Russ, here are a few suggestions that may or may not be realistic, and they may be long shots.
I thought I'd toss them out anyway.
- By chance is there a thin spacer plate between the flywheel covers and the engines? (very long shot)
- Are the flywheel ring gear teeth all there and healthy?
- Are the flywheel ring gears in the correct positon on the flywheels?
- Are the old Prestolite Bendix drive motors extending the pinion gears further than need be, causing your comparison measurements to be off?
- Is there a chance that new HTGR motors have the pinion gears/shafts reversed (re; armature rotation/direction), casusing non-function of the "kick-out"?
As per #5, these new starter motor pinion shafts may be spiral splined (similar to the Bendix idea) causing engagement, in lieu of solenoid lever action.
If so, and if these were accidentally reversed (as per starter and engine rotation) during assembly, the splines would not allow the starter motor rotation to kick the pinion gear out.
Read the Autocosmos PDF page #13, item "I" under the heading
"Bendix-Drive is Slipping: "
This can also occur with the spiral spline scenario if these are reversed.
BTW..... what the Autocosmos people are calling rollers, is actually a form of sprague or sprag clutch.