Starter spinning and not engaging

Hi all,

Now that we are all in lockdown, I finally have some time to work on my Quattroporte III. Yesterday was my first attempt at replacing the starter motor with a RobbMC high-torque #3005 mini-starter. My car is an automatic so the original starter was a Chrysler.

It is a very tight fit and I've had a hard time aligning the mounting bracket with the starter holes, but I finally managed to bolt it in place using the old bolts. I also took this opportunity to replace the starter cable with a brand new one coming from Campana in Italy.

Upon turning the key to start, the new starter does spin but does not seem to engage the flywheel. There is a little click before it starts spinning, which makes me think that the pinion extends and hits the flywheel, and then it spins freely without cranking the engine.

What is your troubleshooting checklist/routine when installing starters?

I have three main hypotheses but wanted to hear suggestions before trying. My hypotheses are:
1. Not enough backlash between pinion and flywheel. That's possible, since the pinion support cone on the starter sits within the mounting bracket, which has its own support pinion cone, and there might be a small interference between the two, which would push the starter inwards (towards the flywheel), thereby reducing the backlash. I don't know if I make myself clear, I could provide an illustration if needed.
2. Excessive end gap. I don't know how likely this is, but basically that would mean the starter's pinion does not extend far enough to actually reach and engage the flywheel.
3. Insufficient voltage and/or current. That's possible but these mini-starters require less current than the original starter, and the battery does provide sufficient current to crank the engine with the old starter so should be more than enough for the new one.

I am 100% sure this starter works with this particular car, since both Thor (Quattroporte3) and MrMark have reported successfully using it with their QP3s, so it's a matter of troubleshooting.

Unfortunately I didn't take many pictures yesterday, but there's this one which illustrates my 3-hour struggle to align the starter hole with the mounting bracket. Either the top bolt would go in, and the lower hole would be misaligned (as in the picture), or the lower bolt would go in and then it's the top hole which is misaligned. I finally found a solution by inserting the lower bolt backwards (i.e. from the outside of the mounting bracket), then use the protruding bit to help me keep the lower hole properly aligned while I screw in the top bolt, and then screw the lower bolt correctly. But it's clearly a very tight fit.

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