Small Engines (Lawn Mowers, etc.): Lawn Chief Rider/ muffler replacement, muffler replacement, pipe nipple

Question
I have a Lawn Chief rider that I bought at Walmart about 4 years ago.  At the end of last summer the end of the muffler fell off.  I can't figure out how to get the rest of the tube unattached to put in a replacement.  The mower still runs fine, but it's louder than crud.  Is there some trick?  There's what appears to be a lock washer/nut that screws around the pipe that tightens down to the engine, but it doesn't really change the movement of the pipe.

It's a briggs and straton engine, the muffler came out of the engine below the air filter on the left side towards the front.  The pipe is 3-4" long and the muffler was welded to the end of that.

I just don't want the neighbors to complain.

Answer
 Hello Jeff:

 The Nut needs to be Loosened from the Engine. This Nut is A Heat Expansion Nut. It Prevents the Muffler Pipe from Loosening up when it Heats Up and Then Virbrating out of the Engine. Once the Expansion Nut has been Loosened, Start the engine and allow it to Warm up for about 3 to 5 min at Medium Engine Speed (1600rpm). Shutoff the Engine. PLEASE BE CAREFUL!!! HOT PARTS YOU WILL BE WORKING WITH. WEAR THICK GLOVES WHEN HANDLING TOOLS AND OLD PARTS!!! Take a Pipe Wrench and Remove the Pipe Nipple from the Engine. The Replacement Muffler May Require a New Pipe Nipple that is Threaded at Both Ends.  Hope this Helps.

 Good Luck

 Respectfully

 John