At the bottom of the cylinder that partially drilled hole is for the oil passage way. If you look at the engin case you will see an oriface and the partially drilled hole will allow oil from the oriface and will transfer to the head bolt and run up the cylinder to the head, cam plate. That is also why you will see o rings, that is the oil return.
Trikes:
1985 ATC 70 mod to 184cc
1984 ATC70 mod to 124cc
1983 ATC 70 Xmas restoration
Quads:
1986 TRX 250R
2002 TRX 400EX
2- LT80 for the kids
1986 LT 50