I appreciate the input. These were new brass freeze plugs. They didn't blow out when the engine was run on the stand after the rebuild for the cam break in. Do you think it would be a good idea to replace them all?
You will want to look at the one that came out and compare to new one to see if you can figure why it dropped out, unless you had water in the lower block and it froze pushing it out.
Like I say those frost plug bores can rust and pit real bad, so you want to make sure they are staight and clean. Then put in another new one and make sure it goes in snug. (They may have cocked the first one bending it out of shape before it went in.)
Then you can go around with your install socket and tap firmly on the others and make sure they don't move. But not hitting them so hard that you drive them in more.
