Mule 620 gas engine blows head gaskets. The motor was rebuilt before I bought it, so I thought I got a great deal. After it blew the 2nd set of head gaskets inspected the heads to discover what looked like gouges from a die grinder. I took the heads to a machine shop. He said the gouges mad the heads out of spec and he milled them for me. After I put them on it blew a head gasket again. The compression seems to be very high. Is there a thicker gasket I can put in? Could I stack 2 head gaskets to lower the compression?