I thought all XB9's (even USA) had the smaller primaries but with the same collector output size as the XB 12's?
I don't know? just asking.
OP: sounds like he gave you a 12 muffler for your 9. It will bolt on fine and work fine as long as you have all the brackets too, just wire the little arm for the valve all the way open (against the spring pressure).
This is a 12 muffler, note the the cable and arm assembly.
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13,14,15,16,17,18 etc.
I have a beautiful 9 muffler (no valve) if you want to buy it.
