diff --git a/motor_base.py b/motor_base.py
index 35b779b..d3d8c2c 100644
--- a/motor_base.py
+++ b/motor_base.py
@@ -31,7 +31,7 @@ SPAR_MORTISE_ANGLE = 45.0           # +-45 deg to the arm axis
 MORTISE_L = TENON_L + FIT       # 4.2
 MORTISE_W = THICKNESS
 
-BORE_D = 4.0             # 1404 bell / boss clearance
+BORE_D = 5.0             # 1404 bell / boss clearance
 BOLT_SQ = 9.0            # 1404/1105 M2 pattern -- the DIAGONAL between
                          # opposite bolt holes, not the square's side
 BOLT_D = 2.2             # M2 clearance