[Bob Putnam]

"If flexible mounts are used, won't the engine torque, through the sidepipes et al tend to twist the sidepipes apart?"

JT,

It depends upon how the side pipes connect to the primaries. A solid connection (flange-to-flange) will eventually break something. The original-style side pipes that most people use are a tube-in-tube slip fit, with tabs to keep everything from coming apart. Just enough freedom to prevent fractures, although there is a small amount of leakage.