[AROWHED]

Scat SFI flexplate on a Turbo 400 with a 10" 2800 stall convertor. Mini-starter on a factory 400 block.
Under load, even just brake-torquing, the flexplate will flex forward enough to rub the ring gear teeth against the ends of the starter gear teeth, making a lot of noise and a few sparks and obviously not doing either any good. That flexing distance is about .050".
Shimmed the mini-starter's motor away from it's mounting block to move the gear further back but it still occasionally touches...need to shim a bit more. So far I'm still getting good gear engagement for cranking. Just wanted to know if this much flexplate flexing is normal.

[chitownTA]

Did you check to see if the pinion gear is retracting? I had one that would come out when I accelerated forward and shoot back in when I hit the brakes.

[AROWHED]

Yes I checked the starter action, and it works as it should. Crank thrust bearing is good too.
What is happening here is that the flexplate is bending forward enough under the torque load reaction from within the convertor, to cause the ring gear teeth to contact the starter gear while it's at rest. It has actually nicked the front edge of a few of the starter gear teeth.

Replicating the flexing action with a crowbar between convertor and trans case, I can even see where the center hub of the convertor has been sliding in\out of the crank, polishing it slightly.

[1969GTO]

does the convertor need to be shimed back ware it mounts to the flex plate?

[ponjohn]

Might want to check trans pressures.

[BruceWilkie]

Starter depth is wrong... retracted should be closer to 1/8" clearance. 0.050" is too close.

Try a stock starter... if clearance stays similar you have a flexplate issue...

[Big Mike]

I would check crankshaft end play. Could be a worn thrust bearing. Also, as wear takes place on the input shaft of the transmission, the converter will place an additional load on the thrust bearing wearing it out sooner.

Several posts on converter web sites discuss this issue. A new input shaft should be used when ever a new or refurbished torque converter is installed. It is even more of an issue with a power glide with a trans brake.

If you can see the flywheel moving back and forth, I'd really suspect too much crank end play. Since you checked end play and it is within specs, then you have too much pressure in the transmission forcing the converter to move forward. If left alone, you will have thrust bearing issues soon enough.