I have an '87 420SEL, and it's about time to replace the timing chain and guides. I have read all the threads about this job that I could find, and unfortunately I'm still confused about the chain guides.

How many chain guides are there, all together, and where are they located? Which of them actually need to be replaced? Only the top ones on the cylinder head? Which guides do you need to remove the timing cover to replace?

Thanks for your help.