This seems to be an ongoing issue. When I first posted about this, it had the old starter/bendix on it. I first replaced the bendix, then the starter. It still does this will all combinations of old and new imaginable. Would filing down the gears at the end of the bendix a millimeter or two help this problem?
Issue may be the ring gear on the flywheel is worn, so that engagement is compromised. Or perhaps the Bendix spring is worn too.
If your replacement starter is good with straight shaft, and your Bendix replacement is good with checked out counterweighted gear and shaft, then perhaps there may be an alignment issue. That can be from gasket at mounting plate of the starter, or ill-fitting hogshead too throwing the starter Bendix gear out of alignment to the flywheel gear.
Terry has your T had this issue since you've owned it? If it hasent something must have gotten changed along the way. Did you change the transmission cover in some way? Was it loosened up and then retightened? Are all the mounting holes in the trans cover, pan, still in good shape and not elongated? Maybe using a thinner or thicker starter mounting gasket would make a difference.
Is the trans cover ear thats directly under the starter have a bolt in it that's a bit to long and poking through just enough to not let the starter bracket be pulled up flush to the cover?
Hope the info may be of some help.
I did notice that the new starter gasket was a lot thicker than the old one. Perhaps that is causing an issue. I may get a new bendix and give that a try, too. Could be that the "new" bendix I put in has a flaw.
Check to make sure your starter ring gear is not installed backwards on the flywheel.