I've slept a few times since I had my timing cover off so I'm not positive if I had differing lengths of slotted screws or not. As long as they are long enough to get a purchase on the threads in the hole and not so long they bottom out it shouldn't make any difference. You can try running the longer screws into all of the holes before putting the cover on and see if they bottom out anywhere.

Save a life, adopt a senior shelter pet.
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Name your dog Naked so you can walk Naked in the park.