I have a chevy big block that I am having problems starting.  Sometimes when I turn the key it bangs (sounds like a hammer hitting the flexplate) and then grinds.  Sometime it just grinds real bad and sometimes it jams and won't turnover at all.  I have tried different variations of shims and it still happens.  It doesn't sound as bad after I take the starter off and put it back on but it goes right back to that horrible sound again after that.  It seems to be worse when the engine is hot but still does it cold.  Any ideas on this would be super helpful.  Thanks.  

When you pull the starter, what do the teeth look like on the plate? What do they look like on the drive gear?  

The teeth on the flexplate look a little worn but not too bad.  I'll have to look at the drive gear on the starter.  One thought is that maybe the starer is the wrong one for the flexplate.  I know the only number of teeth on the flexplate for this motor is 168 so the only thing I can think of is it's the wrong starter.  Any thoughts?  

If shims aren't solving the problem, sounds like you've either got the wrong starter or the wrong flexplate.  

Are the starter bolt holes straight across from one another or offset?  

The starter bolts are offset.  Also I noticed one of the teeth on the flexplate broke off.  The flexplate was new last fall when it was put in.  

For proper starter shimming you should have an 1/8 inch clearance between the starter shaft and the teeth on the flywheel.
 take an 1/8 inch drill bit and try to put it between the starter shaft and the teeth of the flywheel when the starter is bolted to the engine. Sometimes an 1/8 inch allen wrench works better cause its got the 90 degree bend at the one end. I usually use a long needle nose pliers. Its a pretty tight fit to get up to where you gotta measure. If you can't get the bit between the flywheel and the shaft you gotta add a shim, if there is a big gap then you gotta take shims away. They make shims in a variety of different thicknesses so you can get it pretty close to perfect.
Another way to check for proper starter shimming is by measuring the distance from where the bolt head meets the starter to where the face of the starter meets the block. I do it with a digital calipers. I think the spec. is 88 or 106 millimeters. I can't remember specificaly but I can find out. Just let me know and I can get back. measuring with a caliper is easier cause you can do it on the bench. But if material has been taken off the bolt side of the starter it scews your specs.
drill bit between the shaft and the flywheel is a little more foolproof. and just about anyone has those tools.
 hope i am helpfull. good luck.  

Thanks a ton for the info.  I am going to look into it and will post a reply after I check things out.  I still have to get a new flexplate and I believe I need one with 168 teeth.  Is that correct?  From my understanding all big blocks (454) require a flexplate with this number of teeth.  Another thing I might try is to wrap the starter to keep it a little cooler from the heat off the headers.  Should help protect the solenoid some. I figure it can't hurt.  

I am not sure how many teeth you should have on the flywheel. If you have a place localy that rebuilds starters I betcha you could call them and find out those specs.  

Offset bolt starter is correct for 168 tooth, if memory serves me.  

Thanks for the info.