Jaguar Stakedown Kit Information
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.
Coventry West 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.