My buddy, Vic Giles, has this Sonerai homebuilt airplane. It uses a VW engine for power. He's giving me lathe lessons today and shows me some of the VW heads he has that have some cracks in them. The cracks are all in the same place, inside the spark plug hole in the threads. The crack runs from the top of the threads to the bottom and is very, very small as far as width goes. It appears that the crack does not go beyond the threads in depth, although there is some blow-by coming from the combustion chamber, up through the crack showing up by the side of the plug. Vic has redrilled and machined the heads to accept a second plug for each combustion chamber and these holes do not crack. He asked me if I thought I could try to fix these heads up (they cost about $300 a pair with valvetrain). I said that I wasn't sure and that I might ruin them. He said "they're no good now, so how much damage could you do?"
My question is, these are aluminum (I don't know what "flavor") ..my thoughts are to use a carbide burr and grind through the threads and create a U shaped channel where the crack is, then fill the crack with filler, grind it down a bit and rechase the threads for the plugs. Does this sound like a reasonable approach? If I mess it up, Vic doesn't care and I figure the experience might be good. I don't know what filler to try or if there might be a trick to this.
Any thoughts would be appreciated.