Triumph 712903 TR250 TR5 TR6 Water Valve Control Pictorial Type Black Knob
Note there fits on to the standard Lucas 1/4" hexagonal shaft with spring loaded retaining plunger. Knob head size 29.2mm x 29.2mm, Knob depth (27.3mm). As fitted to Triumph TR250, TR5 & TR6 (US markets up to 750000CC onwards).