Last summer I converted the 350 to 383 stroker with roller cam and have 10.43 to 1 compression now. I bought a high torque mini starter (power master) and shimmed it properly. When the car is cold it engages and starts properly. After about 20 minutes of driving when I go to re-start it the starter pinion grinds like crazy like its not engaging the flex plate properly.

What can cause this? Starter over heating? Something wrong with the flex plate?

If it was a compression issue I would think the pinion would engage but would just have a hard time turning without the grinding.

Any advice would be appreciated.

Maybe the solenoid gets weak when it gets hot. Might try putting on a heat shield.

Will try the heat shield. I just checked and I have 12.44 volts to the main windings on starter but only 7.58 volts to the solenoid when I hit the remote starter switch. This doesn't seem right. Maybe there is not enough juice to throw the pinion into the flex plate. Shouldn't this be 12 volts also?

Yes.  The solenoid is basically a heavy duty switch.  Not sure where the voltage drop is coming from but if the starter is getting 12 volts then it should spin the engine. 

My money's on the solenoid's wire connectors not making good contact. Clean all of them up and then see what you have.

Steve 

While I can’t help on the posted issue, I sure would like to learn more about what you’ve done to your car? That pic showing it diving into that corner looks awesome. Love to hear about your suspension and steering setup if you care to share. Thanks.

Bryan

After cleaning I have 7.58 volts to solenoid in park and 8.3 volts when I try and start it in neutral. I'll have to figure out how to test neutral safety switch on the shifter. Might be losing voltage through that some how. (I hate electrical problems as I have no skills in that area)

Thanks Bryan, I haven't raced the car just used it for time trialing events with EMRA. Original suspension set up but new shocks and Borgension power steering unit added.  All the work was in the motor. Conversion from 350 to 383 stroker and new 64 cc aluminum heads.

Is this the voltage at the "S" terminal on the starter while cranking?(Should be the purple wire at the solenoid) You are correct in thinking it should have 12v while cranking. The larger terminal of course should have battery voltage at all times. Have you tested the voltage at the "S" terminal when it actually works, and starts up, to compare it with the voltage when it does NOT start? If there is not a full 12v(battery voltage) at the "S" terminal with the key in the "Start" position, you obviously have a bad connection somewhere in the start circuit when hot, OR you have a faulty solenoid. New doesn't mean it is good.
Your problem could be in the ignition switch, neutral safety switch, or ANY of the connections from the ign. switch down to the starter, including the pass-thru connector at the fusebox/firewall, where the main harness connections are.

You mentioned not having 12v on the starter while using a remote starter switch. Would this remote switch be connected from the positive battery cable to the "S" terminal on the starter? If so, and you have less than battery voltage there, then you have an issue with the positive battery cable from the battery to the starter....or even the ground connections.

Thanks Adam, I have a new neutral safety switch and new blower /starter relay coming. I connected a new ground strap directly from the starter to the frame to insure good ground. I will let you know how it works out.