After years of wear, the shims have a slight hollow on the side towards the valve. If they are turned upside down and replaced, the clearance may be in range initially. However, they will progressively deform (relatively quickly compared with normal wear) as the middle of the shim is beaten upwards by the valve. As this develops, the hollow moves to the other side of the shim, the valve clearance opens up and tappet noise increases.

 

Thanks for the clarification on shims Peter. I think we now have the whole story. I also made some minor edits above to better clarify. I used new shims throughout for better or worse. My engine rebuilder refused to use old shims & had boxes of new ones in all sizes. All he rebuilds are Jaguar XK engines. He also has every Churchill tool for his restoration business.

 

It's important to keep in mind that setting valve clearances down to the odd thousandth of an inch is a relatively high precision process in engine assembly. It's also a fairly elaborate one for the XK engine. I'd say new shims are pretty well essential; certainly never turn them over. Also, carefully line up the camshaft and torque down the bearing caps. Give the camshaft a couple of turns so everything is seated. Keep everything clean and by wiping with an oily rag. That way, it may get done in two cycles and stay done for 200,000 miles ... .

 

Same applies to Alfa's. The Alfa shim is a cup that fits over the valve stem ~ then bucket. (never grind shim cups!)

Out of interest my engine runs the slightly shorter buckets. All part of Lyons deal with our government on our local content program. Blank heads & blocks were sent to South Africa and all machining & assembly done here on the latest state of the art equipment for that era. (paid for by the SA taxpayer)

BTW ~ From February 1957, camshaft rattle during a cold start was reduced by drilling a lubrication hole in each cam. ~ Credit all the major scribes including Taylor.