The tappet takes the place of the valve lifter, and the Cam Follower takes the place of the rocker arm. VAlve adjustment is not possible with the SOHC engine. For some reason when the tappet goes bad the plunger on the tappet collapses and does not extend. This results in the follower being kicked out of position when the engine is reved, simular to what you experienced. Ford recomends replacement whenever a tapping noise first begins.