Simple
No two pieces of metal are the same. The new starter may have a little more out of spec in it's housing than your old one. To install the starter right. You need to measure the spacing between the starter drive gear and your flexplate/flywheel. when you get the right measurement, then you should have nothing to fear. I usually take the starter solenoid off and manually engage the drive to the flexplate gear and use a large paperclip as my feeler gauge. You have to adjust until the depth of the starter drive is at that gap amount. When done measuring, then I re-install the solenoid and hook up the starter. Most starters come with an instruction sheet on how to do this.
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