ORIGINAL: SrTelemaster150
The stock rear cover screws do not go all the way to the bottom of the hole in the case. (at least not on te FA150)
There may be enough thread left to use a longer screw. I think I used the length screw that was used in the FA150 cam housing.
Anyway, check the depth of the hole against the length of the OEM rear cover screw. If there is enough depth, try a srew that is 1mm longer or better yet instal a stud in the hole. No dangerv of bottoming out & breaking off a screw W/a stud.
A ''bottom tap'' may also get you enough additional threads to get a better hold.
Thanks for the idea. I'll take a look at it and see if it's the same on the 91. It would certainly be an easier repair.
In the event it won't work out, I beleive there is enough material in the crankcase to re-tap to a larger size.