They would not have that hole if the operators would keep the oil level checked and get the engines repaired when they notice abnormal oil usage. The 280000, 310000, and 330000 OHVs are have a head design that tends to blow the head gasket between the cylinder and push rod galley. This causes oil to blown into the intake via the breather and burned. Many of today's don't even cause smoking in these cases. Now the older 12.5 280000 L head series don't have this problem.
This is a common enough problem here that I do a leak down test whenever I do a valve adjustment as you can only hear (pinpoint) the gasket leaking while the valve cover is off; although, the normal blow by sound at the oil fill is increased. This is not mention in the service manual. Since I just recently got a proper leak down tester that has a percentage leakage gauge I am still getting a feel for what is normal for this type of failure. My homemade tester did find these leaks too.
One of the problems is the head gasket design in between the cylinder and push rod galley. They don't match the cylinder curvature so instead of fully covering the seating area it only partially does leading to weak area. It also doesn't help to such a wide spacing between the two clamping bolts.
Btw some customers will not tell you that the engines are using oil as I do ask before repairs.
