I did this on a set of 6X heads a couple months ago. In the end it worked. I haven't fired the engine yet.
You definitely have to layer it in slowly over a week or 2. I screwed up the first time. I packed the blind hole full of the stuff from Lowes that comes in a caulk tube. I had it all perfect. It didnt say or i forgot about the part of slowly heating it. I thought Ma's oven at 350 would cure it. The stuff expanded and flowed out out of the hole. Reminded me of 4th of July ash snakes that kids play with. Ma wasn't happy.
After I got all that crap out of the hole I got some premixed in a tub chimney stove cement from Ace. I layered that in a little at a time 1/8 inch or so. I used a small screwdriver as a spatula. I always tried to keep a large hole at opening so I could get the next layer in the next day. I used a propane torch to heat the cast iron then let it set overnight. It took at least a week to fill it all in. Then I baked it in the oven again at about 225.
I sanded it smooth and it looks great. I started with a file then sandpaper. I still plan to use the Felpro 1233 gaskets with my 65 tri-power intake.
Layer it slow. I learned the hard way.
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