Your point is well taken. I'm sure it is dependent upon the initial clearances of the fit, in my application they fit OK after coating without any tweaking, other than fighting to get each individual pipe into the collector in turn. I finally machined a spacer to mimic the thickness of the center of the collector, then inserted it into the center of the four pipes, then put a LARGE hose clamp around them and slipped the collector and sidepipe assembly onto the primary pipes.
Rick
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American Industrial Coatings
7637 Fair Oaks Blvd
Carmichael, Ca 95608
916 944-3300