8 cyl oil pan flap valve

bob ward

I'm cleaning out the oil pan on a 35 8cylinder engine. On the rear oil pipe there is a flap valve which is designed to close when the car is facing uphill. I'm not 100% sure what it does, but I'm thinking the valve is there to stop oil from leaking or siphoning out through the rear main. Is that about it?

Frankvintagefullflowcom

Correct. It acts as a check valve to prevent oil from flowing backward up through the passage that drains the cavity behind the rear main. It would never create a siphoning effect, though.
Since these engines have no rear main seal, there is a direct pathway to the outside world. Under most circumstances, it doesn't come into play but a quick start from a standstill on a significant up-grade might give you a good example of why it's there.
It's possible to retro-fit a rear main seal to these engines but it's not a simple task and certainly not worth the time/expense/effort unless you have some specific need for it.
Fnk