The physical differance in many diesel heads is: frequently a contra piston is used. This thin free sliding piston within the head is located directly above the crank piston and is free to slide up and down. The compression screw above the head (can be a knurled nob or a T bar for turning) will push the contra piston down to increase the compression when the adjustment screw is turned in and vice versa.

With a dedicated diesel (not a conversion) the crankcase and head hold down design will be of a bit stouter construction as well as the head itself compared to a glow engine of similar displacement.

John