Coolant leak found on rear of block

Hey guys, need some help trying to identify the caps in attached picture please.

Background- 2002 Tundra V8 4.7L. Started to run hot - replaced the radiator, water pump, thermostat, coolant pipe O ring. During all this, snowmaggedon hit Georgia. The truck sat outside in 13 degrees for 2 days. Today, I get everything buttoned up. Started to fill the radiator and coolant starts pouring out the rear of the block on both sides.

I put the hose in the radiator, turned on and crawl all over it above and below. I find these caps "blown" out of the rear of the block with water dumping (not leaking) out. These are located below the valve covers, basically behind where the pistons are. I can't find any diagram in the book or online labeling what these are.

This did not happen prior to the cold snap. I had originally tried a flush, then replaced the water pump and those caps were still in place. I assume the lack of coolant and residual water caused the blow out.

Can anyone help identify these and what are they held in with? The caps are very clean around the edges (no sealant or gasket leftover). They're about the size of a quarter. The attached pic is from the passenger side, looking up past the exhaust system. The driver side is basically a mirror image.
Thanks in advance guys. Dave.

 

Freeze plugs. Although not a reliable method to prevent cracked block. I’ve always used Permatex Indian Head shellac in the brown bottle on mine. Best wishes…

 

Thank you!

 

Bigger question is: You should've had pink long life coolant in that engine to avoid issues. What were you actually running in there?

 

I do run the long life pink coolant in it. This happened after I tried doing a flush with mineral water and a coolant flush.

 

Did you use the correct mineral water?

 

Supposed to use distilled water, not mineral water.

 

Someone recently talked about using RO water over distilled; do you see any advantage?

 

Distilled water has always worked for me, and it's what most manufacturer's call for.

In a pinch, for active flushing purposes, I've used city water, so it's not like I can judge anyone on what they use. But distilled water is typically cheaper at the grocery store than mineral water, so that's a motivator for me too.