The crimp is almost always there, especially on 50's Gretsch. It can be hard to spot. Or, if the bolt was forced through the receiver, it can get "cleaned" off (at the expense of the receiver).
Regarding cleaning the receiver threads with a tap, I didn't mean one that had already been stripped...I was referring to preventative maintenance on one that is still in good shape.
I've dealt with a few hundred of these mufflers, and have never seen one that was stripped just from being under tension. There are two reasons they end up being stripped - either they were forced loose and the crimp ruins the receiver, or they were forced loose while frozen from corrosion or rust, and the receiver threads give before the rust does. It should always be easy to remove the knob bolt...if there is resistance than either the crimp is catching, or the corrosion needs to be dealt with.
Here's some pics.
The first shows a standard early style muffler with the thick aluminum bracket. An arrow points to the crimp, and another arrow points to the area where corrosion freezes the muffler.
[IMG]http://www.classicvintagedrums.com/crimp.jpg[/IMG]
This pic shows a die on the bolt, ready to clean the thread and remove the crimp. The thread size for these muffler bolts is 10-32.
[IMG]http://www.classicvintagedrums.com/die1.jpg[/IMG]
This pic shows the wrench on the die for additional leverage. Make sure to use oil with the die.
[IMG]http://www.classicvintagedrums.com/die2.jpg[/IMG]
This last pic shows the clean thread. The best way to deal with any corrosion is PB Blaster...works MUCH better than WD-40 (as any car mechanic will attest).
[IMG]http://www.classicvintagedrums.com/clean_thread.jpg[/IMG]
Thanks,
Bill