The Mechanics of Classic Car Overhead Valve Flat Tappet Cam Engine

High valve spring pressure in classic car overhead valve (OHV) engines creates a high pressure contact zone between the cam lobe and the lifter (a.k.a. flat tappet).

Modern passenger car engines are different. Many are multivalve overhead cam engines with lower spring pressures. Those modern engines that still use an overhead valve arrangement use roller lifters instead of flat tappets.

The high pressure contact zone between the cam lobe and the flat tappet in classic car engines significantly reduces the ability of the oil film to prevent metal to metal contact. This requires the engine oil to be formulated with the proper level of anti-wear chemistry to prevent metal to metal contact. Zinc dialkyl dithiophosphate (ZDDP) has been the predominant camshaft/lifter/valve train anti-wear chemistry in passenger car engine oils for well over 50 years. 

The level of ZDDP anti-wear additive in current API SM/ ILSAC GF-4 passenger car engine oils has been thoroughly tested in the current industry standard wear tests which use modern engine designs (lower spring pressures as compared to classic car flat tappet engines). Modern passenger car engines do not appear to have camshaft or valve train wear related field issues when using the oil specified for the vehicle.

There have been numerous field issues reported when the current API SM/ ILSAC GF-4 passenger car engine oils are used in classic car flat tappet cam engines or in high performance flat tappet cam engines. The current API SM/ ILSAC GF-4 passenger car engine oils with lower levels of ZDDP anti-wear chemistry are suspected of contributing to wear related field issues in classic flat tappet cam engines. Other contributing factors may be incorrect engine assembly, improper cam and lifter lubrication or lack of proper break-in assembly lube, and negligent maintenance of the valve train.