I have tata sierra std 1996 model But from 3 4 months i

I have tata sierra std. 1996 model. But from 3-4 months i am facing engine overheat problem,though i have done half engine overhaul + head overhaul in the last month. But still this problem continues. I can't even able to use AC, because it gets heated quickly on AC than w/o AC.Also i get feeling of lack of power & pickup while driving it. Mechanic told me it's due to piston boare problem. There is some minor gap between piston & boare. its gonna cost me near 20000/- but already i have spend 22000/- for half engine + head over haul. AC condenser is also new so there should not be overload problem. plz tell me what to do?