How engine and transmission parts are made
Most engine and gearbox parts start as a casting or a forging. Pistons are usually cast aluminium alloy; liners and many housings are cast iron; crankshafts, connecting rods, gears and shafts are forged steel. The rough part is then machined on CNC lathes, machining centres and grinders, heat-treated (carburised, induction-hardened or nitrided, depending on the part), finish-ground or honed, washed and inspected. Gears may be hobbed, shaved or ground. Valves and some piston rings are coated or plated.
Each step is a place where a part can go wrong without looking different: a soft case after a short furnace cycle, a ground diameter that drifted as the wheel wore, swarf left in an oil gallery. That is why OE customers run these suppliers through PPAP and control plans, and why aftermarket buyers need an inspector who reads records as well as parts.
Kolhapur in Maharashtra is one documented source. A 2012 TERI cluster profile prepared for SIDBI says most of Kolhapur's foundries supply the automotive sector, including castings for cylinder heads, clutch housings and gears. For the casting and forging stages, see iron castings and steel forgings.
What we check on engine and transmission parts
| Check | How it is checked | What fails |
|---|---|---|
| PPAP pack | Part submission warrant, dimensional results, material and performance results and control plan read against the current drawing revision | Pack for an older revision; characteristics missing from the dimensional results |
| Critical dimensions | Bores, journals, stems and ring grooves measured with calibrated micrometers, bore gauges and air gauges; ovality and taper at several positions | Any critical characteristic outside tolerance |
| CMM results | Factory's CMM reports reviewed for sampled serials; one or more parts measured on the factory's CMM while the inspector watches | Report that does not match the part measured; positional features out |
| Surface finish | Roughness on journals, bores and sealing faces read with the factory's calibrated roughness tester | Ra above the drawing value on a running or sealing surface |
| Hardness and case depth | Surface hardness witnessed on the factory's tester; case-depth records from sectioned samples of the same heat-treatment batch reviewed | Soft spots; case depth outside the drawing range; no record for the batch |
| Crack detection | Magnetic particle inspection of crankshafts, connecting rods and gears witnessed, done by the factory's qualified technicians | Any linear indication on a stressed area |
| Cleanliness | Washing process, the factory's ISO 16232 extraction and particle-count report, and visual check of galleries and blind holes | Particles above the specified limit; swarf in passages |
| Grading and marking | Piston weight classes, size grades and part-number marking checked against the grading chart | Mixed grades in one pack; marking missing or wrong |
PPAP, CMM and cleanliness: reviewing records, not owning rigs
Clear To Load does not own a coordinate measuring machine, a metallurgical lab or a cleanliness cabinet. For these parts we work in three ways.
PPAP review. PPAP is the AIAG process by which a supplier proves to its customer that its production process makes parts to the drawing. The customer approves it, not us. We check that the PPAP pack you hold matches the drawing revision and the parts in the lot, and that the control plan's inspections are actually being done and recorded.
CMM and measurement. We review the factory's CMM reports, check that the CMM and gauges are within calibration, and watch parts we choose being measured. Where results matter, we measure the same parts with hand gauges to cross-check.
Heat treatment and cleanliness. Case depth is found by sectioning a sample part and running a hardness traverse, so it is a batch record, not a check on every part. We trace the record to the furnace batch. ISO 16232 sets out how particulate contamination on vehicle components is extracted, analysed and reported. We review the factory's report and the washing and packing steps that keep parts clean. Independent checks go to an accredited lab through lab testing coordination.
Common defects in engine and transmission parts
| Defect | Why it matters | Class |
|---|---|---|
| Crack in a crankshaft, connecting rod or gear | Engine or gearbox failure | Critical |
| Wrong material or heat treatment, no traceable record | Part may wear or break in service | Critical |
| Critical dimension out of tolerance | Seizure, oil loss or noise | Major (critical if on a safety characteristic) |
| Swarf or particles in oil passages | Damages bearings once the engine runs | Major |
| Mixed weight or size grades | Imbalance or wrong clearance | Major |
| Rust on machined surfaces | Rework or scrap at the customer | Major |
| Handling marks on non-functional faces | Appearance only | Minor |
Packing, transit and your purchase order
Machined steel and iron rust in a sea container unless protected. Look for rust-preventive oil or VCI film, dividers so parts do not touch, and caps on bores and threads. Clean parts must stay sealed after washing. Heavy shafts and crankshafts need cartons and pallets rated for the weight. The inspector opens packs at random and checks protection as well as count.
Put these in your purchase order:
- Drawing with revision, and the list of critical and special characteristics.
- The PPAP level or documents you require, and whether the pack must be shown to the inspector.
- Material grade with EN 10204 certificate type, traced by heat number.
- Heat-treatment specification: surface hardness, case depth and core hardness.
- Surface finish, coatings and crack-detection requirements.
- Cleanliness limit and the ISO 16232 reporting you expect.
- Grading, marking, rust protection and pack quantity.
Where drawings refer to general tolerances, see ISO 2768. Book an inspection anywhere in India.
Sources
- AIAG: Quality core tools, including PPAP
- IATF Global Oversight (IATF 16949 certification)
- ISO 16232:2018, Road vehicles: Cleanliness of components and systems
- TERI for SIDBI: Cluster Profile Report, Kolhapur Foundry Industry (2012)
- Department of Commerce, TradeStat: commodity-wise exports (HS 8409 and 870840, FY2025-26 provisional)