Helping a friend pulled his 450 inch small block and put new 470 inch motor in. The starter will not disengage on new motor . The car has front and mid plate new flex plate. Everything looks correct. Converter not hitting crank . Tried diffrent starters the same. Any ideas

was the transmission case measured with a dial indicator for runout mated to the new block ?
if not, the drive gear could be jammed into the converter teeth causing the starter drive to stick.
just the first thing i thought of.

My thoughts too. Also does the bellhousing have a locating hole for the starter snout?

I have disassembled starters that have suffered heat soak, found corrosion on the plunger on most of them, cleaned the solenoid plunger and bore and coated them with a light coating of synthetic disc brake grease(one of my favorite greases).

I had this happen while using a Ford type remote solenoid. The contacts in the solenoid were sticking. An upgrade to higher capacity part solved the problem.

Mark

One other thing to check.
The starter might need a shim (away from the transmission).

If you’ve swapped starters it is most likely a mounting issue. I’m assuming a mini starter in a 727? If so; I’ve had the starter hit the boss on the block. The one with large vertical hole. It prevents the starter from setting parallel to the crank. Is it an aftermarket/glide/Ultra bell, etc? If so use a tool to pull the drive out into the ring gear. There should be clearance at the bottom root of the teeth. If not the starter needs to move outward from the crank centerline. The last possibility is wiring. Does the trigger wire shut off when the starter button is released?
Doug

Agreed, every time my start wont disengage, it because I left out a shim washer.

if i remember, the DC manual stated the starter drive gear tip to the converter ring gear tooth root was to be around .060 or so.
the way they said to check was to pull the drive gear into the ring gear and use a common paper clip for a clearance check gauge.
i have ran into the mini starter to block casting interference more than a bunch of times over the years.
if one starter hits, the next may not. i blame this on machining tolerance stack up, as well as casting variances on both the starter[s] and the block[s].