Thinner than about 3.40 mm and the cam can clip the edge of the cam bucket rather than ride on the cam.
Thicker than about 4.00 mm and the weight of the shim starts to play a role as it becomes heavy enough to move between the bucket and the cam with the possibility of the shim dislocating at high engine RPM's that could result in engine failure
We did a lot of research on the correct shim thicknesses before machining and found that if you are going to do a job like this you need to find someone who speaks the language. Lucky for us David F. at Ferrari Service of Bedford is so knowledgeable that he is fluent in Ferrari (www.fluentinferrari.com).
David outlined the risks of going too thick or thin and the closeup photos below help illustrate this where it can be clearly seen that if a shim is too thin it can cause the valve lobe to clip the edge of the shim bucket. As for going too thick, it is hard to illustrate so lets just take David's word for it. Trust me this guy knows his stuff and we thank him for sharing his knowledge with us.
Now for some finished shots of the heads with all new solid stainless steel valves (to replace the fracture prone sodium filled Ferrari parts), beryllium valve seats, and freshly skimmed deck surface. Some readers will note that the block was not skimmed where the head gasket mates to it. Again following David's advice, once we determined that it was perfectly flat, we left it alone as machining it would improve nothing and only introduce unnecessary risk.
Lastly some may remember images of the butchered head plug where an SAE plug was forced into a Metric hole. Well Gord B. acquired a special tapered tap, worked his magic and here is the result: a perfectly installed plug.