I have a 12v motor that had never been apart. I installed a new Cummins head a couple of years ago and had grooves cut for firerings. I had a coolant pressure issue when running the stock water pump and had to pull the head off. Noticed my intake valves were barely leaving a mark on all 6 pistons. Checked valve protrusion. The intake valves were at the small side of the spec and the exhaust were at the large side of the specs. Had the intake valves set into the head about .010" to get to the larger side of the spec and fixed the problem.