[ChrisBennet]

When I'm disassembling something I save the fastners so I know what type of fastener to use. I've seen several types of engine perimeter washer&nuts so I think the fasteners may change over the years even for the same application - or someone has worked on the motor.

Here are the types of fasteners you'll see in a 911:
aluminum crush washers: act as a seal, used in conjunction with a locking nut
solid washers: Used with locking nuts or another locking washer.
spring washers: aka wavy washers, a type of lock washer. Used with normal nuts
nylocs: Used with aluminum crush washers
aluminum sealing rings: Used on aluminum (motor) and oil line banjo bolts
copper sealing rings: Used on steel (oil tank)
"under sized" nuts: some nuts take a smaller wrench than normal. 10mm nuts usually take a 17mm wrench but the 10mm nuts on engine carrier and transmission take a 15mm wrench. 8mm nuts usually take a 13mm wrench. The heat exchangers sometimes use 8mm nuts that take a 12mm wrench.

Just to get the ball rolling, here are some combinations I've seen. Keep in mind that they may be wrong i.e. fastener was changed in the past and replaced with the wrong type. Hopefully, someone will jump in here and set things right.
aluminum crush washer + nyloc: low torque gasketed covers
aluminum crush washer + metal locking nut: engine perimeter nuts, 915 transmission perimeter nuts
solid washer + metal locking nut: engine perimeter nuts
aluminum crush washer + nyloc: engine perimeter nuts, 915 transmission perimeter nuts
split lock washers: used internally on 915 transmission, some CV joint bolts, 911 engine carrier


That's all I have time for now...
-Chris