While I know the top part of the plastic cover is important for aligning the marks on the camshafts, what about the rest of the plastic pieces? The rear one seems more trouble than it's worth - of course, I just replaced the entire head and numerous other parts! - and I'm thinking that it might not be necessary to replace it.

I thought the collective knowledge and expertise here would provide the best answer. So, can I leave off the rear and main plastic timing belt covers and just install the upper portion?