1980 Ski-Doo Shop Manual, Page 115Get this manual
SECTION 03 SUB-SECTION 02, (ENGINES)
From top edge of magneto side inlet port, mark crankcase at 49 0
Magneto side mlet port
To correctly install the rotary valve disc proceed as follows:Turning crankshaft counter-clockwise, (drive pulley side) bring magneto side piston to Top Dead Center using T,D,CgaugePosition the rotary valve disc on gear to have edges as close as possible to the marks, NaTE: The rotary valve disc is asymmetrical, therefore, at assembly try positioning each side of disc