Mounting a starter without the proper style and length bolt will cause serious problems. The bolts will flex, and the starter will shift when cranking. One or more of the bolts will break eventually, but well before this happens, the teeth on the starter and flexplate will be damaged. The problem is not the lack of knurling on the bolt, but that the knurling does not protrude past the mounting plate.
As you can see in the picture above, the knurled bolts at least 200 thousandths or more of the knurled section should show when dropping the bolt through the aluminum mounting pad. This knurled section fills the over-sized pilot hole in the block, as the diameter of the knurled part of the bolt is much thicker than the threaded section.
All GM blocks have this un-threaded portion in the starter mounting holes with the intent that approx. 200 thousandths of the knurled bolt will pilot into them. The picture above is exactly what you should see when dropping the bolt through the mounting plate. If you do a Google search on this issue, you will find links to many car forums addressing its importance.
When mounting the starter, be sure the gear mesh is correct, making sure that the gears don't mesh tightly. It may be difficult to see this as the dust cover is usually in the way. The long flat shims will bring the starter away from ring gear if needed. You may need to use these shims if the starter has a loud scratchy sound when cranking. This means the starter gear is meshing too tightly. We recommend removing the starter and installing both of the long flat shims to change the gear mesh. You will notice how much quieter the starter sounds. The other shims provided are for use if your starter gear is sitting too close to the flexplate and possibly touching it when the engine is running.
We have tested our sample starter on several engines with normal fitting flexplates and have found the starter to have a very decent fit right out of the box when using a well used flexplate or flywheel. Those using a new plate may want to use the two flat shims upon the first installation.