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: 31mmTop Diameter: 36mmLength: 35mmTemperature Range: -30°C - 70°CHole Diameter: 5mmPack of 10Size 31 stoppers are ideal to use with Economy NM Flasks 100mL Pyrex NM Conical Flasks 250-500mL and Simax Side Arm 250mL.Please note: Extreme care must be used when inserting glass into the holes in rubber stoppers.