I have replaced my starter with 4 brand new ones within the last year.I have also replaced the flywheel.The problem I keep having is that it starts most times,but always really noizy,metal on metal.Ive had the starters in and out 50 times

Ive been told it needs to be shimmed but where at. Do I shim the starter from the bellhousing? Do I shim at the bolt holes or go all the way around the starter? What do I use for shims? Ive also been told to drill the bolt holes on the starter casing allowing me to rotate it one way or the other. This seems drastic to me and it would void my warranty.My most recent starter is 2 weeks old.Ive replaced alot of things on my truck with no problems but this has got me baffled. I'd appreciate any ideas anyone has. Thanks