Hey all- I plugged 2 of the 4 1/2" holes I drilled in mufflers by just tapping hole with 1/4" pipe tap and screwing self sealing pipe plugs in hole. I thought it would be neat for anyone not liking the sound after drilling. I am just trying it to tune to my liking. The 2 out of 4 holes plugged is better than 4 open. I am going to try just plugging 1 out of 4 tomorrow and see how that sounds. The self sealing plugs should stay in there ok. For those of you not familiar with pipe plugs- 1/4" is for the inside diameter of 1/4" pipe, but outside diameter is near 1/2". The plug and pipe tap are tapered. So only run tap in far enough till when you try plug, it will tighten without threading all the way through. Never tap hole all the way to end of threads on tap or plug will just thread right through the hole. Self sealing plugs have a dryed red sealant on threads. Barbecue Paint will hide these well enough.