diff --git a/docs/linuxcnc-porting.md b/docs/linuxcnc-porting.md
index 1659c67..e9823de 100644
--- a/docs/linuxcnc-porting.md
+++ b/docs/linuxcnc-porting.md
@@ -201,7 +201,7 @@ This matrix tracks LinuxCNC feature coverage for the web/WASM simulator. A featu
 | Spindle speed mode `G96/G97` | covered with Canon `SET_SPINDLE_MODE` preserved as mode state events, including `G96 D...` max-RPM value | `tests/gcode/linuxcnc_spindle_modes.ngc` |
 | Spindle orientation `M19 R... P... Q...` | covered with Canon `ORIENT_SPINDLE` and `WAIT_SPINDLE_ORIENT_COMPLETE` preserved as temporary mode state events | `tests/gcode/linuxcnc_spindle_orient.ngc` |
 | Spindle-synchronized feed and threading `G33/G33.1/G76` | covered through smoke, LinuxCNC native, and source-linked paths with Canon `START_SPEED_FEED_SYNCH`/`STOP_SPEED_FEED_SYNCH` preserved, rigid-tap line numbers normalized, and G76 multi-pass threading smoke-covered | `tests/gcode/linuxcnc_threading_sync.ngc`, `tests/gcode/linuxcnc_threading_cycle.ngc`, smoke API regression |
-| Lathe diameter/radius mode `G7/G8` | covered in smoke parser for X-axis diameter/radius scaling, including interaction with G76 threading geometry; native/source backends use LinuxCNC interpreter state directly | smoke API regression |
+| Lathe diameter/radius mode `G7/G8` | covered through LinuxCNC native/source backends for same-block X scaling and `#<_lathe_diameter_mode>`/`#<_lathe_radius_mode>` readonly named-parameter state; G76 threading geometry interaction remains smoke-covered | `tests/gcode/linuxcnc_lathe_diameter_mode.ngc`, smoke API regression |
 | Digital/analog I/O `M62`-`M68` | covered with Canon digital output, analog output, and input wait callbacks preserved as temporary state events | `tests/gcode/linuxcnc_io_controls.ngc` |
 | Override controls `M48`-`M53` | covered with Canon feed override, spindle speed override, adaptive feed, and feed hold callbacks preserved as temporary state events | `tests/gcode/linuxcnc_override_controls.ngc` |
 | Modal state save/restore `M70`-`M73` | covered for main-program save/invalidate/restore plus O-word subroutine autorestore in smoke parser, with native/source regressions checking restored distance mode, units, and XY plane motion | `tests/gcode/linuxcnc_modal_state.ngc`, `tests/gcode/linuxcnc_modal_autorestore.ngc` |
diff --git a/test-all-native.sh b/test-all-native.sh
index 0b9b4be..8b5a7e0 100755
--- a/test-all-native.sh
+++ b/test-all-native.sh
@@ -556,6 +556,8 @@ if grep -F 'read -r -a python_includes <<<"$(' list-linuxcnc-source-common-cxxfl
     echo "source common cxxflags still hides python-config failures behind read" >&2
     exit 1
 fi
+grep -F 'convert_lathe_diameter_mode() scales G7/G8 X words' test-native.sh >/dev/null
+grep -F 'tests/gcode/linuxcnc_lathe_diameter_mode.ngc' test-native.sh >/dev/null
 
 for script in \
     test-native.sh \
diff --git a/test-linuxcnc-rs274-native.sh b/test-linuxcnc-rs274-native.sh
index e529b10..15a84b9 100755
--- a/test-linuxcnc-rs274-native.sh
+++ b/test-linuxcnc-rs274-native.sh
@@ -141,6 +141,10 @@ if CNC_SIM_RS274_VAR="$var_file" "$build_dir/linuxcnc_rs274_dump" tests/gcode/li
 fi
 CNC_SIM_RS274_VAR="$var_file" "$build_dir/linuxcnc_rs274_dump" tests/gcode/linuxcnc_threading_sync.ngc >"$output_dir/cnc_sim_linuxcnc_threading_sync.json"
 CNC_SIM_RS274_VAR="$var_file" "$build_dir/linuxcnc_rs274_dump" tests/gcode/linuxcnc_threading_cycle.ngc >"$output_dir/cnc_sim_linuxcnc_threading_cycle.json"
+# Source basis: LinuxCNC src/emc/rs274ngc/interp_convert.cc
+# convert_lathe_diameter_mode() scales G7/G8 X words; interp_namedparams.cc
+# exposes #<_lathe_diameter_mode> and #<_lathe_radius_mode>.
