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SECTION A THE ENGINE
GENERAL DESCRIPTION
The M.G. (Series MGA) twin-overhead-camshaft engine is built in unit construction with an 8 in. (20.3
cm.) Borg & Beck clutch.
The valves, which are fitted with bucket-type tappets, are inclined at an included angle of 80° in the
detachable aluminium-alloy cylinder head and are directly operated by the chain-driven camshafts.
The valve clearances are adjusted by means of hardened-steel shims, which are supplied in a range
of sizes. The shims are inserted between the top of the valve stem and the underside of the tappets.
Each camshaft runs in three renewable white-metal bearings and is driven by a fin. pitch duplex roller
chain from a half-speed shaft situated in the left-hand side of the cylinder block. The drive is taken
from the crankshaft to the half-speed shaft through a pair of reducing gears. The tachometer, oil
pump, and distributor are driven from the half-speed shaft.
Two idler sprockets are employed with the camshaft chain, one permanently located and the other
mounted on a fulcrum arm, the position of which can be adjusted by means of a manually operated
chain tensioner.
The aluminium-alloy pistons carry three compression rings and one slotted oil control ring. The
gudgeon pins are of the fully floating pattern and the connecting rods are fitted with renewable lead-
indium- or lead-tin-plated bearings. Three renewable bearings, also of lead-indium or lead-tin, support
the forged-steel counterbalanced crankshaft. The thrust is taken by special washers at the centre
main bearing. The renewable-element full-flow filter is secured by its centre-bolt to the right-hand side
of the cylinder block.
A centrifugal water pump fitted with a fan is driven by the dynamo belt.
The two semi-downdraught H6 S.U. carburetters are supplied with fuel by a large-capacity S.U.
electric pump.
LUBRICATION SYSTEM
An eccentric-type oil pump inside the crankcase is driven from the half-speed shaft by a short vertical
shaft. Oil is drawn into the pump through a gauze strainer and is delivered through crankcase drillings
to a non-adjustable plunger-type relief valve located at the rear of the engine on the left-hand side.
From the relief valve the oil passes through an internal drilling across the rear of the block and
through an external oil pipe to the main oil filter. The filter bowl is filled with oil at full pressure which
passes through the element into the annular space around the centre-bolt and from there into the
main oil gallery: drillings supply oil to the main, big-end, and half-speed shaft bearings.
Oil is fed to the camshaft bearings from the main oil gallery through an external pipe. The timing gears
are sprayed with oil from a small drilling in an oil distributor pillar attached to the engine front plate. Oil
is also taken through pipes from this distributor pillar to the timing chain idler sprocket bearing and the
chain adjuster sprocket pivot bearing.
A.1 DRAINING THE SUMP
The sump on new and reconditioned engines must be drained and then filled with new oil after the
first 500 miles (800 km.) and at intervals of every 3,000 miles (4800 km.). The hexagon-headed drain
plug is at the rear of the sump on the right-hand side. The sump should be drained when the engine is
hot as the oil will flow more readily; allow to drain for at least 10 minutes before the drain plug is
replaced.
The capacity of the sump is given in the "GENERAL DATA section.
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