"White smoke" could be leaking ac refrigerant and the burning smell could have been due to a seezed/leaking ac compressor and overheated/slipping accessory belt and the "chipped piston" could be your mechanics underhanded way of getting alot more $$$$. Hard to say really without a firsthand examination. Only way for a piston to get chipped, I would guess, is if your engine dropped a valve or a sparkplug came apart. This would be accompanied with ALOT of VERY bad sounding mechanical noises and VERY rough running, if at all, engine. If he had the piston in his hand then he's allready well on his way of doing a engine overhaul.