abc wherein the spacer is constructed in the form of a ring a ring of desired thickness such as a thin ring 54 a or a thick ring 54 c is positioned between the center collars 43 prior to insertion of the pipe 3 for example fig1 b illustrates a centralizer and pipe assembly incorporating a spacer 54 b having a medium thickness which forces the bow springs radially inward a greater distance than the small spacer 54 a but more than the larger spacer 54 c as would be understood by those skilled in the art the spacer can take various forms such as simple longitudinal rods 53 as illustrated in fig1 a and 12b the bow springs may be forced radially inward utilizing still additional constructions for example fig8 a and 8b illustrate a centralizer 1 including a plurality of flexible straps 55 and pins 57 which function as u201c tension members u201d so as to pull center collars 43 towards adjacent end collars 9 in still alternative embodiments the pipe may be constructed to include mechanical structures such as projections which lock the end collars into longitudinally extended positions so as to force the bow springs radially inward the projections may be simple pins or bolts not shown affixed to the pipe 39 s sidewall which are positioned to maintain the end collars in an extended position alternatively as illustrated in fig1 a and 11b the pipe 3 may include fixed or adjustable ring like structures 61 which affix to the pipe so as to maintain the end collars 9 in a longitudinally extended condition though not shown in the figures the pipe may include projections such as pins bolts or rings which engage both center collars or a single center collar.