On '85 and '86 model 928s, the cam covers (also called valve covers) are held in place by a two piece bolt. Once part goes on top of the cam cover, and the other is a short "spacer" bolt that goes under the cover and screws into the head. Unfortunately, these bolts are so weak, and snap off in the head so often, that Porsche discontinued them:
Both the top bolt and the lower spacer must be replaced with longer S4 bolts:
You may be able to remove the covers and put them back on without a problem, but if you plan on ever taking out the spacers, plan on at least a few snapping off, and plan on replacing them with the longer S4 bolts. They're $24 per bolt new, but used bolts can be found for as little as $6 each.