Piston and camshaft wear


(Mike Spoelker) #21

It would be my opinion that in the context of the carbureted engines found in the 1950’s XK’s, absent racing or very serious track use, you should see no benefit form this modification.

The tappet guides (the part that is pressed into the head) on Series 3 XJ6 4.2L engines were prone to loosening and floating upwards where they would come into contact with the spinning camshaft. The hold-down kits were developed to prevent this from happening. I have had a S3 XJ6 4.2L engine pass through my hands that had this exact failure.

The tappets in an XK engine are very slightly off center to the cam lobes. This offset imparts a force to the tappet such that it rotates a bit with every camshaft rotation. This ensures that the same spot on the tappet is not constantly exposed to the load from the nose of the cam.

Compared to the camshaft, the tappet guides are relatively soft. When the guides loosen they ride up with the tappet where their inside edge is struck by the spinning cam lobe. When struck by the camshaft two things happen: first, the guides begin to rotate in their bore, making them looser, and second the cam nose begins to gnaw at the inside edges of the guide. The result is a nasty ticking noise and a large quantity of very abrasive metal particles in the oil supply. If left unremedied long enough, you will soon lose a main or rod bearing.


(tony) #22

On some old MKVII heads I have, the cam buckets and retainers were in very good condition, but what I did find was valve seat recession serious enough that would require new valve seats inserts.

An easy way to identify the issue is within Des Hammills XK engine building book.

Mine were marginally outside the spec, using the method he outlined

When I took the heads to an expert XK engine builder & machinist, he took one look and said they were not ok


(Roger King) #23

Thanks chaps. So far, by my own visual inspection, my head will need new valve seat inserts, valve guides, valves (and springs etc.), cam followers and cam bearings. Beside excessive clearance in the valve guides, although the valve seats are very nicely polished evenly, with no signs of burning or corrosion in valves or seats, there has definitely been some recession. This may be due to lower levels of lead in more recent times. I’ll take advice from the machine shop on that as the inserts themselves seem unduly large compared with other aluminium heads I’ve worked on so might take more machining, but obviously that’s not really the issue. A couple of the cam followers have not been rotating as evidenced by polished lines worn by the cam lobes. Lots of varnish around to have caused that. I’ll get a more scientific crack test done too - love the clip in the film where they just dip the head underwater to test for cracks! A bit of TIG around the one open waterway and hopefully, fingers crossed, that’ll be it for the head.