+CNC_SIM_RS274_FILE_MODE=1 CNC_SIM_RS274_VAR="$var_file" "$build_dir/linuxcnc_rs274_dump" tests/gcode/linuxcnc_lathe_diameter_mode.ngc >"$output_dir/cnc_sim_linuxcnc_lathe_diameter_mode.json"
 CNC_SIM_RS274_VAR="$var_file" "$build_dir/linuxcnc_rs274_dump" tests/gcode/linuxcnc_tool_number.ngc >"$output_dir/cnc_sim_linuxcnc_tool_number.json"
 CNC_SIM_RS274_VAR="$var_file" "$build_dir/linuxcnc_rs274_dump" tests/gcode/linuxcnc_program_stops.ngc >"$output_dir/cnc_sim_linuxcnc_program_stops.json"
 CNC_SIM_RS274_VAR="$var_file" "$build_dir/linuxcnc_rs274_dump" tests/gcode/linuxcnc_coolant.ngc >"$output_dir/cnc_sim_linuxcnc_coolant.json"
@@ -377,6 +381,21 @@ if not any(
 if not any(event["type"] == "program-end" and event["line"] == 5 for event in threading_cycle):
     raise SystemExit("missing G76 program end line number")
 
+lathe_diameter_mode = load("cnc_sim_linuxcnc_lathe_diameter_mode.json")
+lathe_feeds = [
+    (event["line"], event["end"]["x"], event["end"]["y"])
+    for event in lathe_diameter_mode
+    if event["type"] == "linear-feed"
+]
+expected_lathe_feeds = [
+    (3, 5, 0),
+    (5, 5, 1),
+    (7, 6, 1),
+    (9, 6, 2),
+]
+if lathe_feeds != expected_lathe_feeds:
+    raise SystemExit(f"unexpected G7/G8 lathe diameter-mode feeds: {lathe_feeds!r}")
+
 tool_number = load("cnc_sim_linuxcnc_tool_number.json")
 if not any(
     event["type"] == "tool-change" and
@@ -1186,6 +1205,7 @@ echo "dumped $output_dir/cnc_sim_linuxcnc_modal_state.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_modal_autorestore.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_threading_sync.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_threading_cycle.json"
+echo "dumped $output_dir/cnc_sim_linuxcnc_lathe_diameter_mode.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_tool_number.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_program_stops.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_coolant.json"
diff --git a/test-linuxcnc-source-link.sh b/test-linuxcnc-source-link.sh
index 63e567f..4895a2b 100755
--- a/test-linuxcnc-source-link.sh
+++ b/test-linuxcnc-source-link.sh
@@ -220,6 +220,12 @@ CNC_SIM_RS274_VAR="$var_file" \
 CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_threading_cycle.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_threading_cycle.json"
+# Source basis: LinuxCNC src/emc/rs274ngc/interp_convert.cc
+# convert_lathe_diameter_mode() scales G7/G8 X words; interp_namedparams.cc
+# exposes #<_lathe_diameter_mode> and #<_lathe_radius_mode>.
