OILWAY PLUG SCREW 5/16" UNC - What's it for?

Plugged oilways are usually the result of providing an internal oilway junction within the casting. Once the drilling has done the pathway through,  the external access is tapped and plugged.  cheers, howard

Hi Martyn....as Howard says this is an internal oilway for all of the Dominator and Commando engines. It is very important in that it runs all the way from the crankcase sump up to the oil pump,  clearing oil from the sump. From the oil pump the route goes  back to the oil tank. (often via a filter).

Because this oilway can not be cast or drilled in a single machining/drilling, it has to be drilled in two external actions. One to the oil pump mounting boss and the other to the sump. On Dominators and Commando engines using a large sump plug a small amount of machining helps to connect the the lower oilway to the sump plug.

The Grub screw you spotted seals the drilling passage for the top oil way. The original grub screw being 5/16" x 18 tpi BSW. Commando engines going  down the UNC route.  There is a similar angled connection and grub sealing screw in the crankshaft where the internal oil passageways connect.

Some people remove the Grub screw and enlarge the 1/4" oil ways to 5/16". This helps the pump to scavenge oil from the sump faster which in turn can diminish exhaust smoking after starting up a cold engine that has not been run for a while.