This threaded muffler is known as the “weenie” muffler in kart racing circles. 

This is one of the first mufflers used in early sanctioned box stock classes. This pipe is rarely used in today’s adult classes but is still used in many restrictor-plate junior classes. The rule organizations made the change to 4104 to allow a little more sound, but consequently, it allowed the engines to make a lot more power changing the class entirely. 

We recommend using the “weenie” pipe for tracks running predator classes as it puts a huge damper on “cheating” and limits gains made from blueprinted carburetors and aftermarket camshafts. This will keep the class running for low-budget and entry-level racers.