The wall of the muffler 'can' may be pretty thin and not hold a threaded nipple in place with vibration and heat. Mufflers designed for smoke nipples usually have additional material added for mounting it. Mahy folks simply use the mounting flange of the muffler for the location of the smoke nipple. This area is plenty thick to accomodate it, and injects the oil into the hottest area of the exhuast. It can be drilled at an angle, away from the engine cylinder, to assist in preventing oil from injecting directly into the cylinder. Start out low on the pump setting, and make small adjustments up from there until you get where you need to be.

Chad