I replaced the starter about 1 year ago with a high torque aftermarket one from Gustafson Machine Gear Reduction Starters. Prior to October it was working fine. Then it started to occasionally spin without cranking the engine. From the attached drwg you can see that even when fully engaged the pinion just barely makes contact. I rechecked the ring gear dimensions at several points and they all were about 1.53" to 1.55" from the bell housing to the front face of the gear. There is no visible  damage to the gear teeth or obvious displacement. So either 1) the ring gear has shifted aft .250" all around or 2) the starter solenoid is not extending completely or 3) the starter needs to be mounted further aft.

  Based upon Greg's  reply I  now  tend to think the ring gear position is OK which  would be great as since the clutch  and rear main are fine I would rather not pull the transmission!  It is possible the  pinion never fully engaged the ring gear  since new.

Attachment: Ring Gear Drwg.jpg (Downloaded 59 times)