XK Cylinder Head Repair
by Richard Maury & Chris Engelhorn
Loose Tappet Guides: The most common repair the XK cylinder
heads require is due to one or more loose tappet guides. The tappet guide is a
steel sleeve, machined to press-fit into the head as a guide for the valve
tappet. It is a tight fit and under normal circumstances does not move. The
symptom is a clatter, usually evident in the exhaust cam cover area, although
the symptom can appear on the intake side as well. This occurs due to the
engine being overheated, which then causes the aluminum of the head to expand
away from the guide allowing the guide to back out of the head along with the
tappet, bringing the guide into contact with the cam lobe.(Tappet starting to come up below far left picture)
Removal of the cam covers is necessary to confirm the problem. If the owner is lucky and did not run the engine too long while the symptom occurred, he might not need to have any tappets replaced. At this point, the factory method can be used to lock down the guides without removing the cams. However, if the guides appear damaged or broken the cams will have to be removed in order to inspect the affected guides (Severe case with Damaged Valve Guide above far right). Also, since the valve covers are off this would be a good time to adjust the valves.
Inspection of the guides with the cams removed will reveal only certain scenarios. The lobe might mark one or more tappets, leaving them essentially undamaged. Sometimes the guide will be raised slightly and look higher than the others, and if so, it should be tapped back down into place before being locked down. Impact by the lobe might wear away some of the rim of the guide, leaving a jagged edge. This does not necessarily cause a problem, so long as the tappet moves smoothly in the guide and the edge of the guide can be utilized for one of the locking methods describe below. If the guide is broken then another will have to be installed in its place.
Used guides are easier to install than new ones. This is because the new ones are oversize on the outside and undersize on the inside. This requires special machining of the head and then of the tappet guide once installed for proper clearance. Not all machine shops are capable of doing this job properly so a good used guide is usually a much less expensive choice.
Jaguar has a special lockdown kit for holding the tappet guides down (Center Picture above). The plates are set over the guide edges and do not rest on the aluminum of the head below the guides. Boltholes for securing the plates to the head are aligned and marked through the holes in the plates. Holes are drilled to the size necessary and self-tapping screws are used. We at Coventry West drill and tap threaded holes above the guide in an area both where the aluminum is substantial and where oiling is not inhibited. We then install cap screws to secure the guide.(Picture above second from right) When the guide is clattering it is basically "floating" out of the head's casting, so the head of the screw is sufficient to hold down the guide. We continue to use our method as we developed it years before Jaguar came out with a fix for the problem. We naturally believe our method is better but both methods work equally well. Locking the tappet guides down is recommend as a matter of preventative maintenance. If you decide to adjust your valves or if your cam covers are off for cleaning or polishing, then this would be a perfect time to lock down your tappet guides.
Cracks Due to Overheating: Before work on
a head begins, the first problem area to check is cracks. At the outset, it
must be stated that the XK head rarely cracks and it usually happens from a
severe overheat. Cracks are difficult to perceive with the naked eye, so we seal
the water passages of the head and pressurize the water jacket with air while
the head is submersed in water. If the head has a crack it shows up as fine
bubbles escaping from the affected area.
Cracks appear in typical places. One area often seen is a hairline branching out from the expansion plug (commonly referred to as freeze plugs) between cylinders 3 and 4 and going into the spark plug hole. (See picture above Far Left) The early heads have a threaded plug here instead of an expansion type. These early heads seem less prone to cracking. Cracks are also found between a valve seat and its adjoining spark plug hole.(See Picture Second from Left Above ) Almost any crack can be repaired but the cost of repair must be compared against the value of the head to justify the job. Early heads are usually numbered and matched to a car where as the later heads are not. It may be worthwhile to repair an older head in order to maintain originality. Also, the highly visible cracks, such as the ones on the top of the head, are the most difficult to repair if appearance is an issue.
Corrosion Issues: Corrosion is a problem
on old heads and must be addressed before the head is rebuilt. It is found
anywhere there is water, which includes the steam holes and water passages
between the head and the block, the passages from the head and the intake
manifold, and on rare occasions a head can develop a porous spot. We've seen
porosity form in the intake passages. This can be found only by pressure
testing as described above. Welding aluminum into the affected area is the only
long term proper repair for corrosion. Almost all XK heads require welding on
the sealing surface to the block, especially the late heads with the small steam
holes. Welding up the corrosion minimizes the amount of material that must be
removed during resurfacing to obtain a flat surface. (See Picture Below Left)
Corrosion in an aluminum head is directly related to the level of maintenance done to the engine during its lifetime. Most people don't realize that antifreeze has more than the antifreeze and antiboil functions. It also has anticorrosion properties, especially necessary for aluminum. Therefore, proper maintenance of an engine entails flushing the cooling system and installing fresh antifreeze regularly. The newer, longer life antifreeze will help minimize corrosion for a longer period. Keep in mind, however, that “Lifetime Antifreeze” really means "for the life of the antifreeze", not "for the duration of your life". It still must be changed on a regular basis.
Outer Surface Issues: These heads have been in existence for a long time, and have been worked on by many people. Needless to say, they need a great deal of attention in order to be made presentable again.
