1997 4.3 Volvo Penta Starter Issues

[Rocklin Freebirds]

In the past couple years I have broken 2 starter bolts for my 19? Wellcraft with a 4.3 Volvo Penta engine. My boat always had sounded rough when starting the engine and sometimes it takes 5 ? 10 seconds to start. I was told that I should add shims to the start to align the starter and flywheel. But, I am unsure what thickness to try. I just removed the starter and noticed a 9 / 16 bolt that was not screwed in. The bolt is threaded in the engine and when you tighten or loosen the bolt is separates the engine and flywheel. First question Does anyone know what this bolt is for and how to know the correct setting? Could this be the cause of breaking starter bolts? Second question I current have 1 shim, how would I know if I need to add more shims to align the starter and flywheel?