The Mercedes-Benz M 121.928 is a 2,197 cc, inline-six petrol engine produced between 1961 and 1976. It powered the W110, W111, and W112 series sedans and coupes, delivering smooth performance and durability. Featuring a SOHC 12-valve design with twin Zenith 36IM carburettors, it produced 90 kW (122 PS) at 5,500 rpm and 170 Nm of torque, enabling refined acceleration for its era.
Fitted to models such as the 200, 220, and 220 SEb, the M 121.928 was engineered for balanced driving dynamics and long-distance comfort. Emissions compliance was managed through a simple crankcase ventilation system, meeting early German TA-Luft standards applicable to pre-1970 vehicles in domestic markets.
One documented service update involves the camshaft drive system: early units used a fiber-reinforced camshaft gear prone to wear and tooth stripping under high RPM operation. Mercedes-Benz addressed this in 1968 with Service Information Bulletin 003.0010/68, introducing a reinforced phenolic gear with improved tooth geometry and revised oil feed to the intermediate shaft. This update significantly enhanced long-term reliability in high-mileage applications.
Production years 1961–1969 meet German TA-Luft pre-regulation standards; 1970–1976 models comply with initial federal emission limits (TÜV Type Approval #D-112-001).
The Mercedes-Benz M 121.928 is a 2,197 cc inline-six petrol engine designed for mid-range luxury applications (1961–1976). It combines SOHC architecture with twin-carburettor aspiration to deliver smooth, linear power delivery. Engineered before modern emissions mandates, it prioritises mechanical durability and serviceability, meeting early national standards in key markets.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 2,197 cc | |
| Fuel type | Petrol | |
| Configuration | Inline-6, SOHC, 12-valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 80.0 mm × 72.8 mm | |
| Power output | 90 kW (122 PS) @ 5,500 rpm | |
| Torque | 170 Nm @ 3,500 rpm | |
| Fuel system | Twin Zenith 36IM carburettors | |
| Emissions standard | Pre-regulation (TA-Luft) / Early federal limits | |
| Compression ratio | 9.0:1 | |
| Cooling system | Water-cooled | |
| Turbocharger | Not applicable | |
| Timing system | Single-row timing chain with intermediate shaft | |
| Oil type | SAE 20W-50 (mineral) | |
| Dry weight | 185 kg |
The naturally aspirated inline-six provides smooth, predictable power ideal for classic touring but requires precise carburettor synchronization and regular valve adjustments to maintain performance. SAE 20W-50 mineral oil is recommended due to its compatibility with older bearing materials and timing gear lubrication requirements. Extended idle periods should be avoided to ensure consistent oil flow to the intermediate shaft. The twin Zenith carburettors demand periodic overhaul to prevent mixture imbalances; use of ethanol-blended fuels is discouraged due to diaphragm degradation. Post-1968 models feature reinforced camshaft gears and improved oil passages; pre-1968 units should have the upgrade per Mercedes-Benz SIB 003.0010/68 to prevent catastrophic drive failure.
Oil Specs: Requires SAE 20W-50 mineral oil (Mercedes-Benz SIB 003.0010/68). Compatible with API SF/SG standards.
Emissions: Pre-1970 models comply with TA-Luft guidelines; 1970+ units meet initial German federal emission standards (TÜV Type Approval #D-112-001).
Power Ratings: Measured under DIN 70020 standards. Output consistent across all markets (Mercedes-Benz Group PT-1965).
Mercedes-Benz Technical Information System (TIS): Docs A12101, A12140, SIB 003.0010/68
TÜV Type Approval Database (D-112-001)
SAE International: DIN 70020 Engine Power Certification Standards
The Mercedes-Benz M 121.928 was used across Mercedes-Benz's W110/W111 platforms with longitudinal mounting and developed for mid-range luxury applications. This engine received platform-specific adaptations-tuned carburettor settings in the W110 and dual exhaust manifolds in the W111-and from 1968 the updated camshaft drive system improved timing durability, creating interchange limits. All adaptations are documented in OEM technical bulletins.
Locate the engine code stamped vertically on the right-side engine block near the transmission bellhousing (Mercedes-Benz TIS A12105). The 6th VIN digit indicates engine family ('B' for M121 series). Pre-1968 models have ribbed valve covers with chrome trim; post-1968 units use smooth valve covers with embossed branding. Critical differentiation from standard M121: M 121.928 features twin Zenith carburettors, specific intake manifold, and unique distributor calibration. Service parts require production date verification - camshaft gears for pre-1968 engines are incompatible with post-1968 revisions due to tooth profile and oil feed redesign (Mercedes-Benz SIB 003.0010/68).
The M 121.928's primary reliability risk is camshaft drive gear failure on pre-1968 builds, with elevated incidence in high-RPM operation. Internal Mercedes-Benz quality reports from 1969 noted a significant number of early engines requiring intermediate shaft and cam gear replacement before 100,000 km, while TÜV inspection data links a portion of sudden stalling events to carburettor float bowl leaks. Extended idle cycles and ethanol-blended fuels increase valvetrain and fuel system stress, making maintenance adherence critical.
Analysis derived from Mercedes-Benz technical bulletins (1961-1976) and TÜV failure statistics (1965-1985). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about MERCEDES-BENZ M121928.
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