+CNC_SIM_RS274_FILE_MODE=1 CNC_SIM_RS274_VAR="$var_file" \
+    "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_lathe_diameter_mode.ngc \
+    >"$output_dir/cnc_sim_linuxcnc_source_lathe_diameter_mode.json"
 CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_tool_number.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_tool_number.json"
@@ -1073,6 +1079,21 @@ if not any(
 if not any(event["type"] == "program-end" and event["line"] == 5 for event in threading_cycle):
     raise SystemExit("missing source-linked G76 program end line number")
 
+lathe_diameter_mode = json.loads((OUTPUT_DIR / "cnc_sim_linuxcnc_source_lathe_diameter_mode.json").read_text())
+lathe_feeds = [
+    (event["line"], event["end"]["x"], event["end"]["y"])
+    for event in lathe_diameter_mode
+    if event["type"] == "linear-feed"
+]
+expected_lathe_feeds = [
+    (3, 5, 0),
+    (5, 5, 1),
+    (7, 6, 1),
+    (9, 6, 2),
+]
+if lathe_feeds != expected_lathe_feeds:
+    raise SystemExit(f"unexpected source-linked G7/G8 lathe diameter-mode feeds: {lathe_feeds!r}")
+
 tool_number = json.loads((OUTPUT_DIR / "cnc_sim_linuxcnc_source_tool_number.json").read_text())
 if not any(
     event["type"] == "tool-change" and
@@ -2469,6 +2490,7 @@ 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"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_threading_cycle.json"
+echo "dumped $output_dir/cnc_sim_linuxcnc_source_lathe_diameter_mode.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_tool_number.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_program_stops.json"
 echo "dumped $output_dir/cnc_sim_linuxcnc_source_coolant.json"
diff --git a/test-native.sh b/test-native.sh
index 41b926b..1264eee 100755
--- a/test-native.sh
+++ b/test-native.sh
@@ -140,6 +140,9 @@ grep -F 'expected LinuxCNC INI PARAMETER_FILE to restore numbered parameters' co
 grep -F 'expected LinuxCNC config INI_FILE_NAME to back #<_ini[]> named parameters' core/tests/cnc_sim_api_linuxcnc_rs274_smoke.cpp >/dev/null
 grep -F 'interp->ini_load(options.ini_file_name.c_str())' core/src/linuxcnc_rs274_backend.cpp >/dev/null
 grep -F 'ScopedEnvironmentVariable ini_file_env("INI_FILE_NAME", options.ini_file_name)' core/src/linuxcnc_rs274_backend.cpp >/dev/null
+grep -F 'convert_lathe_diameter_mode() scales G7/G8 X words' test-linuxcnc-rs274-native.sh >/dev/null
+grep -F 'convert_lathe_diameter_mode() scales G7/G8 X words' test-linuxcnc-source-link.sh >/dev/null
+grep -F 'tests/gcode/linuxcnc_lathe_diameter_mode.ngc' docs/linuxcnc-porting.md >/dev/null
 grep -F 'rs274ngc_pre.cc ini_load() consumes INI_FILE_NAME' test-web-wasm-node-smoke.cjs >/dev/null
 grep -F 'rs274ngc_pre.cc ini_load() consumes INI_FILE_NAME' web/test-browser-wasm-smoke-linuxcnc-sections.js >/dev/null
 grep -F 'metadata:src/emc/nml_intf/emcops.cc:source basis for native source-link EMC status constructor support object' linuxcnc-rs274-source-files.txt >/dev/null
diff --git a/tests/gcode/linuxcnc_lathe_diameter_mode.ngc b/tests/gcode/linuxcnc_lathe_diameter_mode.ngc
new file mode 100644
index 0000000..09e9115
--- /dev/null
+++ b/tests/gcode/linuxcnc_lathe_diameter_mode.ngc
@@ -0,0 +1,11 @@
+G21 G90
+F100
+G7 G1 X10
+O10 if [#<_lathe_diameter_mode> EQ 1]
+G1 Y1
+O10 endif
+G8 G1 X6
+O20 if [#<_lathe_radius_mode> EQ 1]
+G1 Y2
+O20 endif
+M30