Plastic washers will not rust like metal. The rubber bungs slide into the holes perfect and when you tighten the bolts they squash up in the holes making a good tight fit. The rubber grommets in the kit have a threaded section so you can just push them into the holes in the top of the toilet pan and when you tighten the screw it expands and tightens itself into the hole, without the need of a bolt on the other end of the screw. Suitable for 14-19mm hole for 2 toilet seats, universal uses. They are very easy to fit done in a matter of minutes, simply remove old bolts clean around the area and fit the new bolts. Supplied with 4 soft plastic washers, when you tighten the bolts the washers crush down into the holes and you need to be careful you do not tighten the bolt so much it pushes through the washer. Screw Diameter: 5.7mm Long: 48mm Tube Nut Diameter: 18mm (thickest), 14mm Long: 40mm Washer Diameter: 14mm Color: black Material: Rubber + metal Package Contents: 4 x toilet fixings Only the above package content, other products are not included. Note: Light and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.