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Most Twin Cam Engines use a direct acting tappet which is commonly adjusted with shims and this is the classic defitiniton of a 'flat' tappet engine.
The XK Jag engine is typical as is the Lotus Twin Cam, Ford BDA, DFV, most Coventry Climax Race engines. They are commonly adjusted using a ground shim between the tappet and valve. the Alfa sud engine is unusual in that it uses a flat tappet that combines a screw adjuster.
I think the standard 996 has a hydraulically adjustable direct acting tappet.
The 906 Solid lifter is a variation on this design as it uses a small lash cap for adjustment. This style of lash cap has become quite a common replacement for the heavier tappet in a wide range of race engines.
The reduction in ZDDP that ocurred in most oils around 5 years ago does cause flat tappet engines specific problems.
I believe that around 1200ppm is needed to ensure that adhesive wear does not take place.
I believe that current SAE standards limit this to around 800ppm which is just not enough.
(I hope my figures are correct)
If modern oils are used during 'break-in' then damage is very likely and either cam lubes are essential or carefully formaulated Break In oils are vital.
We used to use Valvoline 20/50 racing and for years had no issues. The new VRI had a roughly 50% reduction in ZDDP and since it introduction we swapped to Joe Gibbs Drive XP4 and again have had no issues.
There are now several high ZDDP Race Oils on the market as well as additive concentrates from Lucas Oil, Redline and a variety of other companies.
Some care is needed as excessive levels of ZDDP can cause bearing pitting and other damage.
I think the maximum level is around 2000ppm but I am sure that there is better information than this with a little more research.
I am sure that this is quite a problem for the classic car industry and we all need to be using the correct grade of oil with sufficient anti wear and anti scuff additives.
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