On this parts fiche the Bolt,flange,6x25 is the most specified component.
Some items from this specific schematic: Tube,5x240, O-ring, 21.9x2.3, Bolt,flange,6x50, O-ring 13.5x1.4.
Of all the components that make up this Thermostat - Water Pipe the Bolt,flange,6x25 has the lowest price.