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Some bots and PA have stats for tearing. Thats basically what you are looking for.
As for making up rules, I would just say its double a basic punch, but without strength bonus.

I'm pretty sure that Rifts or Robotech covers this, but I know for a fact this is in HU: Armor for robots and cyborgs is specifically listed by body part. If the character is attempting to "rip an arm off", but only wants to tear off the armor, then he has to completely deplete the SDC/MDC of the armor itself in order to rend it from the main chassis. Which means, essentially, that you'll need to know how much SDC/MDC that particular portion of the armor has.

I'd also say that the character would need to roll a strike roll, using the PS damage bonus to strike instead of the normal strike bonuses. Why? Well, he's gripping the ends of the armor and attempting to rip it off. He wouldn't need to "strike" it, per se, as he's already gripping it. But I would think that he'd still need to make a roll of some kind to see if he can do damage to it, and I like the PS bonus instead of the strike roll as it really is a strength check he's making.

Damage I'd say would be whatever normal, hand-to-hand damage for the character would be. Again, PS bonuses would apply.