This paper reviews historically-significant valve train developments and their combustion system/engine installation implications. It describes the ROTULAR TAPPET™, a simple invention which allows angularity between the valve stem axis and the operational plane of the valve actuator to produce hemispherical chambers with four radial valves. It compares modem conventional gasoline and diesel engines to upgrades utilizing this device. It concludes that a new prechamber on an advanced IDI engine, with DOHC, hemispherical chamber, four radial valves and variable effective compression ratio could provide the optimum solution for small automotive diesels. The new tappets can make these engines possible.