I can hand turn the engine backward (CCW) but not forward 2000 Windstar 3.8 liter

Yah. Thats not good. I'd be looking at the connecting rods or the heads. Since you were able to turn it over 8 rev. backwards I'd pull the oil pan off and check the cods. If it was a broken or bent valve or some other blockage it should stop crank rotation in both directions and you should be able to make on full turn.

Did the motor fail while running? Or during a start up. The loud clank did the motor turn breifly or did the starter instantly shed gears.

Oh yah. Just incase it is a broken rod do not try to start it (if you get a new starter) or put to much pressure on it with the wrench, If it is a rod thats a good way to make a crappy situation much worse and punch a hole in the block. Been there, done that.

Motor failed during start up. As far as I can tell, it did not crank at all. I tried it twice and got a clank each time. After that, only the starter solenoid clicked, as the starter motor was apparently jammed with the broken teeth. I assume the engine was locked up somehow and the starter motor teeth broke as a result. Could it be something that runs by a gear off the camshaft, like the oil pump?

:) Not going to take a chance on destroying another starter motor until I'm sure what the problem is.

Doubt it. How much force did you put on the crank (how long of handle)? OK you've turned it 8 turns CCW and it stopped, then 1/2 turn CW. Will it go back CCW? Does it lock up solid or does it gradually lock up?

Starter is cheap compared to the engine block!!! If teeth broke off the starter or flexplate it is something major!

It would probably go past 8 revs backward. I just voluntarily stopped there. It locks up suddenly going forward. I used a 17-inch cheater bar to turn the crank.

Ok. When your done cussing at it can you check. I need to know with any motion CCW what is the most that you can turn the crank CW. The cam and everything after it move at a 2:1 ratio to the crank. If it's locking and cannot make a full turn forward from any point I'd start poking in the bottom end. Also does it stop dead? You need a small tipped borescope to peek in the oil pan via the drain plug and or the cylinders via the spark plug holes. Some mechanic's have them but the good ones are pricey!!!

need a motor...

Thanks. Might take a while for all that. I need to go back to work now.

http://www.ebay.com/itm/2M-USB-Waterproof-Endoscope-Borescope-Snake-Inspection-pipe-Video-Camera-4-LED-/360779956065?pt=US_Security_Cameras&hash=item5400293f61#ht_3132wt_1105 Holy crap. I'm buying one. Theses are cheap and will fit in zee hole. ? on the resolution?

Friend thought it could be the timing chain. Anything internal on this engine is had to get to. Comments?

Anything at this point. But I would doubt is was the timing chain. It would slowly bind and you should here it being right there with the breaker bar. What is the sub-frame like under that van. Can the oil pan be removed? if you can't find a borescope. My first thoughts were and still are a broken rod. You could do a simple check for that. Remove all of the spark plugs. Use a piece of semi-ridgid hose (something soft so the pistions, cylinders or plug threads do not get damaged) and slowly turn the motor over. If all the pistions move I'm wrong and it could be the timing chain or valve-train.