20mm. This DORMAKABA BTS Extended Spindle is suitable for the Dorma BTS80F concealed floor spring. The extended spindle provides greater floor clearance if required for different surfaces, such as carpeted floors, or doors with a threshold. It is the spindle insert only and is non-handed for convenience.. Extended spindle to suit the Dorma BTS80F concealed floor spring. Provide greater floor clearance. Ideal for carpeted floors or doors with thresholds.