Thread Tap Size

Google - bolt thread chart.

I believe it is 3/4 NPT.....I remember years ago using a piece of it to deflect exhaust...you can bring the muffler to a plumbing shop or home depot to try matching it up

NPT national pipe thread ,briggs motors use pipe thread fitting for exhaust. The same pipe sizes that plumbers use for threaded water lines and natural gas lines. Like MARK(LI) said take the the old muffler to a plumbing shop or hardware store ,and match up the size pipe you have . There are taps available for different pipe sizes, so you can clean out the threads in the briggs engine block.