Freeze Plug Blowout


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By Florida_Phil - 7 Years Ago
I was driving my '55 Bird yesterday when the engine started puking antifreeze.  Pulled off the side of the road an found a freeze plug laying on the passenger side of the frame.   Two weeks ago I upgraded this car to an aluminum radiator and electric fan.  The car runs cool all the time and it's was a huge improvement.  I'm sure the new radiator cap has a much high pressure setting.  Could that cause my freeze plug to blow out?  The engine was rebuilt by a previous owner.  The block and freeze plugs are not rusty.
By Ted - 7 Years Ago
Brass, steel, or stainless steel on the freeze plug that came loose?  Brass typically does not have the same press fit characteristics of the factory steel plugs so they are suspect from the git go.  My own preference leans towards the stainless steel freeze plugs as they fit tighter within the core holes while eliminating corrosion issues down the road.  I suspect the new radiator and cap simply upped the pressure on the cooling system just to the point that the freeze plug which was marginal in ‘hold strength’ simply let go.  You didn’t mention what pound cap was on the car before replacing the radiator but that’s likely at the top of the list for why the freeze plug did come loose.  Freeze plugs properly fitted to the holes should take all the normal operating pressures you can throw at them.
By Florida_Phil - 7 Years Ago
The freeze plug that feel out was a factory style steel plug.  I agree the old plug probably wasn't seated correctly and the higher pressure caused it to blow out.  The old radiator cap was a stock 8 pound cap.  I do not know the pressure of the new cap as it came with the aluminum radiator.  My guess is 20 pounds or more.  I'll install a new plug and check the others.  Triple AAA is a good thing....  
By KULTULZ - 7 Years Ago
Florida_Phil (4/6/2018)


The freeze plug that feel out was a factory style steel plug.  I agree the old plug probably wasn't seated correctly and the higher pressure caused it to blow out.  The old radiator cap was a stock 8 pound cap.  I do not know the pressure of the new cap as it came with the aluminum radiator.  My guess is 20 pounds or more.  I'll install a new plug and check the others.  Triple AAA is a good thing....  


Most likely the new pressure cap is 13-14#. That is a lot on an old system which probably needs servicing to bring it up to speed. Unless replacing all plugs, maybe consider the stock pressure cap (less pressure) until a complete flush/plug replacement.
By slumlord444 - 7 Years Ago
Think I would replace them all while I was at it.
By PF Arcand - 7 Years Ago
Until you can replace all the frost plugs, most parts dealers have 7 lb caps available.. 
By Florida_Phil - 7 Years Ago
The freeze plug that blew out was behind the starter.   I pulled the starter, installed a new freeze plug and reinstalled the stock radiator cap.   The car is back on the road.    When I swapped on the G heads I could see the pistons were .040 over.  This engine was rebuilt some time in the past.  How long ago, who knows?  It runs great and I don''t feel like pulling it out right now.  The freeze plugs are not rusty.    I agree they should be replaced.  I'm thinking about building a new short block.  Maybe this summer.
By 56_Fairlane - 7 Years Ago
Whenever I replaced freeze plugs on any block, I always swabbed some brown Permatex around the edge of the cup type plugs. I never had one blow out.