Unsolvable 351W starter motor problem!

Hi Guys, I'm a Newbie to this great looking site and arrive with a drama nobody can explain.........l

In my boat I have a 650 hour, marinised, twenty year old Ford 351 Windsor V8. Due to a cracked cylinder head, I had it rebuilt locally with a new head, pistons, rebore etc. etc. All went well and it started up first go. After returning the boat back to my jetty, I ran it at fast idle, various RPM for about two hours before deciding to change the old starter motor that I know had a dicky solenoid. It had probably performed about ten to fifteen trouble free starts during this period.

No problem with the new starter initially but after about three or four starts it decided to start crashing the pinion gear into the ring gear! Aaaaggggghhhhhh!

I figured I'd put the old one back in as possibly the dealer had given me the wrong model hence the non-engagement issue.

Nope, wrong! Same problem with the gear crashing on the old one! So now I have an old and a new starter producing the exact same symptom.

After much head scratching, the usual "advice" from friends, neighbours and the mechanic who said "not my problem Mate" I started looking at the electrics as I suspected possibly the voltage/amperage was down and therefore the pinion gear was not going far enough up the drive shaft and therefore not engaging. Had both relatively new batteries charged up by local auto electrician, load tested and pronounced in A1 working condition.

I replaced all the pos/neg battery cables, the wire to the solenoid and any other "stuff" that was looking a bit sus! Ran a multimeter across the solenoid during the start cycle and a health 11.5V. Used a screwdriver to short out the pos/neg on the solenoid terminals, same deal! Paralleled both batteries with HD jumpers, again no joy!

Even a badly worn ring gear wouldn't suddenly stop working overnight, as this one apparently did! There is little or no scuffing on the entire ring gear and that confirms the pinion is simply spinning round and round with just the very extremities of it's teeth striking the ring gear's teeth!

So how in hell have these two moved "away" from each other such a huge distance? You can cross off the ring gear as no way it can go back any other way than correctly and the holding bracket for the starter is bolted firmly to the back of the motor and similarly immovable, I guess. There are NO shims on either the old or new starters!

The ring gear is the original and was not found to be excessively worn and no work was carried out on it during the rebuild.

Triple checked the battery isolation switch connections plus every other connector and bit of wiring in the boat - almost! Got excited a few times but all false alarms ..........

OK, moving on to the starters themselves, as you would expect the teeth on the pinion gear took a bit of a beating. On the new one, the very tip of each cog was burred but to such a small degree it was pretty hard to see. The old starter, being well worn in I guess, showed no significant damage to its pinion gear!

So it appears, unbelievingly, that whilst the pinion gear is in the right position on the drive shaft to engage, it is sitting "too far away" from the ring gear to even look like doing so? If you start measuring the depth of the cogs it is crazily about a good eighth of an inch short of the ring gear, and that is impossible! On the premise that the ring gear teeth go into the valley of the pinion gears cogs about two thirds during a normal start, mine is short by this ridiculous eighth of an inch!

To get this "distance" between both sets of teeth either the ring gear is incorrectly positioned - impossible - or the starter motor has "moved away" from it's correct position - impossible! The two bolt holes align perfectly and it's rock solid! The ring gear was not worn significantly and even were it so, that wouldn't just happen "between two starts!"

I'm missing something here guys! Sorry if I haven't explained it all that well but nothing makes sense here! It see,a to be me that it was only AFTER I fitted the new starter that the problem started by how come that when the old one goes back in, the problem is still there? Surely there is nothing I have "left off", dropped or misfitted?

Any suggestions would be very, very, very much appreciated!