▁be ▁slightly ▁less ▁than ▁the ▁diameter ▁of ▁the ▁wheel ▁centre ▁on ▁which ▁it ▁is ▁mounted , ▁to ▁give ▁an ▁inter ference ▁fit . ▁ ▁The ▁t ire ▁is ▁fitted ▁by ▁he ating ▁to ▁a ▁controlled ▁temperature , ▁avoid ing ▁over he ating . ▁ ▁This ▁causes ▁the ▁t ire ▁to ▁expand . ▁ ▁The ▁wheel ▁centre , ▁usually ▁already ▁mounted ▁on ▁the ▁ax le , ▁is ▁lower ed ▁into ▁the ▁t ire ▁which ▁is ▁fl ange - side - up . ▁ ▁The ▁t ire ▁co ols , ▁and ▁the ▁retain ing ▁ring ▁( a ▁sh aped ▁steel ▁bar ▁rolled ▁into ▁a ▁ho op ) ▁is ▁fitted ▁into ▁the ▁gro ove . ▁ ▁H yd ra ul ically ▁operated ▁roll s ▁sw age ▁the ▁gro ove ▁down ▁onto ▁the ▁retain ing ▁ring . ▁ ▁The ▁t ire ▁is ▁primarily ▁held ▁in ▁place ▁by ▁its ▁inter ference ▁fit . ▁The ▁shoulder ▁on ▁the ▁outside ▁and ▁the ▁retain ing ▁ring ▁also ▁keep ▁the ▁t ire ▁in ▁place ▁if ▁the ▁inter ference ▁fit ▁is ▁lost . ▁ ▁This ▁is ▁most ▁often ▁due ▁to ▁severe ▁drag ▁bra king ▁down ▁a ▁gradient , ▁or ▁due ▁to ▁an ▁error ▁in ▁the ▁mach ining . ▁ ▁Rem oval ▁of ▁a ▁worn ▁t ire ▁is ▁by ▁mach ining ▁out ▁the ▁retain ing ▁ring ▁and ▁he ating ▁the ▁t ire ▁to ▁relax ▁the ▁inter ference ▁fit . ▁ ▁Some ▁steam ▁locomot ive ▁whe els ▁had ▁t ires ▁bol ted ▁through ▁the ▁rim , ▁or ▁with ▁a ▁second ▁smaller ▁shoulder ▁mach ined ▁on