The gauge should be a friction fit - if not adjust the wheels on the axles or replace if necessary. TT Wheel Gauge - Back to Back.
2 Pack - OO Gauge (1:76) Back to Back Wheel Gauge Tool 14.4mm
2 Pack - TT Gauge (1:120) Back to Back Wheel Gauge Tool 10.24mm
Back To Back Wheel Gauge 14.4mm For OO Scale
2 Pack - N Gauge (1:148) Back to Back Wheel Gauge Tool 7.65mm