General Head Preparation:
Cleaning: All heads are steam cleaned and bead blasted. The large cylinder head stud passages are exposed to engine coolant and usually coated with corrosion. These are rebored. All stud holes are chased and blown out with compressed air.
Broken Studs: Any broken studs must be removed and damage to the holes repaired. (See Picture Below Right)
Damaged threads: All damaged stud holes are drilled oversize and steel threads, commonly referred to as Heli Coils, are installed.
Spark Plug Hole Damage: These, too, must be drilled oversize and replaced by a steel insert. Spark plug hole repairs can be avoided by using anti-seize compound on the threads when the plugs are changed.
Tool Marks and Other Abuses: Heads can be difficult to remove. Anyone who has tried to remove one knows this, whether it's the hobbyist working at home or the professional. They will resort to any and all techniques, no matter how destructive, to get the head off. Hammer and prybar marks must be smoothed and filled in with filler such as JBWeld before the head is painted. (See Picture Second From Left below)
Scrapers quite often mar the cam cover surface. I do not like the idea of grinding this surface down with hand tools because hand tools leave the surface rounded. I prefer to fill in the marks and then paint the surface lightly. The paint is just for appearance, and the home mechanic would not need to apply paint here, but filling in the scrape marks will give a better sealing surface for the cam cover gasket. Do not worry about paint on gasket surfaces, we've found that it has no negative effect on the ability of a gasket to seal.
Another area that
requires attention is gouging of the washer
surface for the acorn nuts that hold the cylinder head down. This gouging can
only be caused by someone in the past tightening the head down without flat
washers installed. It is astonishing that any mechanic would do this, but we
see it all the time. (See Picture Below)
Please note that if you are doing your own work, the XK engine used different lengths of cylinder head studs, depending on the position and type of hoist brackets (assuming it came with hoist brackets). If you have engine hoist brackets, you will notice that a thinner washer is used on them. The positions of the hoist brackets vary, too, depending on the model of Jaguar. Be sure to make note of the bracket and washer placement when disassembling your engine.
Cylinder head build process: After cleaning and preparation, the head is sent to the machine shop for surfacing, valve guide replacement, and valve seat grinding.
Head warpage and its effects: It is common knowledge that if a head is overheated, it is probably warped and therefore must be resurfaced. However, what is not common knowledge is that head warpage not only affects the sealing surface; it can affect the valve adjustment, too. When a head warps, the whole head warps. When a head is surfaced at the machine shop, it straightens only the surface that mates to the block. The rest of the head can still be slightly warped in the cam area. Once the head installed on the block, the cam sprockets are in place, and the chain tightened, the cam can pull down too far and the valves for number one cylinder can become too tight. We compensate for this when adjusting the valves on the bench and by using new cam bearings on the journals closest to the front of the engine. We recommend checking the valve adjustment after both cams are installed. Be sure your number one cylinder (the one at the front) is at TDC on the compression stroke (the distributor rotor pointing to the five o'clock position) when you install the cams, and don't let the cams 'snap' ahead allowing the valves to hit the number 6 or number 1 piston during installation!
Upgrades: There are upgrades that can be done to the heads that offer more performance and also improve reliability and longevity.
Steam Holes: Later XJ saloon heads came with steam holes between the combustion chambers, which mate to holes in the block. The early 4.2L engines did not have the holes at all. If an early head is installed on later block, the engine can overheat. This is because the heat around the cylinder walls will create steam pockets which have nowhere to go and will force the coolant out of the system. This will further reduce cooling capacity causing further overheating. We drill steam holes into the early heads. This also allows an early head to be installed in place of a later head without risk. A later head can be installed on an older block, with no adverse effect, as the holes will be redundant.
Enlarging the Valves: Carbureted heads had smaller intake valves than did the fuel injected heads. Most machine shops can upgrade these to accommodate the larger valves, providing a nice increase in performance.
Valve Guide Replacement: We replace the guides on the pre 1968 heads with new bronze style guides. The later guides have a groove in them that holds the valve guide seal in place. They also have a retaining ring to locate the guide in the head. The earlier guides were held in place through an interference fit only and seals were not used.
Camshaft Selection: Around 1968, Jaguar changed the camshaft profiles and the adjustment specification. These camshafts produce a lot less valve clatter and do not hurt performance. They can be put in to the earlier heads requiring nothing more than a valve adjustment at a different specification. These cams do not have an oil hole on the bottom of each lobe as the earlier cam do. This does not seem to hurt anything as the tappet valley is filled with oil with bleed off from the cam bearings and this is more than adequate for lubricating the tappets.
Multi Angle Valve Seats: This type of valve grind is commonly referred to as a “three angle” valve grind or a “performance” valve grind. This type of grind involves cutting the seat in at least three different angles. The valve is cut at 45 degrees and the main contact seat also is 45 degrees. The other two angles are on the seat and are usually 30 and 60. Some can even have more angles. This lets the air flow over the seat flow easier and as such lets in and out more air. This in turn helps the engine produce more power. A side benefit is that this type of job usually last longer than a single angle grind. The contact surface of the seat is consistent all the way around so the valve cools evenly. If one side had a wider contact surface, the valve would cool more on that side and would tend to warp causing loss of compression.
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