diff --git a/nuts_and_bolts.scad b/nuts_and_bolts.scad
index 95c275a..acb651c 100644
--- a/nuts_and_bolts.scad
+++ b/nuts_and_bolts.scad
@@ -145,7 +145,7 @@ METRIC_BOLT_CAP_DIAMETERS =
 [
 	-1, //0 index is not used but reduces computation
 	-1,
-	3.8,
+	3.8,//m2
 	5.50,//m3
 	7.00,//m4
 	8.50,//m5
diff --git a/units.scad b/units.scad
index 61d0d7f..38c427e 100644
--- a/units.scad
+++ b/units.scad
@@ -17,6 +17,7 @@ X = [1, 0, 0];
 Y = [0, 1, 0];
 Z = [0, 0, 1];
 
+M2 = 2*mm;
 M3 = 3*mm;
 M4 = 4*mm;
 M5 = 5*mm;