1 3/8" Screw-On Adapter For 1" Exit Bolt-On Drivers On A 4 Bolt 4" Diameter Circle Converts Bolt On Driver 

To Accept Screw on Horn -1 3/8" For Large Heavy Duty Drivers 4 Bolt Holes 1/4" on 4" Diameter Circle -

Overall Diameter : 5" (Metal). 

We Also Have A Smaller Adapter