diff --git a/vitamins/rail.scad b/vitamins/rail.scad
index b40112f..431f952 100644
--- a/vitamins/rail.scad
+++ b/vitamins/rail.scad
@@ -145,6 +145,7 @@ module rail(type, length, colour = grey(90), use_polycircles = false) { //! Draw
     height = rail_height(type);
 
     vitamin(str("rail(", type[0], ", ", length, "): Linear rail ", type[0], " x ", length, "mm"));
+    assert(rail_end(type) < (rail_pitch(type) - rail_bore(type)) / 2, type[0]);
 
     color(colour) {
         rbr = rail_bore(type) / 2;
diff --git a/vitamins/rails.scad b/vitamins/rails.scad
index 2fc3ce9..3a84856 100644
--- a/vitamins/rails.scad
+++ b/vitamins/rails.scad
@@ -20,11 +20,11 @@
 //
 // Rails
 //
-//                Wr  Hr    E     P   D    d    h                                       go    gw
+//                Wr  Hr  Emin    P   D    d    h                                       go    gw
 MGN5 = [ "MGN5",   5,  3.6,  5,   15, 3.6, 2.4, 0.8, M2_cs_cap_screw, M2_cs_cap_screw,  1,    1 ]; // Screw holes too small for M2 heads
 MGN7 = [ "MGN7",   7,  5,    5,   15, 4.3, 2.4, 2.6, M2_cap_screw,    M2_cs_cap_screw,  1.5,  1.5 ];
 MGN9 = [ "MGN9",   9,  6,    5,   20, 6.0, 3.5, 3.5, M3_cap_screw,    M3_cs_cap_screw,  1.5,  1.5 ];
-MGN12 =[ "MGN12", 12,  8,   10,   25, 6.0, 3.5, 4.5, M3_cap_screw,    M3_cs_cap_screw,  2.25, 2.75];
+MGN12 =[ "MGN12", 12,  8,  7.5,   25, 6.0, 3.5, 4.5, M3_cap_screw,    M3_cs_cap_screw,  2.25, 2.75];
 MGN15 =[ "MGN15", 15, 10,   10,   40, 6.0, 3.5, 5.0, M3_cap_screw,    M3_cs_cap_screw,  2.5,  2.75 ];
 SSR15= [ "SSR15", 15, 12.5, 10,   60, 7.5, 4.5, 5.3, M4_cap_screw,    M4_cs_cap_screw,  2.5,  2.75 ];
 //