I changed ****** with new flexplate when cranking over starter and flex plate grind. could i have installed flex plate backwards

that is huge pain , no? sorry it happens to you, really am.....
causes,
bolts missing holding it (end to end, or loose)
top cause. not set to torque spec. bolts.
a bad clutch TC can do it. or wrong one there, or was drop damaged.
some odd starter problem that has wrong tooth count on the starter? pinion.? making jam up cranking and each time overflexes the flex plate
or the flex plate if from cloners in china made from old beer cans.
buy only real TOY plate.
I call wrong parts or car or clones.
I call bolts loose.

possible incorrect pitch of the ring gear of the flex plate to the pinion gear of the starter
not all flex plates have the correct ring gear for the starter

Typically this sounds like a chewed up ring gear or flex plate but if you recently replaced that, the likely culprit is the pinion gear on the starter. It could be chewed up or the starter is not throwing it out enough. Also could be that the starter isn't properly shimmed.

Replace the flex plate. I can guarantee, that if you turn the flex plate enough you find a spot or two worn more than the rest of it. Missing teeth will cause a bendix grind, But if the teeth on the flex plate come to a point or even close point. This will kick the bendix back to the starter. The spring in the starter won't hold the pressure.

This can happen for a number of reasons, especially after a replacement is installed on the transmission side. From my research online and doing this irl, ive discovered that if the torque converter is not seated properly, it can push on the flexplate and cause it to hit the starter. This can also happen if different year transmissions are used, as Ive discovered there is about 1/16 inch difference in them despite being the exact same transmission/engine and model. What I have seen for a resolution to this issue, is to either replace the starter, or pull it out and grind off the 1/16 inch from the blades so they no longer "zing" across the flexplate while it is running/starting. There is also the suggestion to shim the starter to fix the issue, but to my mind, shims would push it further from the flexplate in the wrong direction, away from where they mate. It seems like shaving off some of the distance on the starter teeth would be the best way to fix it.
Additionally a number of folks have bought starters from AutoZone that were just garbage and were not disengaging properly after engaging the flexplate. I think the least likely of scenarios, although still something to be aware of, is that any of the flexplate or torque converter bolts are loose or not torqued to spec will cause very slight wobble in the rotation and make a similar grinding sound.

This is a big job, this is listed as a 2004 ford taurus you will need a engine support bar and a ****** jack, a car lift will also be needed, doing this on the ground can be done but makes things alot harder. the trannys electricl connectors unhooked and also the battery removed for room. next you will want to drain the ****** oil.install the engine suppurt bar , this bar will hold the engine and ****** in car when the lower frame is removed. you will remove the air filter box. raise car and remove flex plate cover and starter. next remove bolts that hold flex plate to ****** converter.with engine support in place you will unbolt and remove ****** mounts frt ,rear and leftside. note the only thing holding the engine is the rightside mount and bar.remove the drive axles, unbolt lower control arms at lower balljoint and remove axle nuts, then axles will come out. next is to unbolt steering rack from the frame.dissconnect ****** lines from ******.remove exhaust system front pipes with converters, (Y-PIPE). next is to remove the k frame. this is the frame that holds engine and ****** inside car, becarful as engine and ****** will fall out if support bar is not installed correctly.you will need to double check for any connections in the way, with all clear and someone to help you lower the frame you will unbolt and remove it. next you will lower adjusting rod on the engine support bar to tilt engine and ****** down, the ****** will tilt down, next remove all ****** to engine bolts except one. with all bolts out except one you will install a ****** jack to hold the ******. next remove the last bolt and look around for any connection that may need to be removed. split ****** away from engine slowly always checking that all is clear. with ****** removed roll off to side or back and now you will be able to remove the flexplate. remove the bolts holding flexplate to engine mark a hole to install new one the same way. most flywheels (flexplate) only go on one way but just incase mark it. install new plate and reverse steps to but back together. note always install new starter when replacing a flexplate, this is to avoid damage to new flexplate. good day. also this job would take a tech 8 hrs.

the flexplate may not be sitting flush against the crank, or the crank may be out of balance causing a vibration and breaking the flex plate. try turning the crank by hand and feeling for any irregularities and look for signs of heavy wear on the broken plates.

It sounds like the starter is not engaging fully and you should change the starter.When you remove the starter check the flywheel\flex plate(Flexplate for automatic trany) for damage.

Have you chk. the starter switch on the steering colume under dash? Or is starter drive kicking in but not engaging teeth in flexplate? If its not engaging teeth on flexplate starter needs to be shimmed, or if its missing the teeth altogether on flexplate or bearly catching the teeth, top of starter nose wear it bolts up needs to be filed down to engage the teeth on flexplate.

absolutely a known issue...cracked flexplate...pull starter and inspect flexplate very carefully for cracks before installing new starter and thinking its a done deal