I recently picked up a '73 RS that I believe still has the original L65. It's got an oil leak and I thought I had it fixed when I found the dipstick was loose. I can't get a good look at the back of the motor/heads, but caught this in a video clip. I know i need to fix the valve cover gasket, but this opening is on the backside of the driver's side cylinder head. What size and thread count plug do I need for this?