Here are instructions on valve cover gasket replacement (so you won't cut one again):
Remove old gasket. Clean valve cover and head with solvent. Must be absolutely clean and DRY, no oil, not even a film. Install gasket on valve cover, it pushes on.
Install valve cover to head, making SURE that the gasket didn't slip anywhere. Lift cover and thump firmly down on head. Repeat until the muffled "thumpf" becomes a distinct "clop", like a horse's hoof on the radio. You will hear it when it happens. This seats the gasket properly.
Install and tight the bolts ONLY using the head of your ratchet, not the handle. Not much pressure is required, and if you turn them down "snug" (that is, bottomed out), you will cut a nice slice competely though the gasket near the bolts. I know, I've done it.
You can always tighten them a bit more later if you need to.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!