In this Tip of the Month, we will attempt to explain what a stakedown kit is and why you would want to install one on your six-cylinder XK engine. First, we must delve into a bit of the design history of the XK engine. The classic dual overhead cam Jaguar XK engine design dates to the early 1940s and how it came to be is a rather interesting story. During the Battle of Britain, the British government enacted a regulation which required manufacturers to guard their factories around the clock, primarily for the purpose of raising the alarm if Nazi incendiary bombs were to cause fires which could then spread to neighboring factories and businesses. In a move that was typical of his way of thinking, Sir William Lyons, head of Jaguar, reasoned that if he was going to have to keep personnel on the factory premises all night, he would schedule all of the design team members on the same night. This way, rather than simply biding their time in “firewatching,” his designers could while away the nighttime hours in planning for the new cars Jaguar would build after the war. The first drawings for what became the XK engine were put to paper during these wartime firewatching sessions. At that point in time, once the dual overhead cam layout was decided upon, the engine designers were not comfortable in allowing the cam followers or tappets to ride directly in bored holes in the aluminum head. They interposed interference-fit cast iron sleeves or tappet guides into the cylinder head for the tappets to ride in. (In engines designed in later years, by Jaguar and others, allowing the tappets to bear directly upon the aluminum head became accepted practice).
Referring to the picture of a sectioned XK cylinder head above, the tappet guide sleeve is visible below the RH camshaft; the tappet itself is just visible directly under and touching the camshaft. On the LH side, the tappet and tappet guide are sectioned exposing the spring and valve adjustment shim. For most of the XK engine’s production, the tappet guides worked fine and gave no problems. When engine tailpipe emissions became an issue, and the XK engine was tuned to run on substantially leaner air-fuel mixtures than when first designed, a problem began to appear with the tappet guide system. The leaner air-fuel mixtures resulted in higher engine temps and since aluminum’s rate of thermal expansion is almost twice as large as that of cast iron, a tappet guide (almost always on the exhaust side of the head) would occasionally loosen and begin to rise and fall along with the tappet itself. As the picture above shows, with the tappet guide fully seated there is very little clearance between the nose of the camshaft and the guide, about 1/16 of an inch. If the guide rises above its normal position, the camshaft (which is much harder than the tappet guide) will damage the top edge of the guide. See the picture below for the type of damage which occurs.
The random “chamfering” of the inner edge of the guide is bad enough, but if pieces are broken off the guide (as has occurred in the above picture) and make their way into the timing chest at the front of the engine, things can get very expensive very quickly when the chunks get caught in the timing chain and gears! To prevent this problem from happening, a number of ways have been developed to positively retain the tappet guides. One method is commonly referred to as a stakedown kit. This is a set of machined plates which are screwed into the top of the cylinder head and which bear on the edges of the tappet guides, preventing them from rising above their normal position. The main advantage of the Stakedown Kit plates is that they can be installed on the head with the engine in the car and the camshafts in place. See below for a sample plate showing how it fits between a pair of guides (the plate shown does not have the screws in place). The second photo shows a set of stakedown plates in place (the cam is removed even though this is not necessary to install the plates).
The stakedown kit plates are just one way to secure the tappet guides. All Coventry West rebuilt XK cylinder heads are machined to use cap screws to positively retain the tappet guides on both intake and exhaust tappet guides, even though it is very rare for an intake side tappet guide to come loose. See picture below. Special tooling is required to utilize this method of tappet guide retention.
A third method of retaining the tappet guides is to drill and tap holes from the outside of the head between the cams towers at 90° to the axis of the guide and then to install set screws to lock the guides in place. This method works but the set screw holes can be a source of oil leakage. The bottom line about tappet guides is that it is a very wise move to install a stakedown kit on the exhaust side tappet guides if one is not present or if one of the alternative retention methods is not in use. One can usually confirm if a tappet guide retention method is in place through the oil filler hole. A further point is that one should never ignore tapping noises from the valvetrain, especially if one’s tappet guides are not known to be retained by one of the methods discussed above.