What do you call this piece? If it popped out, how can I replace it?

[Smuller9]

What do you call this piece? If it popped out, how can I replace it?

[roadnsky]

Freeze Out Plug
Available HERE by our host (MMI)

[Peter]

It is a freeze out plug. It pops out when your engine freezes - hopefully it pops out and your manifold does not crack.

https://moyermarine.com/product/freeze-out-plug-78-oblk_15_126/

Recommend you call Ken at Moyer to ensure you get the correct plug.

Hope that helps,

Peter

PS - Jerry types faster than me

[Al Schober]

Well, it's actually a core plug. The casting process uses cores to get internal cavities. That plug covers the hole where the core was removed after the casting cooled. You'll find some in the head too.
They do tend to pop out when internal fluid freezes and expands.