Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 16 Times in 11 Posts
Re: freeze plug
The core support plugs, AKA freeze plugs, are the three large ones. When the block is cast the sand cores that form the water jacket void are supported in the mold through the holes. After the iron solidifies the block is vibrated. That breaks up the sand which is dumped out through the core support holes. Then, to close them off, they are machined and steel plugs pressed in.
They get their common name from being pushed out when the block freezes, but if there were another way to support the cores, they wouldn't be there at all. They are a potential trouble spot and don't usually save the block anyway.
If the engineers were really concerned about protecting the block there would be five plugs, one between each pair of cylinders, and probably another five on the other side. That might do it.
The two smaller holes are threaded bosses where temperature sensors for the engine management system are screwed in. They are probably cast as solid bosses, then drilled and tapped.