I had an RJC seep before under boost between #5 and the coolant passage at the back. That was on a stock short block with factory heads that had been ported and surfaced. I bought a large 1/2" thick piece of glass on line somewhere and used some 3m spray adhesive, and adhered some 220 wet sand paper to it. The piece of glass was sized to put 3 pieces side by side in the short direction. The floating process for making glass makes it about the flattest thing you can buy. With the block on the stand I started sanding the deck. You could immediately see the high and low spots which were numerous, and it was low between #5 and the coolant passage. It took a while to sand each side flat. I also checked the heads on the glass by sanding a little, heads were flat. After cleaning everything up, I reassembled with cometics. I followed what some other guys on the board did, I think it was Jerryl who recommended the method. I took the cometics apart, washed them, and sprayed hylomar on each layer both sides, and assembled. No leaks of any kind. After my current build, I did the same with the cometics, but the block was surfaced at the engine shop, no leaks this time either.
