Farmall M Crankshaft/Flywheel question

We had a 51 Farmall M that was running until one day the starter was unable to turn over the motor. We can still turn the fan pulley/crankshaft and everything functions including the flywheel turns. However, when we turn the flywheel, the crankshaft does not turn. What do you think is going on here?
Thanks!

best guesses would be either a broken crankshaft end or the bolts that hold the flywheel to the crankshaft sheared off.

If the ring gear is loose on the flywheel it will act like that. If you put it in High gear (5th) and rock the tractor back and forth hard, the engine should move (fan turn) with the motion. if it does, it is the ring gear. If not it is the crank to Flywheel, or broken crank.
With the starter off, doing the above check will allow you to see if it is the Flywheel or loose ring gear. If ring gear, the tractor needs to be split to replace it. Not hard to do once split. Jim

Can you tell me will the crankshaft still be able to turn the flywheel if the crankshaft is broken? The Flywheel gear is not slipping.
Thanks again!

Do you know if the crankshaft will still be able to turn the flywheel if the crankshaft is broken? The Flywheel gear is not slipping.

We had an engine on a dynamometer test stand for the military. I was on the midnight shift and about 3 am it made a noise to wake up the dead, but it kept running and lost power. I shut it down and called the project engineer and he said
to wait until he arrived. He told me to start the engine and the dynamometer did the cranking
and about the time the engine started he was yelling to shut it down, but you had to wait until the dynamometer stopped.

After getting an oil sample and draining the oil
we pulled the oil pan and found the crankshaft broken in the counterweight. We disconnected all the instrumentation and pulled the engine off the test stand. The test engineer didn't want the engine torn down until the photo lab opened so he could get pictures. My shift was up and I received a call at home later that afternoon to come in at midnight. The crankshaft was replaced by the time I arrived, the engine was back on the test stand and running. No more failures after the new shaft was installed. Hal
PS: We had heavy mobile dynamometers for drawbar and cooling tests. We tested the M1A1 Abrams Tank before it was fielded.

Ive been around Ms all my life, and have never heard of one breaking a crankshaft, not saying it wont happen, but Ive never seen it. I also dont think youve sheered the bolts on the flywheel, thats 4 bolts and two dowels, but again, it can happen. I would drop the oil pan and see whats going on. Ill ask, how are you turning over the flywheel to determine that the rest of the engine isnt spinning? What happens att he flywheel if you spin the fan the opposite direction?

Thank you so much for responding.
To turn the flywheel, I take off the starter and turn the flywheel by pushing the gear with my pliers handle.
I'm glad you asked about turning the fan. I went back to check this more closely, and when I turn the fan either direction, the flywheel turns maybe a 1/4 turn then stalls out. It acts like it wants to go but just "stutters" then stops after the 1/4 turn. Then, when I turn the fan the opposite direction, same thing happens.
Also, I dropped the oil pan and did discover that a part of the lower piston in cylinder 2 had chipped away near the connecting rod. We found the pieces in the bottom of the pan.

Thanks again!

(quoted from post at 10:47:10 10/03/13) Do you know if the crankshaft will still be able to turn the flywheel if the crankshaft is broken? The Flywheel gear is not slipping.

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Probably not.

If the crankshaft is broken, it's broken somewhere between the flywheel and the fan pulley. Turning the fan pulley won't turn the flywheel anymore.

BTW, I have seen one broken crankshaft on this type of engine, on a Farmall 400.

IIRC, it was broken between the #2 and #3 cylinders.

My guess is broke at the center main journal. Take the spark plugs out and gently turn the crank in case a piston would contact a valve. See how many pistons come up when turned from each end.
If broke in center also check for block cracked at the center main webbing.

Time to pull the engine to see if the block was damaged. Hal