Yanmar 3gm30 breaking starter motors

Hello. This is my first time posting any thread, anywhere, so forgive me if I do anything dumb. I am a semi-skilled amateur, not a mechanic, but I am working with good mechanics.
I have a 3GM30F with a V-drive. I bought and installed the motor after it was submerged and revived by good professional mechanics. I bought a used V-drive and installed that myself--the motor previously had a regular transmission. I have replaced the alternator and starter and have had the injection pump rebuilt. The motor starts easily and runs well, but when I go to re-start it, it will barely turn or not at all. The mechanics have followed several theories but we just discovered that it broke the first new starter motor, and it has just broken a second one, seemingly immediately in the attempt to re-start the motor. It probably did this the first time the first starter tried to re-start the motor. It is the cast end of the starter that holds the end of the starter shaft that breaks. Before we realized the first starter had broken, we did find that the motor started easily when cool. It appears that something is happening to make the engine hard to turn when it is warm--maybe the starter breaks because of this, or something is damaging the starter while the engine runs? When I installed the second starter I rotated the engine by hand and could not hear anything on the flywheel hitting the starter. We would greatly appreciate any suggestions for how to diagnose the source of the problem.

Dan
I've had the same problem. The original starter was fine untill prolonged starting damged it. I didn't have the choice of 10 sec intervals, just needed to get the motor going. The problem was an air leak I then bought a "high quality" new starter from the USA (origin china) paid the postage and the tax into France and after two or three excellent starts the 3GM30 broke the end casting as you've described. The subsequent replacement went the same way. I figure that the Chinese casting just isn't strong enough.....
So I'm hunting down a rewind company to fix the original one

My engine also feels a tadge "stiff" but I haven't been able to compare other motors. There doesn't appear to be any reason for it unless a piston is coming up to TDC on compression
I suppose that if the engine isn't being cooled then expansion could cause a tightening. Apart from the starter motor problem the engine sounds sweet when she's running
Your's

LOA

I've had the same problem. Switched from after market to OEM. I was told it is back presure although I feel it was a cheap casting also. Has there been any further update on the problem? I'm waiting to see how it is this season. I want to carry a spare but would like an after market because of price.