I believe it was mentioned here, but just go ahead and tap that nipple for 1/8" NPT and put a real pipe plug in it. Then use the dorman cap just for the aesthetics of a finished edge. I ran that on my 318 for at least 7 years.
There is no nipple on the manifold for a return that I can see. There is just that boss where an AC compressor would mount. My M1 intake has that extra nipple where I could feed it back to the manifold.
I'm talking about tapping the ID of the water pump heater hose nipple to 1/8" NPT. The one you put a rubber cork and all-thread on.
Yep, now I understand. I just need to get some NPT taps.
Just remember NPT threads are just that, tapered. If you tap too far, you stand the chance of the plug not getting tight anymore. Sneak up on the depth you tap it and don't go crazy on torque, the cast aluminum won't like that. A dab of RTV or some teflon tape, snug it up and go.
For what it's worth I tapped mine with the pump on the car. Shavings were minimal, I flushed out the pump the best I could with the lower radiator hose off and went on my way. Pump never did give me a problem.
