Neutral, or in your case, park safety switch or related wiring.
Solenoid
Bad spot in armature
Worn armature bushings allowing armature to touch a field
Smacking starter with a hammer sometimes wakes up or changes things in the starter/solenoid & it starts, for a while. But it will fail again if that's it.
This is a reasonable list of possibilities, but doesn't give any diagnostics.
NSS - first and easiest test is to hold the key in the start position and work the shifter back and forth through park and neutral a few times. If this makes it start or at least spin, the switch is bad or out of adjustment. If no response, a test light will show if power is getting to the small terminal on the solenoid when the key is turned. No power means bad wiring, bad ignition switch, or bad NSS. More probing with a test light will isolate the problem.
Solenoid - they almost never go bad on these starters.
Worn bushings - Starter would still click, but crank slow or not at all if this were the problem
Tapping on starter makes it work - This indicates worn brushes in starter. If the brushes in a GM starter don't make contact with the armature, the solenoid won't pull in even though power is getting to it. The starter motor provides a current path for the solenoid winding that pulls in the core. There is a second winding that holds it in once the starter begins spinning. The reason tapping works is that it may bring the brushes into contact with the armature. This usually only works a few times so it is a good indication that the starter needs to be rebuilt.
Heat soak - I have only seen this a couple of times and both were on big blocks in very hot weather. I suspect that the solenoid simply didn't create enough magnetic force to pull in because of the high temp, but I am not sure.
Jerry
