Stuck rings or hole in piston, BUT if massive blow, ----gotta be big to let that much blow-by.
What engine does it have in it? I assume also its a diesel?
Exhaust back pressure? Can't think of much else that would cause that. Head gasket wouldn't cause anything like that, I don't think.
Crankcase vent ?

"A machine you build yourself is a vote for a different way of life. There are things you have to earn with your hands."