Add modular piston and connecting rod assemblies

Author: defencore

README.md

@@ -71,6 +71,7 @@ The library uses independent part modules, including the requested bearing famil
 | Linear motion           | Ball screw linear axis: six prototype presets with dual profile rails, moving table, fixed/floating bearing supports and cutaway view                                 |
 | Motion                  | D25 L30 jaw coupling with independent bores, clamp screws and elastomer spider; six dimensioned bevel gear pairs; miniature m0.5 pinions                              |
 | Servo linkages          | Single, double, cross, six-arm and disc horns; clamping arms; servo spline gears; pushrod, female/male threaded and cable clevis ends                                 |
+| Pistons & rods          | Compressor/engine hollow pistons and pneumatic disks; separate rings, seals and wrist pins; rods with bushings, bearing shells and removable big-end caps             |
 | Enclosures & structural | Open enclosure, L bracket and round spacer                                                                                                                            |
 
 Standard fastener selection starts with the available catalog thread and length sizes. **Custom dimensions** exposes thread pitch, handedness, coverage, start offset from the tip, length and smooth shoulder diameter. Choose a modeled helical thread, a smooth thread envelope or no thread. Countersunk nominal length includes the head; other headed fasteners use length under the head. Irrelevant fields disappear when a variant changes. Switching head or point type initializes sensible editable dimensions.
@@ -95,6 +96,8 @@ The eight supplied reference images are available from the relevant presets. The
 
 The servo linkage extension adds 28 horn references, 16 servo gear presets (13 actual ServoCity catalog sizes and three prototype examples), and 24 clevis presets. Find them under **MOTION → SERVO LINKAGES**, or search for **servo**, **horn** or **clevis**. The 18 supplied images remain linked from the corresponding references. See [servo linkage coverage](docs/servo-linkages.md) for source dimensions, modeling limits and native FreeCAD verification.
 
+Find **Piston** and **Connecting rod** under **MOTION → PISTONS & CONNECTING RODS**. Piston catalog presets include all eight supplied nominal sizes: 42, 47, 48, 51, 65, 70, 80 and 90 mm. Other dimensions, pneumatic configurations and connecting-rod presets are editable prototype examples. Both packages support assembled, exploded and body-only states, with movable FreeCAD components. See [piston and connecting-rod coverage](docs/piston-linkages.md) for reference evidence, editing and verification.
+
 The [native validation report](data/reference-expansion-native-validation.json) records FreeCAD solid checks, STEP round trips, sampled clearances and preview comparisons for the reference expansion. Repeatable ball-screw checks are documented in the [validation guide](docs/ball-screw-catalog.md#repeating-native-validation).
 
 The [assembly export guide](docs/freecad-assemblies.md) explains component editing and the guided ball screw axis. Its [native audit](data/freecad-assembly-validation.json) covers 39 complete macros, independent component movement, STEP/FCStd reload and rollback cleanup.