Played around with muffler on my 266 a couple of weeks ago. 1" 316 s/s tubing .040 wall thickness.
Plated the side with 20 ga. s/s and closed off the original exhaust on top. Re tuned it and it runs great.
I did notice a slight bit of scoring on the piston approx. 3/16" wide x 1/2" long...