The Bendix engages the pinion gear and ring gear at near zero rotational speed while at the same time allowing the starter motor to spin up for a flying start.
When the starter begins spinning, inertia keeps the counter weighted pinion gear from turning as the helical shaft spins through it forcing the gear toward the flywheel ring gear. About the time the two gears mesh the pinion gear hits the Bendix stop nut and is locked to the shaft transferring all of the energy from the spinning starter to the flywheel. When the engine starts and the power is cut to the starter the pinion gear decelerates and spins along the shaft away from the flywheel.
The Bendix assembly is connected to the starter shaft through a spring, which uncoils absorbing the initial torsional impact of the spinning starter and then rewinds applying its energy to help crank the engine.
A drift pin and spring inserted into the counterweight rides on the ridge of one of the helical threads until it drops into a recess in the spring end of the shaft keeping the gear from drifting into the ring gear. The gear is trapped in that position until the next time the starter is engaged. If the pin is misaligned and slips into the groove between the threads it will jam the gear so that the Bendix will not engage.
The Bendix part number for a Ford Model T is L10FA. “L” for left hand, 10 designates a 10 tooth gear. The FA apparently references the extra-long stop nut. All of the drives I have are stamped L10D.
Next time you are at a swap meet sorting through a box of parts remember that Henry Ford was left-handed. The Model T Bendix is also left-handed. Here’s how to identify a left-handed Bendix. Notice how the spring curls the same as the index finger on your left hand.
Hold the spring in your left hand as shown in the photo. The spring eye should curl the same direction as your index index finger. Hold the Bendix shaft by the stop nut. Rotating the gear to the left will cause the gear to climb toward the spring.
Remove the Bendix drive cover. If the inside cover-mounting hole is slotted then loosen but don’t remove the mounting screw. Now remove the other three screws. With some finesse the cover can be removed and reinstalled without removing the inside screw. It is a bear to reinsert this screw so leave it in place if possible. I use 6mm x12mm socket head machine screws for this purpose. A long Allen Key holds the screw better than the blade of a screwdriver.
With the cover removed bend back the washer lock tab and remove the Bendix head, spring bolt. This allows you to remove the drive head and extract the Woodruff key. The Bendix assembly can then be removed. I have never had a problem sliding the Bendix off the shaft. Back in the day there was a special puller designed to extract a stuck Bendix.
It is always best to remove or install the starter when the Bendix assembly is not attached to the armature shaft so you don’t damage the magneto field coils.
When reinstalling the Bendix leave the Bendix head detached from the spring. Make sure the inner spring bolt is tight and the lock washer tabs are properly bent. Slide the assembly on the shaft then insert the woodruff key and slide the Bendix head over the key. Rotate the assembly until the head indexes with the head sleeve and line up the spring with the bolt hole. Position the lock washer and insert the bolt. When you have tightened the bolt make sure to bend the tab on the lock washer to lock it in place. It is wise not to reuse the lock washers. They are inexpensive so it is a good idea to keep several in your parts supply, just in case. (keep a couple of Bendix lock lock washers and a spring in your touring emergency kit)
It is not uncommon for the counterbalance to become separated from the pinion gear. These were often welded or brazed back in place. I repaired a couple of these gears only to find that the drift pin hole was out of alignment and the pin would not ride on top of the helical thread. New gears are not that expensive. http://www.modeltford.com/item/5021G.aspx
On a recent tour the owner of a beautiful fordor heard a terrible crashing sound in his transmission. He drained the oil and a found a piece of the counter weight in the drain pan. Intermittently during the tour other pieces of the counterbalance could be heard crashing around in the transmission.
To replace the gear you must first remove the stop nut which threads onto the Bendix shaft. It is staked to the shaft by swaging a piece of the shaft into the holes bored in the nut. To remove the shaft, first carefully run a drill through the holes. This will remove most of the staked metal. The nut is very hard. I find the easiest way to remove it is to clamp the shaft in the vice and lock onto the nut with Vice Grips. Clamp down hard and work the nut back and forth until it frees up. The nut is so hard that it is unlikely to be marred by the Vice Grips. It may be quite a challenge to free up the nut. Reinstall the gear remembering that the counterbalance goes toward the spring. Tighten the nut. I use a bit of high strength Loctite and then with a sharp punch, stake the shaft to the nut.