{HEAD ~ The outlet shell} which defines {TAIL ~ the outlet chamber} is adapted to engage the inside of the pair of shells which form the catalyst chamber 
defines
{ HEAD ~ The outlet shell } which defines { TAIL ~ the outlet chamber } is adapted to engage the inside of the pair of shells which form the catalyst chamber
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH