Here is what could happen to Knucklehead barrels. They do not have the reserve of strength that panhead cylinders had. The 2 blown cylinders that I have seen - 1. had 1 dark piston and one light so may have had air leak and 2. Point gap was off so maybe one cylinder was out of time.
To put pan barrels on a knuck you have to trim ot remove the firering on the cylinder and move the bolt on the head.
Sorry Jon for using you as an example. This happened late last fall.