When I rebuild my drill battery pack with new C nicads do I have to stay
in the same mAh rating. The originals are 1300mAh, I can replace them
with 2300mAh cells with tabs. Or even 4000mAh ones. Will more mAh hurt
the drill?

I'd read somewhere that as mA rating goes up for a given cell size the
insulators get thinner and the battery is more at risk from heat (during
charge and discharge) than lower mA cells.

Someone more knowledgeable re. battery chemistry can probably answer here or
at

sci.chem.electrochem.battery

Good luck.