Tech File - XK Cylinder Head Repair 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 aluminium 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.
Alternative method of tappet guide lock down. (Started on right side and finished on left).
Tappet starting to come up
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. While the camshafts are off this; would be a good time to adjust the valves.
Excessively Chipped Tappet Guide
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 described.
Jaguar factory method of locking down the tappet guides.
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.
THE OFFICIAL MONTHLY MAGAZINE OF THE JAGUAR DRIVERS CLUB OF SA
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