# Freeze plugs



## orelog (Dec 21, 2011)

Does anyone know why Pontiac changed from 2 freeze plugs to 3 freeze plugs on the side of the blocks beginning in 1967?


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## Rukee (Feb 8, 2007)

Better protection maybe?


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## orelog (Dec 21, 2011)

Rukee, I thought of that, but it seems like it may make the block weaker in that area. I have a chance to buy a 400 block that was made in Nov. 1966 but that was in a 1967 GTO and it only has two freeze plugs. I want to make sure it is a good decision.


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## BearGFR (Aug 25, 2008)

First thing to realize is that those aren't "freeze plugs" --- their purpose is not to provide protection from freezing, contrary to popular belief. They are required as part of the block casting process to support the inner part of the molding sand and also provide a way to remove it once the metal has hardened. It just so happens that 1967 was the year that all the engines "changed". The 326 became the 350, the 389 became the 400, and the 421 became the 428. I suspect that in the process of designing the new blocks the engineers decided that they could get better, more consistent pours by supporting the inner structure in three places instead of two.

Bear


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## rickm (Feb 8, 2012)

no worries. i think in '68 they equipped blocks with pet cocks for draining.


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## jmt455 (Mar 26, 2012)

BearGFR said:


> First thing to realize is that those aren't "freeze plugs" --- their purpose is not to provide protection from freezing, contrary to popular belief. They are required as part of the block casting process to support the inner part of the molding sand and also provide a way to remove it once the metal has hardened. It just so happens that 1967 was the year that all the engines "changed". The 326 became the 350, the 389 became the 400, and the 421 became the 428. I suspect that in the process of designing the new blocks the engineers decided that they could get better, more consistent pours by supporting the inner structure in three places instead of two.
> 
> Bear


Bear is absolutely correct. 
This was a change for casting/foundry purposes, apparently due to the casting pattern revisions that made the 1967 blocks more rigid than the earlier style.
Though everyone calls them "freeze plugs", they are called "core hole plugs" in the Factory Service Manuals.


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## rickm (Feb 8, 2012)

BearGFR said:


> First thing to realize is that those aren't "freeze plugs" --- their purpose is not to provide protection from freezing, contrary to popular belief. They are required as part of the block casting process to support the inner part of the molding sand and also provide a way to remove it once the metal has hardened. It just so happens that 1967 was the year that all the engines "changed". The 326 became the 350, the 389 became the 400, and the 421 became the 428. I suspect that in the process of designing the new blocks the engineers decided that they could get better, more consistent pours by supporting the inner structure in three places instead of two.
> 
> Bear


thats interesting, didnt know that. they are referred to as "expansion plugs" and upon a water filled block freezing, they will push out.


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## jmt455 (Mar 26, 2012)

rickm said:


> thats interesting, didnt know that. they are referred to as "expansion plugs" and upon a water filled block freezing, they will push out.


Though they might pop out when a block freezes, that is not their purpose. 

They are there to allow the foundry core sand to be removed from the cooling system passages in the block casting.


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## BearGFR (Aug 25, 2008)

rickm said:


> thats interesting, didnt know that. they are referred to as "expansion plugs" and upon a water filled block freezing, they will push out.


Sometimes they will, sometimes they won't, sometimes they will and the block will crack anyway. Personally, I'd never depend on them to "save" an engine. 

That's really not what they're there for. The purpose of the holes is to support the inner mold structure while the block is cast, the purpose of the plugs is to seal the holes afterwards so the cooling system doesn't leak.

Bear


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## geeteeohguy (Feb 2, 2008)

I have never seen core plugs pop out and save an engine. Every freeze-cracked block I've seen has the core plugs still intact and happy. And I'm going all the way back to 100 year old engines I've looked at. Core plugs are for casting, as stated.


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## rickm (Feb 8, 2012)

back in the days when junk yard owners allowed customers to rummage around scrownging for parts i have seen expansion plugs that have popped out of blocks. i doubt they saved the block from cracking.


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