Anyone know why someone might have done this to the crank?
I think I know how it can be fixed. My thought was to put something to block where the key goes and weld it up and then file it down.
Anyone got any other ideas?
I honestly have no idea how someone could have damaged the taper of the crankshaft like that. Whatever it had for a flywheel has to also be junk. The flywheel fits on a machined taper on the crankshaft. There should not be any wobble or out of round condition to either of these surfaces. It would probably be best to just swap the crankshaft out. Welding the taper where the heavy flywheel is supposed to fit sounds like a disaster waiting to happen. Finding a good replacement crankshaft and flywheel for the TE, TF, THD and TJD should not be difficult. Those are common engines.
Replace the crank (if it is a standard one)? That might be easier. Otherwise, I think your plan will work if you have the facilities (and skill) to do it. If you are a machinist, you can probably re-cut or at least clean out the key way, although you might have to buy/make a custom cutting tool.
Looks like somebody already had plans to weld up and file the damaged shaft, and what we see is the grinding prep ?
A possible problem with that plan is that distortion from the welding heat could leave you with a crooked shaft that no amount of filing will fix.
That's not acceptable where you'll be spinning a flywheel at engine speed.
I myself have fixed a few cranks with bad or worn journals or key slots. I used a mig welder set up as if I where gonna weld 1/8" stock and slowly fill the hole in, then weld up the whole shaft where I planned to cut it back down to size. Then cut a new key slot. Of course the crank was out of the motor. Also, because you can only chuck the crank on the opposite side, just take your time when cutting it back down to size. I have even repaired worn crank tapers that way as well. Just my .02
My guess is both the crank and flywheel will be wiped, yes you can repair anything with enough time but I'd be looking for replacements of both. If you don't get the taper restored to like new condition on both parts it will happen again.
The motor in the pic is a 2 cylinder tjd. Not sure on the HP. I bought a ditch witch with the motor already on it and they gave me another motor to swap it out. I am in the process of swapping them now with a thd. The thing is I am having trouble with the other motor. Valve was stuck so had to remove the cylinder head and work it out. As I was putting the head back on I broke a head bolt. Went to rural king and got some grade 8's and plan on replacing them all. Anyone know the torque specs on the cylinder head btw? I did 35lbs and broke a bolt .
Make woodruff key out of a piece of carbon (from a motor brush) put the carbon piece in what is left of the key way then weld the crank , then put the crank in the lathe tram the taper with the compound and then cut the taper knock out the carbon and you should be good to go , the flywheel nut needs to be tighten to 65# the flywheel nut was not tighten that was what caused the damage . Repaired many cranks this way