Thread Forming Grease Fittings also referred to as spin drive grease fittings are used to save tapping cost and have special tapered drive threads that reshape the material in an untapped hole to provide a leakproof seal.  Spinning these fittings into an untapped hole provides the most effective installation, but they may also be driven directly into an untapped hole.  Thread Forming Fittings may be replaced by standard fittings that have the same thread size.