is it possible to overtighten a metal yoyo . (trinity, dv888, superstar) etc.
what can happen if i over tighten it ???

It can happen, but you’re going to have to be pretty darn tough to over tighten a metal yoyo. I know quite a few people who really tighten their metal yoyos, and they are just fine.

If you don’t try to over tighten it, you’re probably not going to.

You could strip the axle or the part the axle goes into. That’s what could happen. Or, in the case of a bi-metal or plastic, maybe rip the mount out. Either way, you’re going to have to make an effort to strip an axle. But, also keep in mind that they sell replacement axles. With taking the yoyo apart, over time, it’s possible to strip the axle just through this wear. I would find it rather unlikely though.

When I tighten up my DM2, I let it come together through just spinning it. Then I just give a tiny last turn, probably not even a degree. That’s just to let me know it’s on tight. I don’t put a lot of force into it. If I don’t do this, I find that after I play for 5 minutes, one end comes spinning off. If you’ve played a DM2, you know the bearing doesn’t seat tight, so that means one half goes one way, the other half on the floor, and if I’m lucky, the bearing stays on the string and I’m spinning a yoyo-less bearing! Usually, the bearing falls off too! It’s actually kind of funny. Forget 1/2A, this is ZILCH-A.

It’s actually not hard to over tighten a yoyo. I’ve been sent several that were stripped due to over tightening.