I read that the reason for them is to relieve stress in the block and prevent it from cracking. Do freeze plugs serve any other purpose? How can the freeze plugs go bad though, and why are they replaced in an engine rebuild? If you have freeze plugs out, can you remove all the dirt from antifreeze passages in the block? How hard is to replace freeze plugs, any special tools required? How many freeze plugs does an LT1 has? Shouldn't they rust to the block after 10 years of car sitting outside? What are the symptoms when freeze plugs go bad, or when they pop? I have a friend dropping a motor for head/cam change, clutch change, rear seal change, he is thinking also of new oil pump and maybe new freeze plugs. I did search on freeze plugs, but there isn't much posts talking about them.

 

"freeze plug" is somewhat a bad / misspoken term in this day and age. They are actually core plugs that are needed to remove core material after the block is cast. Yes they will most likely pop out if the water freezes solid in the block. But with alumimun heads and so on you will most likely have other damage. You can tell they are going bad if they are leaking. They tend to rust over long periods of time if the coolant is not kept in good shape. Yes if they are all out you can clean the setteled crap in the block they is why they are normally replaced during rebuild. That and the block cleaning processes usually destroy them or the sealant used on them.

 

The cores which are used to form the water passages in the block are located by "core prints" or extensions of the core to the outside of the casting. They make the holes which are also used to remove the broken up core after the casting solidifies. The holes are plugged, usually with a press in plug (aka freeze plug) or a threaded plug in high-end blocks. If another process were used to cast the block, we wouldn't have core prints or "freeze plug" holes.

Here's a description of the casting process:

http://www.efunda.com/processes/meta...ting_intro.cfm