The International Cessna^® 170 Association

The O-300 parts manual (figure 9 index 35) shows a pipe plug installed in the crankcase cover under the starter, between the mags, and above the tachometer cable adapter. My cover (early style as shown in the figure) does not have a plug installed or a hole where a plug could be installed. Does anyone know if any early crankcase covers had just a plug. Also, what was the purpose of the hole that the plug was filling? It almost looks like a plug that could have been removed to prime the oil pump.

Later engines had a right angle starter drive and a pad for a "wet" vacuum pump. That plug was replaced with a pipe to hose adapter for the vapor return from the pump. If you check your parts book closely you'll probably notice a different part number for the accessory case assemblies for the later engines.

The oil system can be primed through the front of the crankcase. There are plugs in the front of each oil gallery.

When reinstalling such plugs, remember that tighter is not better.
Don't try to crack the case!