebuild with the duplex chain, matched sprockets, and tensioner supplied as a complete kit.
Vernier-Adjustable Camshaft Gears
Camshaft timing is conventionally set during engine assembly through the geometry of the timing chain and gear positions, with the result being whatever the production tolerances of the components happened to produce on that particular engine, and on a standard engine the cam timing can only be set to the nearest tooth on the gear, a coarse adjustment that may place the actual valve timing several degrees away from the ideal for a given combination of camshaft, cylinder head, and intended use. Vernier-adjustable camshaft gears use a multi-piece sprocket with the toothed ring rotatable relative to the hub through small increments, allowing the cam timing to be advanced or retarded by precise amounts during engine assembly, typically in increments of one degree, enabling the valve events to be optimised for the specific engine build, advancing the cam slightly for improved low-end torque or retarding it for better top-end power. This level of control is an essential tool on any engine where a performance camshaft or Stage II cylinder head has been fitted, the result being an engine set up to run at its optimum cam-timing setting rather than at whatever the production tolerances happened to produce. The vernier sets are typically supplied complete with both gears, the duplex chain, an Allen key for adjustment, and a timing disc for setting the valve events accurately during assembly.
Supporting Components & Fitting
Timing-chain tensioners are stocked as renewable components, the original-specification tensioner being a slipper-type or spring-loaded design that wears progressively over engine life, with renewing the tensioner during any timing-gear work being standard practice. Crankshaft and camshaft oil seals, the rotating seals at the front of the engine where the timing cover meets the rotating shafts, are stocked alongside the timing-gear kits as they are typically renewed at the same time, and the timing cover itself where worn or damaged is available as a renewed casting. The timing marks on the sprockets must be correctly aligned during fitment, as incorrect timing causes poor running, loss of power, and in severe cases valve-to-piston contact that destroys the engine, so care is essential during reassembly. The dedicated Engine Tuning section covers the broader range of components needed for any performance build, and the technical team is available to advise on the right specification for a specific application and intended use.