[Uhehner]

I cleaned my heat exchangers by removing them and flushing them with diluted swimming pool acid. Worked great. Then I installled new gaskets and zincs. There is a product called Barnacle Buster that I have not tried. Check into it. You may also need new elbows and risers if they are over 5 years old.. Be sure to use the correct gaskets and blocking plate. On my 454's I used the one water hole gasket on the elbows and installed it at the top. The three hole gaskets go at the manifold.
Capt. George