The engine is sealed by these two-piece caps over specific bolts. The bolt slides through a hole in the bottom half of the cap. Once the bolt is properly torqued into place, the top half of the cap is popped in. The top half of the cap is designed so that once in place, it cannot be removed without distorting the metal. This should make it easy for a tech man to spot an engine that has been tampered with. Caps are used on the front cover (so you can't get the camshaft out), the intake manifold (so you can't get the intake or heads off), and the oil pan (so you can't get to the crank or rods).