Rebuild 4

Article 37 Engine Rebuild Part 4

Last month’s article addressed rebuilding the Jaguar XK engine. Since there is a lot to discuss regarding engine rebuilds we are
continuing with a fourth article. As we discussed last month, my engine is a 4.2 litre XK engine for my 1967 E-Type 2+2.
Dis-assembly of the engine revealed damage to the combustion chambers, rings, pistons, and cylinder walls. I decided to send the
engine to Dick Maury at Coventry West for a professional rebuild, not to mention repair of the damaged parts so that I could
continue with a “numbers matching” engine combination. When we left off in last month’s article, Dick's team had finished
preparation of the block and was ready for the head.

In case you have lost track, some of the combustion chambers on my head looked pretty rough. Looking at the pistons, the rings
were broken and in some cases missing. Small pieces of the rings had been pounded into the head. Not pretty! Note that as
Jaguar developed the XK engine from its original 3.4 liter displacement up to 4.2 liters, the diameter of the cylinder bores came to
exceed that of the combustion chambers. This was not all bad, as it created a "squish zone" where the piston develops a very
narrow space around the edge of the combustion chamber in the head, which squishes the fuel/air mixture violently towards the
center of the chamber, increasing turbulence and promoting better burning of the fuel. I know, engineering stuff, but you will find
that many of the performance gains in modern engines over the years since the 60's have been realized by clever configurations in
the cylinders to promote more turbulence. So Jaguar was on to something. One downside is that any small item that finds its way
into the combustion chamber, like a screw from your throttle butterflies, will manage to beat up this small space pretty badly. My
broken ring parts certainly had an adverse affect!

The sight of this carnage caused me to contact Dick for advice. He advised me that the damage could be welded up and the
original head preserved. The heads are made out of aluminum so an expert welder was needed. Dick "has people", just like H&R
Block, and was able to get the damage repaired. Another common area of damage is at the passageways for the coolant. This is
caused by folks that go for years without changing their anti-freeze. The anti-freeze anti-corrosion properties fade in a few years
and aluminum heads are especially venerable. Can you remember the last time you changed your antifreeze? If not, change it!
Damaged spark plug threads can also be repaired. For all rebuilds, Coventry pressure checks the water passages in the heads at
60 psi to verify that there are no cracks, corrosion, or other hidden damage. If the head passes this test, it is ready to be rebuilt. A
very light skim cut will be made across the gasket face of the block to make sure it is perfectly flat.

The XK engine is a dual overhead cam engine with 2 valves per cylinder. It is also a “cross flow head” which means the fuel/air
enters on one side and the exhaust exits the other side. Very efficient but, strangely, not a common design feature in the 60’s.
Dual overhead cams allow separate timing profiles, if required, for the intake and exhaust valves. Overhead cams also simplify the
valve opening/closing mechanism. The crew at Coventry inspects all of the components for the head, those being valves, valve
seats, valve guides, cams, cam followers, and cam buckets and replaces any parts that are damaged or out of spec. All of these
parts are then cleaned and readied for assembly.

An important step is to ensure that the surface between the valves and their seats are in perfect contact. Machining of the surfaces
may be required and in most cases the valves are “lapped” to ensure perfect contact. Lapping is simply a process where the valve
is positioned into its seat with some “lapping paste”, a slurry of liquid with fine abrasive powder, applied to the contact surface.
When I do this at home (and you can try this at home!) I use a small rubber suction cup mounted on the end of a dowel which I
stick to the flat end of the valve. I then spin the dowel in the palms of my hands, which rotates the valve against the abrasive slurry
and brings the contact surfaces into perfect contact.

Dick notes that if requested, Coventry can source valves from a later XJ6 head, which are slightly larger and flow a little more air.
Once the valves and guides are ready to go, each of the 12 valves has its valve spring and keeper installed. Coventry locks all of
the valve tappet guides into place using small screws. Tappet guides have been known to work loose in some XK engines. I believe
the later engines from the Series 2 cars are more prone to this due to their increased operating temperatures from efforts to meet
emissions regs.

Now the cam shafts can be installed, along with new camshaft bearings. If requested by the customer, later camshafts can be
sourced and installed which have superior profiles for quieter operation, although there will be no change to engine performance.
Note that these later camshafts have substantially different cold gap requirements so if you have this done try and placard the
engine, discretely, for the benefit of future owners and mechanics. Dick’s crew will of course set the cold cam clearances spot on
and they should require zero maintenance for years to come, as the clearance properties of an overhead cam engine are very
stable.

The renewed head is now almost ready. The correct color of paint is applied to the spark plug valley, in my case gold. A new head
gasket is laid on the block and the head is positioned into place. The studs from the block stick up through the head and are
tightened up in a methodical uniform way to the final “torque” or tension value. Be prepared to spring for new chrome acorn nuts for
your head stud bolts. They are not cheap but are a very distinctive part of the XK engine.

Next, the timing chains are loosely assembled into place onto their sprockets on the ends of the camshafts. The relationship of the
chains to each cam is very critical to overall engine performance. This is referred to as setting the cam timing, which I discuss in
more detail on my website. Dick’s crew will get it right! Finally, new valve cover gaskets are put into place and the valve covers
installed. Again, spring for new chrome acorn nuts. With the installation of the front breather cover and properly gapped spark
plugs, your rebuilt engine is done! It will be safely ensconced on a shipping pallet and securely wrapped for shipment by truck. Be
sure and have Coventry arrange the truck shipment, they get a much better deal than I could arrange!