Flywheel
 

Have a 2000 Discovery 2. when starting engine, the motor locked up. I replaced the starter with new one, after a while the head on the starter busted. I pull the starter again and found busted teeth on the flywheel. I pull the engine and replace the flywheel with a new one. I got everything back together and the motor was starting fine the first few times. after about the six time I started the motor, it locked again. I pull the starter again and found broken teeth on the new flywheel. What' causing this.
I do not want to go through that again.

Here's a pix from the oil pan side. The drilled reluctor disk is where the crank position sensor lines up, but that tiny thing would be sheared off by contact, and then no spark. Seems like plenty of space for clearance. Seems like if you had a wrong size bolt somewhere it would not have run OK several times. You do have a puzzle.

Check your cylinders for coolant.

The problem is that the teeth on the flywheel are breaking off

A head gasket leak can cause a cylinder to fill with coolant, which won't compress, it can do considerable engine damage. Called a hydrolock. Pull spark plugs one by one and look for moisture. Could also cause extreme wear on other parts. However, it would seem if a cylinder was full of water you might also have misfires, codes, running rough, loss of coolant, steam out the tail pipe, etc.

You've got to clarify several things. Lock up means.....?
a. with the starter engaged to the flywheel and turning over, it actually stops
cranking at some point.

b. the starter upon turning the ignition engages the flywheel and even fails
to turn the flywheel at all.

When the engine is running, does it run well.
Any Codes?
Any occasional grinding sound ( like your starter's gears engaging the flywheel?
Have you checked your starter to make sure the gear retracts back and stays
back ( so it doesn't interfere with the flywheel ) after there is no power to the
starter?
Check the flywheel with the broken teeth. Check the teeth well. It may give you
clues to whether the teeth fractured from an impediment or being over-driven.

Regarding hydrolock, coolant or oil entering the cylinder in sufficient volume will
cause the cylinder to lock up on compression.... serious engine damage.
When the engine starts ( before the hydro-lock ), it may cause an abrupt engine stop.
If during start up, it hydro locks before ignition is achieved, you will not get any ignition
at all.
Doubt if this is a jumped timing chain. Maybe someone else can add.

Post you findings.
We'll try to help.

If a cylinder is full of coolant the starter will try to turn the engine over but once the cylinder with the coolant in it hits the compression stroke the engine will stop dead, the starter will still try and turn the flywheel, stripping or breaking teeth if you are lucky.
If you are not lucky you will bend a connecting rod.

And where are these broken teeth ending up?

In the bottom of the bell housing, out of harms way as the bellhousing is just a empty space.

I pretty sure that water is not getting in the cylinders. When cranking the engine, and the starter reaches the bad spot on the flywheel, it locks. If I turn the engine back from the bad spot far enough, the engine will crank up and run with no problems. The problem is I just replaced the flywheel and I have only crank the engine several times, but have not driven the car. But teeth have already broken on the new flywheel. It's like the starter is engaging too far into the flywheel.