Morris Engines the X Series
By Neil Carns
The XPAG engine as found in quite a number of M.G. cars from 1938
to 1955 shares
much with the family of Morris engines it grew from. For those unaware of the
engines parentage, the designation XPAG can be read as Morris 'X' Series engine,
(known as the short-stroke Morris Ten engine) the P standing for push-rod unit,
the A being the bore of . . . and that 'G' indicating the engine is destined for
In my last little article I mentioned that M.G. used corporate parts from the
Nuffield Company parts bin, especially so from 1935. As the 1140cc X series
engine was intended for the Morris 10/4 and the Wolseley Ten/40, the engine was
designed as suitable for family saloon car use with a mild camshaft. When M.G.
came to use the engine, it was toughened up and bored out to 1250cc, but much of
the other parts were as in the standard family saloon unit. This particularly
applies to the engineering of the camshaft, and with the over-head-valve
components is perhaps one of the engines weak points.
The word "weak" here is not in the way of easy to break, but prone to a high
wear rate. One has to remember that the ohv engine was not common then on
bread-and-butter British saloons, ( sedans.) Only Vauxhall produced an ohv
engine for its smaller saloons pre-war, most other volume car builders still
fitted side-valve engines, ( and this even included Morris on their 918cc 8HP
cars.) So the XPAG as fitted to the TB, then the TC and YA, was very
up-to-the-minute in motor vehicle engineering. Add to this the hydraulic brakes
and independent front suspension of the YA, YB, YT and TD, TF, along with
rack-and-pinion steering, and those M.G.s were very modern in their day. The
styling was in another world, with the upright sit-up-and-beg bodies and very
high drag wings.
The camshaft has very narrow lobes, and initially not a very mathematical design
on the "19 thou" and early "12 thou" tappet clearance version. The later
equal-overlap 5-45-45-5, "12 thou" tappet clearance ones were better, but still
had a very small wearing area on those narrow lobes. The camshaft centre-line
ran a few thou off-centre of the cam-followers. This is intentional as it makes
the follower rotate in its bore as the lobe lifts it, evening out wear.
The bottom face of the follower is ground very slightly convex to assist this.
The cam follower (or tappet) lifts a steel push-rod to operate a rocker on the
top of the engine. The rockers function is to reverse the action of the
camshaft's up-lift, to a down-push on the valve stem. The action required to
operate the valve goes through a 180-degree reversal. This is not very efficient
and the reason for the over-head-camshaft (ohc) engine's superior performance.
But it is far better from the asthmatic side valve engine that preceded it. That
of the XPAG is about the best of the type of its date, with good porting and
slightly angled valves.
Whilst the rotating and reciprocating parts of the X series of engine get a
pressure feed of oil, the camshaft lobes rely entirely upon splash-fed-oil. The
camshaft is one of the more highly stressed parts of the engine, and this is
particularly true of the "ramp" on the camshaft lobes. A ramp is the bulge on
the camshaft that first lifts the valve open, then permits it to close under its
own spring pressure. The lobe has to force the valve open against those quite
strong springs, double springs on the M.G. models, but single on the Morris and
Wolseley. Consequently the wear rate of the camshaft lobes in the M.G. engine is
pretty high. Add to this the high wear rate of the rocker shaft as the rockers
bite into its underside, and that typical ticking-tappet noise of the M.G. can
be understood. It is far better to be able to hear the tappets, than to adjust
them so you cannot. The design of the camshaft has to have the "19" or "12" thou
clearance. Close it up and you will burn out an exhaust valve. The exhaust valve
relies upon sitting on its seat to get rid of much of its heat. Reduce this
time, and the valve rim will overheat and burn away.
The XPAG is supposed to rattle! Morris were well aware of the high rate of wear
of the ohv design, and so the rockers have their own oil pressure feed. This is
designed to spray oil everywhere, ( something you soon find out if you run the
engine hot, without the rocker-cover fitted.) This high oil flow has two
purposes. One to lubricate everything, and two, to remove heat from the valves.
The valve stems are shrouded for this purpose, and have stem oil seals, to try
to stop oil getting down those inlet valve guides.
The rocker shaft and camshaft went through the odd redesign, so there are three
different basic camshafts, and two different rocker shaft lengths. It is not
critical which you fit, as long as the rocker shaft is assembled correctly, (
the rocker-arms are all "handed") and you use the correct clearance with the
The camshaft is heat treated, and is not case-hardened as on modern cars. It is
pretty hard all the way through. This does not stop the lobes wearing away, and
the cam-followers getting hollow on their underside, (concaved.) The engine will
run quite satisfactorily with lobes well worn, followers concaved, and rocker
arms flopping all over the place, The signs are easy to look for, as performance
will drop off as valves are not lifted fully open; tappet clearances will be one
size one minute, and different later. The whole lot will rattle and tick away
like a diesel engine. Often at about 30,000 miles the rocker shaft needs
replacing, and sometimes the rocker arm bushes, ( but not always, oddly.) On
engines with well worn camshafts, it is possible to observe the worst lobes as
they hardly move their respective valve. A worn rocker shaft will hardly affect
oil pressure, as the feed pipe to the head, along with the banjo-bolts, act as
pressure reducing restrictors. In the rocker shaft there is only about 10 psi.
This camshaft and rocker wear is more often than not, the reason that two
apparently identical M.G. models will give different performance on the road.
Whilst on the flat the cars will run together, but once a hill is reached the
car with the worn camshaft lobes will struggle to keep up with the other, and
use more fuel in doing so. The engine has asthma. Often, it is the same car that
belches blue oily smoke after a trailing throttle on a long down-hill run. Here
the valve guides are worn, and with the throttle closed but the cars weight
driving the engine, oil gets sucked down the inlet-valve-guides. Open the
throttle, and the view behind disappears in blue smoke.
Sometimes just new seal will cure this, and an old trick is to feed thin rope
into the plug hole. Remove the rocker shaft and turn the engine over until the
piston firmly holds the valves shut on compression. Now use a split-tyre lever
to compress the inlet-valve springs, and fit a new seal. Those from the MG
Midget 1275cc engine are best, as they clip over the valve stem. Do not be
tempted to do this to the exhaust valves, as they will soon seize up. One hopes
you left one end of the rope outside the plug hole, as you now need to remove it
When the camshaft of the XPAG is placed next to that of the 1275cc M.G. Midget,
or the 1798cc MGB, it looks like a bit of bent wire with lumps on. The later
M.G. engine's camshaft is much meatier and the lobes are much wider, and with a
larger base-circle. But then again they do have the advantage of having learned
from those 1930s designs. (The A & B series engines are in fact Austin designs,
but Austin also used tiny lobes pre-war.)
Do not stint on the engine oil quality. Some parts of the engine really do need
good oil. The first bit of an engine to suffer with low grade oil is the cam
follower underside, where the contact is an infinitely thin line. The follower's
lower face pockets and then breaks up, chewing away the lobe. Today we have
pressure fed, hydraulically adjusted, camshaft followers, ( tappets ) that last