Best quality red rubber. Shore hardness in range 46-55. Bottom diameter moulded on top face. Resistant to dilute acids alkalis and ammonia.Bottom Diameter: 13mmTop Diameter: 16mmLength: 24mmTemperature Range: -30°C - 70°CPack of 10Size 13 stoppers are ideal to use with the Pyrex HD NM Conical Flask 50mL or 16mm diameter test tubes.