diff --git a/Marlin/src/Marlin.cpp b/Marlin/src/Marlin.cpp
index b409ff0909..b3f9cf243f 100644
--- a/Marlin/src/Marlin.cpp
+++ b/Marlin/src/Marlin.cpp
@@ -355,10 +355,6 @@ bool pin_is_protected(const int8_t pin) {
   return false;
 }
 
-#if ENABLED(DELTA) || ENABLED(Z_DUAL_ENDSTOPS)
-  #include "gcode/calibrate/M666.h"
-#endif
-
 #include "gcode/control/M211.h"
 
 #include "gcode/config/M220.h"
diff --git a/Marlin/src/gcode/calibrate/M666.h b/Marlin/src/gcode/calibrate/M666.cpp
similarity index 83%
rename from Marlin/src/gcode/calibrate/M666.h
rename to Marlin/src/gcode/calibrate/M666.cpp
index a79e3b90b3..2dcdc76b90 100644
--- a/Marlin/src/gcode/calibrate/M666.h
+++ b/Marlin/src/gcode/calibrate/M666.cpp
@@ -20,15 +20,24 @@
  *
  */
 
+#include "../../inc/MarlinConfig.h"
+
+#if ENABLED(DELTA) || ENABLED(Z_DUAL_ENDSTOPS)
+
+#include "../gcode.h"
+
 #if ENABLED(DELTA)
 
+  #include "../../module/delta.h"
+  #include "../../module/motion.h"
+
   /**
    * M666: Set delta endstop adjustment
    */
-  void gcode_M666() {
+  void GcodeSuite::M666() {
     #if ENABLED(DEBUG_LEVELING_FEATURE)
       if (DEBUGGING(LEVELING)) {
-        SERIAL_ECHOLNPGM(">>> gcode_M666");
+        SERIAL_ECHOLNPGM(">>> M666");
       }
     #endif
     LOOP_XYZ(i) {
@@ -44,7 +53,7 @@
     }
     #if ENABLED(DEBUG_LEVELING_FEATURE)
       if (DEBUGGING(LEVELING)) {
-        SERIAL_ECHOLNPGM("<<< gcode_M666");
+        SERIAL_ECHOLNPGM("<<< M666");
       }
     #endif
     // normalize endstops so all are <=0; set the residue to delta height
@@ -55,12 +64,16 @@
 
 #elif ENABLED(Z_DUAL_ENDSTOPS) // !DELTA && ENABLED(Z_DUAL_ENDSTOPS)
 
+  #include "../../Marlin.h" // for z_endstop_adj
+
   /**
    * M666: For Z Dual Endstop setup, set z axis offset to the z2 axis.
    */
-  void gcode_M666() {
+  void GcodeSuite::M666() {
     if (parser.seen('Z')) z_endstop_adj = parser.value_linear_units();
     SERIAL_ECHOLNPAIR("Z Endstop Adjustment set to (mm):", z_endstop_adj);
   }
 
 #endif
+
+#endif // DELTA || Z_DUAL_ENDSTOPS
diff --git a/Marlin/src/gcode/gcode.cpp b/Marlin/src/gcode/gcode.cpp
index 3546a706e3..30433e1316 100644
--- a/Marlin/src/gcode/gcode.cpp
+++ b/Marlin/src/gcode/gcode.cpp
@@ -151,7 +151,6 @@ extern void gcode_M502();
 extern void gcode_M503();
 extern void gcode_M540();
 extern void gcode_M605();
-extern void gcode_M666();
 extern void gcode_M702();
 extern void gcode_M900();
 extern void gcode_M906();
@@ -544,9 +543,7 @@ void GcodeSuite::process_next_command() {
       #endif
 
       #if ENABLED(DELTA) || ENABLED(Z_DUAL_ENDSTOPS)
-        case 666: // M666: Set delta or dual endstop adjustment
-          gcode_M666();
-          break;
+        case 666: M666(); break;  // M666: Set delta or dual endstop adjustment
       #endif
 
       #if ENABLED(FWRETRACT)