Just curious what bit everybody prefers for a green horse transistioning from a mechanical hack (what the previous owner used) to a regular bit. She is still learning to turn and stop. I currently have her in a two piece full cheek and Blue just chews and chews (and tries to spit it out) constantly. I was thinking about changing to either copper or sweet iron, any thoughts?
First make sure the bit is positioned properly in her mouth and that could help with the chewing and chomping. I think you might have some success with the mylar bits...they are really great for young, unschooled horses. A mylar snaffle is soft in a horse's mouth. The bad thing is that trying a bunch of bits can get expensive...so it might be nice to see if you can borrow (and clean and disinfect) bits from other people until you find what you like.
I have a well broke gelding and the previous owners rode him in a tom thumb. I started riding him in a regular d ring snaffle...and he HATED it. Chewed and gaped his mouth forever...so I thought I needed to just go back to the tom thumb. But I waited it out and it took some time and gradually he adjusted to the snaffle just fine.
He may not be used to it yet. And I agree with Cobalt - be sure the position is right for him (some horses need it more loose, some more tight) as well as the size of the bit. I personally like copper over stainless steel, and my horses like it much more too - give some taste.
I find that some horses take several days to really get used to the feel of a bit...so if she's only had it in a couple of times, just keep putting it on her for a while and see what happens. Don't try to ride her until she is okay with it though...as in, she stops chewing and gaping; you could cause more confusion for her by trying to guide her while she is still unconfortable with the peice in her mouth.
I may be wrong but I believe a mechanical hack works on poll pressure, like a shanked bit. If she was only ever trained on a mechanical hack, chances are she will respond best to another shanked bit. If not, you may have to give her time to adjust to and relearn how to ride on a regular snaffle.
I would not recommend a happy mouth bit myself. We got one with our pony when we bought her and discovered that after she'd been chewing on it for a while, she had created scores and rough spots in the plastic. I positively hate to think how that could have cut and chafed into her gums! We immediately switched to one similar to the KK training bit. The metal bits stay solid and smooth which it just what you need.
I'm really afraid you have to explore several options before you find the one she likes. Horses are really different, it's hard to give recommendations. My paint likes O-ring. My qh hates O-ring and goes in eggbutt (she didn't like D-ring either). Can you just borrow bits from your fellow boarders/riders to give it a try and see how she likes different ones?