My notes say:
Crankshaft Bolt (LH Thread): CUMMINS POWER GENERATION INC. Cage 44940
NSN 5305-01-361-6562, SCREW,CAP,HEXAGON HEAD, Onan 186-6196, Cummins 186-6196 Left hand threads, Class 12.9, Size looks to be an m16x1.50, about 30 millimeters long not counting the head...