diff --git a/test-linuxcnc-source-link.sh b/test-linuxcnc-source-link.sh
index e7b9cec..9a6d6ee 100755
--- a/test-linuxcnc-source-link.sh
+++ b/test-linuxcnc-source-link.sh
@@ -225,6 +225,12 @@ INI_FILE_NAME="$multi_spindle_modes_file" CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_multi_spindle_all_direction.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_multi_spindle_all_direction.json"
 # Source basis: LinuxCNC src/emc/rs274ngc/rs274ngc_pre.cc init reads
+# [TRAJ] SPINDLES from INI_FILE_NAME, and interp_convert.cc M3/M4 without a $
+# selector start only the default spindle.
+INI_FILE_NAME="$multi_spindle_modes_file" CNC_SIM_RS274_VAR="$var_file" \
+    "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_multi_spindle_default_start.ngc \
+    >"$output_dir/cnc_sim_linuxcnc_source_multi_spindle_default_start.json"
+# Source basis: LinuxCNC src/emc/rs274ngc/rs274ngc_pre.cc init reads
 # [TRAJ] SPINDLES from INI_FILE_NAME, and interp_convert.cc M5 without a $
 # selector stops every configured spindle.
 INI_FILE_NAME="$multi_spindle_modes_file" CNC_SIM_RS274_VAR="$var_file" \
@@ -1867,6 +1873,22 @@ if multi_spindle_all_direction_states != expected_multi_spindle_all_direction_st
         f"{multi_spindle_all_direction_states!r}"
     )
 
+multi_spindle_default_start = json.loads((OUTPUT_DIR / "cnc_sim_linuxcnc_source_multi_spindle_default_start.json").read_text())
+multi_spindle_default_start_states = [
+    (event["line"], event["tool"], event["spindle"], event["reserved"])
+    for event in multi_spindle_default_start
+    if event["type"] == "set-spindle" and event["line"] in {5, 6}
+]
+expected_multi_spindle_default_start_states = [
+    (5, 0, 1200, 1),
+    (6, 0, 1200, 2),
+]
+if multi_spindle_default_start_states != expected_multi_spindle_default_start_states:
+    raise SystemExit(
+        "unexpected source-linked default M3/M4 spindle states: "
+        f"{multi_spindle_default_start_states!r}"
+    )
+
 multi_spindle_default_stop = json.loads((OUTPUT_DIR / "cnc_sim_linuxcnc_source_multi_spindle_default_stop.json").read_text())
 multi_spindle_default_stop_states = [
     (event["line"], event["tool"], event["spindle"], event["reserved"])
@@ -3811,6 +3833,7 @@ echo "dumped $output_dir/cnc_sim_linuxcnc_source_rtcp_controls.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_spindle_direction.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_multi_spindle_direction.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_multi_spindle_all_direction.json"
+echo "dumped $output_dir/cnc_sim_linuxcnc_source_multi_spindle_default_start.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_multi_spindle_default_stop.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_spindle_modes.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_spindle_orient.json"
diff --git a/tests/gcode/linuxcnc_multi_spindle_default_start.ngc b/tests/gcode/linuxcnc_multi_spindle_default_start.ngc
new file mode 100644
index 0000000..474b80e
--- /dev/null
+++ b/tests/gcode/linuxcnc_multi_spindle_default_start.ngc
@@ -0,0 +1,7 @@
+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 M3/M4 without $ starts only the default spindle.)
+S1200 $-1
+M3
+M4
+M30