Do you need a muffler for this exhaust pipe? You can find it here...   https://www.ebay.com/itm/276093278000

Or:  eBay item number: 276093278000

Heavy duty threaded exhaust flange for use on Harbor Freight Predator301cc & 420cc engines, GX 240, 270, GX 390... Great for custom exhaust setups on generators, go karts, mini bikes, or garden equipment. Flange is CNC cut from heavy 1/4" steel plate to prevent warping, outlet pipe is 1" pipe thread that matches exhaust port perfectly for maximum flow. Length of outlet pipe is around 2" Comes complete mounting hardware and gasket. Material type: 304 stainless steel threaded, mild steel exhaust flange. Finding the Correct Pipe Thread Size National Pipe Threads (NPT) Pipe sizes do refer to physical dimensions, but the plumbing industry standard for the sizes of pipe is not always as straight-forward as measuring the pipe. Measuring the pipe will oftentimes mislead you into choosing the wrong pipe, so do take care. The "pipe thread size" as shown in column 3 of the table below is measured and based on the inside of the pipe. But, to actually determine the size of a pipe, the outside diameter of each pipe or fitting must be measured and compared to the table for size identification. For example, a 3/4" NPT pipe thread has an outside diameter of 1.050 inches. Each thread size has a defined number of threads per inch (TPI). The 3/4" NPT pipe thread has 14 threads per inch. Both the TPI (threads per inch) and OD (outside diameter) of the thread are required for positive identification of thread size because several sizes have the same TPI.