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