You have finally put together your rebuilt MGB engine and the day has come to fire it up for the first time. WTF… there isn’t any oil pressure! Guess what? There’s a good chance you used the wrong oil pump gasket in your engine and if you haven’t killed your newly rebuilt engine yet you’ll be cursing when trying to remove the oil pan with the engine in the car.
So, to save you some trouble pay attention. When you buy a gasket set for your MGB you will be getting 2 oil pump gaskets. That sounds very cool doesn’t it? A spare for future use? Let’s find out.
Unless you have a very early 3 main bearing engine you will be using the larger gasket. A good way to remember this, 5 is larger than 3. I think what catches people off guard is that the large hole in the 5 main bearing gasket doesn’t go anywhere as can be seen in the image below. So why is this additional hole here? I really don’t know! What I do know is that the corresponding hole on the oil pump body gets sealed off by this larger gasket. I expect when you use the smaller gasket air is sucked into the pump rendering it useless so of course oil pressure doesn’t build.