Timing Chain Cover w/ Oil Pump Fits 02-12 Chevy GM Hummer Isuzu 2.8 3.5 3.7 4.2L. The timing chain cover is responsible for keeping dirt and debris from entering the inside of the engine. Due to the fact that the timing chain and belt are housed inside the engine, the timing cover even helps to keep the timing belt or timing chain solid and secure for smooth operation. Although the timing cover is not a moving part, from time to time damage may be done to the cover or the gasket inside that will require service or replacement. Be sure to have your engine timing cover replace with our aftermarket replacement timing cover as soon as it starts to give you trouble.