There is a difference between Radiator Stop Leak and Block Sealer.

If there is a crack in the Head Block Sealer (has Water Glass in it; can't remember the Scientific name for it)would be the most likely one to work. But, I have read you should keep it out of the Heater Core and some things I read about it has that you should keep it inside of a running Engine longer than the Instructions say.

What I don't know about Block Sealer is if it can be used as a preventative before there is a leak. However, I don't think you can use it as a preventative. I believe you already need to have a leak.

Stop Leak is merely a bunch of particles of something circulating around the Cooling system with some Chemicals that might help the Particles glue together as that block a hole.

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84 300D, 82 Volvo 244Gl Diesel