diff --git a/test-linuxcnc-source-link.sh b/test-linuxcnc-source-link.sh
index 508850a..f365683 100755
--- a/test-linuxcnc-source-link.sh
+++ b/test-linuxcnc-source-link.sh
@@ -174,6 +174,7 @@ 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"
+multi_spindle_threading_file="$(pwd)/tests/gcode/linuxcnc_multi_spindle_threading.ini"
 output_dir="$build_dir/outputs"
 mkdir -p "$output_dir"
 cp "$linuxcnc_root/tests/halui/jogging/sim.var" "$base_var_file"
@@ -279,6 +280,12 @@ fi
 CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_threading_sync.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_threading_sync.json"
+# Source basis: LinuxCNC src/emc/rs274ngc/rs274ngc_pre.cc init reads
+# [TRAJ] SPINDLES from INI_FILE_NAME, and interp_convert.cc G33/G33.1
+# assign active_spindle from the $ selector before START_SPEED_FEED_SYNCH().
+INI_FILE_NAME="$multi_spindle_threading_file" CNC_SIM_RS274_VAR="$var_file" \
+    "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_multi_spindle_threading.ngc \
+    >"$output_dir/cnc_sim_linuxcnc_source_multi_spindle_threading.json"
 CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_threading_cycle.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_threading_cycle.json"
@@ -1943,6 +1950,29 @@ if not any(
 ):
     raise SystemExit("missing source-linked G33.1 rigid tap motion line")
 
+multi_spindle_threading = json.loads((OUTPUT_DIR / "cnc_sim_linuxcnc_source_multi_spindle_threading.json").read_text())
+multi_spindle_sync_states = [
+    (event["line"], event["reserved"], event["feed"], event["tool"])
+    for event in multi_spindle_threading
+    if event["type"] == "comment" and event["reserved"] in {3300, 3301, 3302}
+]
+expected_multi_spindle_sync_states = [
+    (6, 3301, 1, 1),
+    (6, 3300, 0, 0),
+    (8, 3301, 1, 1),
+    (8, 3300, 0, 0),
+]
+if multi_spindle_sync_states != expected_multi_spindle_sync_states:
+    raise SystemExit(
+        "unexpected source-linked G33/G33.1 selected-spindle sync states: "
+        f"{multi_spindle_sync_states!r}"
+    )
+if not any(
+    event["type"] == "linear-feed" and event["line"] == 8 and event["end"]["z"] == 0
+    for event in multi_spindle_threading
+):
+    raise SystemExit("missing source-linked G33.1 selected-spindle rigid tap motion line")
+
 threading_cycle = json.loads((OUTPUT_DIR / "cnc_sim_linuxcnc_source_threading_cycle.json").read_text())
 if not any(
     event["type"] == "comment" and
diff --git a/tests/gcode/linuxcnc_multi_spindle_threading.ini b/tests/gcode/linuxcnc_multi_spindle_threading.ini
new file mode 100644
index 0000000..fb8b9de
--- /dev/null
+++ b/tests/gcode/linuxcnc_multi_spindle_threading.ini
@@ -0,0 +1,2 @@
+[TRAJ]
+SPINDLES = 2
diff --git a/tests/gcode/linuxcnc_multi_spindle_threading.ngc b/tests/gcode/linuxcnc_multi_spindle_threading.ngc
new file mode 100644
index 0000000..533a67c
--- /dev/null
+++ b/tests/gcode/linuxcnc_multi_spindle_threading.ngc
@@ -0,0 +1,9 @@
+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 G33/G33.1 with $ selects active_spindle.)
+S500 M3 $1
+G0 Z5
+G33 Z0 K1 $1
+G0 Z5
+G33.1 Z0 K1 $1
+M30