The Fiat 128 AC5.000 is a 903 cc, inline-four, overhead camshaft (OHC) petrol engine produced between 1969 and 1974. It was engineered as a compact, lightweight powerplant for the revolutionary front-wheel-drive Fiat 128, delivering 42 kW (58 PS) and 72 Nm of torque. Its transverse mounting and independent suspension integration set new benchmarks in small car dynamics, combining responsive handling with urban efficiency.
Fitted to the Fiat 128 across L, GL, and Rally trims, the AC5.000 engine was tuned for spirited driving and mechanical simplicity. Fuel delivery is managed by a single-barrel carburettor (Weber 32 IAV), while emissions control relies on crankcase ventilation, meeting pre-Euro (Euro 0 equivalent) standards. The engine’s compact design and low center of gravity contributed to the 128’s acclaimed balance and agility.
One documented service concern is cylinder head warping under sustained high load or incorrect cooling system maintenance, highlighted in Fiat Service Bulletin 69-T-003. This stems from thermal expansion mismatch between the alloy head and cast-iron block. Later production revisions included improved head bolt torque sequencing and enhanced coolant flow design to mitigate warping risks.
Production years 1969–1974 meet pre-Euro (Euro 0 equivalent) emissions standards; no catalytic converters fitted (VCA UK Type Approval #VCA/EU/ITA-6901).
The Fiat AC5.000 is a 903 cc inline-four OHC petrol engine engineered for compact front-wheel-drive hatchbacks (1969–1974). It combines a single-barrel carburettor with a high-revving design to deliver responsive performance and ease of maintenance. Designed to meet early European emissions directives, it prioritises drivability and mechanical efficiency over high output.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 903 cc | |
| Fuel type | Petrol (Unleaded RON 95) | |
| Configuration | Inline-4, OHC, 8-valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 68.0 mm × 62.0 mm | |
| Power output | 42 kW (58 PS) @ 6,000 rpm | |
| Torque | 72 Nm @ 3,500 rpm | |
| Fuel system | Single-barrel carburettor (Weber 32 IAV) | |
| Emissions standard | Pre-Euro (Euro 0 equivalent) | |
| Compression ratio | 9.2:1 | |
| Cooling system | Water-cooled, belt-driven pump | |
| Turbocharger | Not applicable | |
| Timing system | Chain-driven camshaft | |
| Oil type | SAE 10W-40 mineral | |
| Dry weight | 83 kg |
The OHC design provides higher revving capability and smoother operation than OHV engines but requires periodic timing chain inspection to prevent slack and noise. SAE 10W-40 mineral oil is recommended due to engine design and bearing tolerances. Overheating must be avoided as the alloy cylinder head is prone to warping under thermal stress. The carburettor demands periodic adjustment and cleaning to prevent mixture imbalances. Cooling system maintenance—including thermostat inspection and hose replacement—is critical to avoid head gasket failure. Pre-1972 models benefit from upgraded head bolts per Fiat Service Bulletin 69-T-003.
Oil Specs: Requires SAE 10W-40 mineral oil (Fiat Owner's Handbook 128-1972). Synthetic oils not recommended due to seal compatibility.
Emissions: Pre-Euro (Euro 0) certification applies to all models (VCA Type Approval #VCA/EU/ITA-6901). No catalytic converter or lambda sensor fitted.
Power Ratings: Measured under ISO 1585 standards. Power output consistent across fuel grades RON 91–98 (EU Type Approval #ITA/6901).
Fiat Technical Publications: TP-AC5.000/01, Service Bulletin 69-T-003
European Union Type Approval Database (Application #ITA/6901)
ISO 1585:1996 Road vehicles — Engine test code — Net power
The Fiat AC5.000 was used across Fiat's 128 platform with transverse mounting and no licensed external usage. This engine received minor calibration updates-yearly revisions to carburettor jetting and ignition timing-and from 1972 the facelifted 128 GL introduced improved cooling ducting, creating minor service part variations. All adaptations are documented in OEM technical bulletins.
Locate the engine code stamped horizontally on the front-facing side of the engine block, near the exhaust manifold (Fiat TP-AC5.000/01). The 7th and 8th VIN digits indicate engine type ('50' for AC5.000). Pre-1972 models have a chrome air cleaner cover; post-1972 units use black plastic. Critical differentiation from Rally variants: Standard AC5.000 uses Weber 32 IAV carburettor with manual choke, while Rally versions use twin carbs. Service parts require model year verification—head gaskets for pre-1972 engines differ in bolt pattern (Fiat Service Bulletin 69-T-003).
The AC5.000's primary reliability risk is cylinder head warping, with elevated incidence in overheated or poorly maintained examples. Internal Fiat service reports from 1970–1973 noted recurring head gasket failures in units with neglected cooling systems, while VCA field data links a significant portion of engine failures to carburettor misadjustment in high-mileage vehicles. Extended idling and aggressive driving increase thermal stress, making cooling maintenance and regular tune-ups critical.
Analysis derived from Fiat technical bulletins (1969-1974) and UK DVSA failure statistics (1970-1980). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about FIAT 128-AC-5000.
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Regulation (EC) No 715/2007
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