The Bendix drive itself only spins with the starter's shaft. Sometimes the flange breaks, which might cause the problem you describe. You can usually hear a metallic tinging noise when this happens. The starter can be removed easily and quickly, so have a new spring and special bolts with tang lock washers on hand. Only one size spring is available anymore, so hopefully it will match yours. Examine the Bendix drive flange to ensure it is not broken free. Then install the new spring, put the starter back in place and see if anything has changed. If not, THEN order a new Bendix drive. If you're feeling unlucky, order all the parts at once so that you'll be prepared. You can keep the original drive as a spare, should it be o.k. after inspecting it.
Addressing a bent starter shaft: This normally causes the Bendix drive to jam into the flywheel's ring gear, locking up the engine. I suppose it's possible that a bent shaft could cause intermittent operation, disengaging for no apparent reason. But the usual result of a bent shaft is jamming, which most of us have experienced one time or other. The shaft gets bent by people forgetting to retard the spark lever when starting the engine. The engine kicks back because the ignition timing is too far advanced with the lever pulled down. If you remove the Bendix drive to check the shaft's straightness, put the starter on the floor and hook up a six-volt battery. The shaft will spin very fast, so you want to hold onto the starter for dear life. It will want to move on its own. If the shaft spins without any blurring, it's o.k. If the shaft is blurry and the starter wants to jump all over the place, the shaft is bent. It can be straightened by trial and error in a bench vise.
Marshall
