D2-D311 engine only rotates partially

Check inside the clutch compartment for anything that may be catching.

I had one doing the same thing. Turned out that mice had come in via the exhaust pipe and built a nest. Some nut shells and other debris had fallen into the cylinders through open valves, and enough had gotten compressed when trying to turn the engine over to prevent a complete revolution.

Could also be a thrown rod - they typically have this exact symptom. Easy to verify by removing the two crankcase side covers and shining your flashlight in there. All you need is a 9/16" wrench__PRESENT

Sounds like a valve stuck open to me.

have you got a borescope if not there not expesive use it to look down inside the pilot exhuast to see if its rusted out if rusted something may have gotten into the cyl.
then do the same with the exhuast manifold check the ports for signs of obstruction. as neil suggested pull the side covers to check for busted rods etc.

check if you can turn the water pump by hand

finally if finding no damage or blockage give the cyl which have open valves a real good squirt of possum piss (inox rp-7 etc) or 50/50 diesel and oil mix let it soak for a day or 2 try turning it gently with a breaker bar rocking it back and fowards.

if that dont work pull the heads and place a block of wood in each cyl then fill with the diesel mix let it SOAK for a few days, top up the deisel daily and give each wood block a tap.

if the rings are stuck that will free them remember gentle taps.

I had a mouse nest in a cylinder once. I was able to blow through the open intake valve with a shop vac and by working the exhaust valve open and closed via the rocker spit the debris out.

Hi Team,
an easy way to check if debris is in a cylinder is to remove the exhaust manifold and see if anything is in a port--if so then a inspection camera from an el cheapo speed shop or such, as mentioned above, would be the next step, then you can decide which way to go then. You may be lucky enough to blast the debris out via the engine inlet and exhaust valves as described above also.
Cheers,
Eddie B.

Great thoughts-thank you guys very much!

Got into it some today. Took the exhaust manifold off, and put the shop vac on each exhaust port, while opening the exhaust valve with a pry bar. I sucked a few corn kernels and seeds out. It has a straight /open exhaust pipe and it must of not been covered-I think the mice made a doomsday stockpile in at least one cylinder. 😞 I tried turning the engine over after sucking out the debris. It turned over, but now its stuck, can't turn it back. I also took the air cleaner intake off, tried the shop vac on it, and nothing came out of the intake.

I assuming when the piston is near TDC, I can only open the exhaust valve a very small amount. Is that normal, or are the seeds in the cylinder going up against the valve?

I don't have a flexible borescope/camera but that is a good idea to maybe get a better idea of what is going on before jumping into taking the head off.

a old farmer told dad and I always put a tin over the pilot and main engine exhausts as the pilot exhausts through the intake the pipe is only thin and can rust out dusting the motor or vermin contamination happens. exhuast flaps only keep out water they wont keep out vermin