Is this a 4.3? I show a 5.7L, 7.4L, and a 6.5L Diesel for a 3500 Express.
Maybe this is a type-o (you or me) or you have the wrong parts?
Continuing,
Have you tried starter shims?
The block surface and starter housing surfaces may be too close together (factory machining). This would make the starter gear and flywheel gears be too close together. Even if it gets thrown in and turns, the gears would be in a bind. Shimming the starter down some might solve the problem.
Sometimes trying another brand of starter might solve the problem (if you have not already).
Your turn,
The Medic
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Monday, April 9th, 2018 AT 6:12 PM