diff --git a/test-linuxcnc-source-link.sh b/test-linuxcnc-source-link.sh
index 598f074..508850a 100755
--- a/test-linuxcnc-source-link.sh
+++ b/test-linuxcnc-source-link.sh
@@ -173,6 +173,7 @@ make --output-sync=target -j"$build_jobs" -f "$source_link_makefile"
 var_file="$build_dir/rs274ngc-source.var"
 base_var_file="$build_dir/rs274ngc-source-base.var"
 ini_named_parameter_file="$(pwd)/tests/gcode/linuxcnc_ini_named_parameter.ini"
+multi_spindle_override_file="$(pwd)/tests/gcode/linuxcnc_multi_spindle_override.ini"
 output_dir="$build_dir/outputs"
 mkdir -p "$output_dir"
 cp "$linuxcnc_root/tests/halui/jogging/sim.var" "$base_var_file"
@@ -257,6 +258,12 @@ done
 CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_override_controls.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_override_controls.json"
+# Source basis: LinuxCNC src/emc/rs274ngc/rs274ngc_pre.cc init reads
+# [TRAJ] SPINDLES from INI_FILE_NAME, and interp_convert.cc M51 applies
+# speed override changes only to the selected $ spindle when present.
+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"
 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"
@@ -1846,6 +1853,22 @@ expected_override_states = [
 if override_states != expected_override_states:
     raise SystemExit(f"unexpected source-linked M48-M53 override states: {override_states!r}")
 
+multi_spindle_override = json.loads((OUTPUT_DIR / "cnc_sim_linuxcnc_source_multi_spindle_override.json").read_text())
+multi_spindle_override_states = [
+    (event["line"], event["reserved"], event["tool"], event["feed"])
+    for event in multi_spindle_override
+    if event["type"] == "comment" and event["reserved"] == 51
+]
+expected_multi_spindle_override_states = [
+    (4, 51, 1, 0),
+    (5, 51, 1, 1),
+]
+if multi_spindle_override_states != expected_multi_spindle_override_states:
+    raise SystemExit(
+        "unexpected source-linked M51 selected-spindle override states: "
+        f"{multi_spindle_override_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"])
diff --git a/tests/gcode/linuxcnc_multi_spindle_override.ini b/tests/gcode/linuxcnc_multi_spindle_override.ini
new file mode 100644
index 0000000..fb8b9de
--- /dev/null
+++ b/tests/gcode/linuxcnc_multi_spindle_override.ini
@@ -0,0 +1,2 @@
+[TRAJ]
+SPINDLES = 2
diff --git a/tests/gcode/linuxcnc_multi_spindle_override.ngc b/tests/gcode/linuxcnc_multi_spindle_override.ngc
new file mode 100644
index 0000000..ad81626
--- /dev/null
+++ b/tests/gcode/linuxcnc_multi_spindle_override.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 with $ selects one spindle override.)
+M51 P0 $1
+M51 P1 $1
+M30