OK this is my first time forgive me, I have a 1976 f100 custom single cab 2wd 302 c4. It has 3 cylinders with 0 compression the other 5 cylinders have 120 psi , I pulled the heads and checked the pistons, no holes in any of them cylinders were clean with no ring groove, I had the heads rebuilt and installed new lifters and push rods no change still no compression in 3 cylinders it has 40 pounds of oil pressure at idle when its warm starts easy every time pretty much just bump the key it doesn't smoke or burn oil either could it be a flat cam, if not how would I know if the rings are bad, the engine was rebuilt when I bought truck it is bored .040 thanks for any info