If you have an automatic transmission, it the ring gear is welded to a stamping that is bolted to your torque convertor. It is replaceable -- but requires the removal of the transmission (actually, at least sliding it back from the engine a few inches).
On a clutch-equipped engine -- the ring gear is welded or interference-fit to the flywheel -- so the flywheel is replaced.
If you have a diesel, they 'park' in repeatable spots due to the extreme compression -- so it is more usual to wear four spots on the ring gear.
Ring gears can work successfully with 25-50% of the tooth are worn down, providing that the starter is regularly engaging without a horrible grinding noise (and accompanying mis-match of the gears).
I assume that your discovery of damage to the ring gear was due to some problem -- and if the starter drive gear has not been engaging properly, you first need to ensure that the starter gear-engagement mechanism is fixed, else you'll just repeat the problem.
The teeth of the ring gear are straight-cut spur gears -- with flat (as opposed to rounded) edges.
The end of the starter drive gear is angled something like 45 or 60 degrees.
Two things happen simultaneously when you hit the starter key:
a) the motor starts to spin
b) the drive is thrust into the ring gear
Everything happens so quickly, that the angle of the starter drive gear, being angle-cut, allows for the gears to nicely mesh due to the angle of the starter drive gear guiding the gears into mesh while the gear is turning -- with the planned result being an engagement prior to applying torque.
One additional feature of the drive gear is that as it is thrust towards the ring gear, it rides on a spiral shaft so that the rotation of the motor is timed with the speed of the engagement so that the spiral adds a bit of time needed for the gears to mesh before effectively locking the starter drive gear to the starter motor shaft (as it is when it gets to the end of the shaft).
Starter problems occur when bushings which take up the side-thrust of the pinion gear wear out -- and, more likely, when the starter drive (pinion) gear does not slide/move freely and quickly on the motor shaft (common causes are no lube on shaft; galled shaft; sticky/hard grease on shaft; rounded pinion gear).
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Y2Kô Jim - N8JG@Hotmail.com
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