Tractor Repair: Rebuilt Ford 3000 has low compression, camshaft timing, piston rings

Question
I just rebuilt a 3000 Diesel boring and sleeving the block, new pistons and rings, head worked, new injectors. I know the timing and valve clearance is correct. It compression test around 250, the book says it should be 410-520. Any ideas what might be causing my low compression?

Answer
  Were the valves and seats ground when the head work was done?  If the camshaft timing and valve clearances are correct, there are still many other things that can go wrong during a rebuild such as incorrect pistons or piston rings, rings installed incorrectly, rings broken during piston installation, incorrect head gasket, air intake or filter plugged, camshaft and lifters worn, etc.