An inner housing defines {TAIL ~ a cylindrical outer wall} and, together with {HEAD ~ the substantially cylindrical inner wall}, forms a substantially cylindrical ring gap 
An inner housing defines { TAIL ~ a cylindrical outer wall } and , together with { HEAD ~ the substantially cylindrical inner wall } , forms a substantially cylindrical ring gap
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH