diff --git a/test-linuxcnc-source-link.sh b/test-linuxcnc-source-link.sh
index f17a8bc..6c1e1c4 100755
--- a/test-linuxcnc-source-link.sh
+++ b/test-linuxcnc-source-link.sh
@@ -292,6 +292,12 @@ CNC_SIM_RS274_VAR="$var_file" \
 INI_FILE_NAME="$multi_spindle_override_file" CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_multi_spindle_override.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_multi_spindle_override.json"
+# Source basis: LinuxCNC src/emc/rs274ngc/rs274ngc_pre.cc init reads
+# [TRAJ] SPINDLES from INI_FILE_NAME, and interp_convert.cc M51 without a $
+# selector applies speed override changes to every configured spindle.
+INI_FILE_NAME="$multi_spindle_override_file" CNC_SIM_RS274_VAR="$var_file" \
+    "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_multi_spindle_override_all.ngc \
+    >"$output_dir/cnc_sim_linuxcnc_source_multi_spindle_override_all.json"
 CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_modal_state.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_modal_state.json"
@@ -1991,6 +1997,24 @@ if multi_spindle_override_states != expected_multi_spindle_override_states:
         f"{multi_spindle_override_states!r}"
     )
 
+multi_spindle_override_all = json.loads((OUTPUT_DIR / "cnc_sim_linuxcnc_source_multi_spindle_override_all.json").read_text())
+multi_spindle_override_all_states = [
+    (event["line"], event["reserved"], event["tool"], event["feed"])
+    for event in multi_spindle_override_all
+    if event["type"] == "comment" and event["reserved"] == 51
+]
+expected_multi_spindle_override_all_states = [
+    (4, 51, 0, 0),
+    (4, 51, 1, 0),
+    (5, 51, 0, 1),
+    (5, 51, 1, 1),
+]
+if multi_spindle_override_all_states != expected_multi_spindle_override_all_states:
+    raise SystemExit(
+        "unexpected source-linked M51 all-spindles override states: "
+        f"{multi_spindle_override_all_states!r}"
+    )
+
 control_mode_tolerances = json.loads((OUTPUT_DIR / "cnc_sim_linuxcnc_source_control_mode_tolerances.json").read_text())
 control_mode_tolerance_states = [
     (event["line"], event["reserved"], event["feed"])
@@ -3770,6 +3794,7 @@ echo "dumped $output_dir/cnc_sim_linuxcnc_source_spindle_orient.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_multi_spindle_orient.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_io_controls.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_override_controls.json"
+echo "dumped $output_dir/cnc_sim_linuxcnc_source_multi_spindle_override_all.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_modal_state.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_modal_autorestore.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_threading_sync.json"
diff --git a/tests/gcode/linuxcnc_multi_spindle_override_all.ngc b/tests/gcode/linuxcnc_multi_spindle_override_all.ngc
new file mode 100644
index 0000000..19b0767
--- /dev/null
+++ b/tests/gcode/linuxcnc_multi_spindle_override_all.ngc
@@ -0,0 +1,6 @@
+G21 G90 G17
+(Source basis: LinuxCNC src/emc/rs274ngc/rs274ngc_pre.cc init reads [TRAJ] SPINDLES from INI_FILE_NAME.)
+(Source basis: LinuxCNC src/emc/rs274ngc/interp_convert.cc M51 without $ applies to all configured spindles.)
+M51 P0
+M51 P1
+M30