1979 Ski-Doo Supplementary Shop Manual, Page 89Get this manual
SECTION 04
SUB-SECTION 02 (ENGINES)
To install shaft assembly and oil seal, use no420876 600 pusher
From top edge of magneto side inlet port, mark crank case at 50 0
Rotary valve adjustment with replaced crankcase
having to timing marksTo correctly install the rotary valve proceed as follows:Turning crankshaft counter-clockwise, (drive pulley sidel bring magneto side piston to Top Dead Center using T.D .Cgauge- Position the rotary valve on gear to have edges as close as possible to the marksAngle finder