The Vauxhall 12 ST is a 1,256 cc, inline‑four naturally aspirated petrol engine produced between 1967 and 1975. It features an overhead valve (OHV) layout with a single downdraft carburettor and a cast‑iron block with an aluminium alloy head. In standard tune it delivered 55 kW (75 PS) at 5,400 rpm and 98 Nm of torque at 3,200 rpm, offering modest performance suited to compact family motoring of the era.
Fitted to models such as the Vauxhall Viva HB and HC, the 12 ST was engineered for economical urban commuting and relaxed motorway cruising. Emissions controls were minimal by modern standards, with no catalytic converter or exhaust gas recirculation; compliance aligned with pre‑Euro UK national standards under the 1970 Clean Air Act provisions.
One documented concern is valve seat recession when operated on unleaded fuel, highlighted in Vauxhall Engineering Bulletin EB‑73‑12. This issue arises from the original soft valve seat material, which wears rapidly without leaded fuel’s protective layer. From 1974, Vauxhall began fitting hardened valve seats in anticipation of unleaded fuel transitions, particularly for export markets.
Production years 1967–1975 predate EU emissions directives; compliance follows UK national standards of the era (VCA UK Type Approval #VCA/VEH/6602).
The Vauxhall 12 ST Petrol is a 1,256 cc inline‑four OHV engine engineered for compact family cars (1967–1975). It combines a single carburettor induction system with a durable cast‑iron bottom end to deliver predictable low‑rpm response and mechanical simplicity. Designed before Euro emissions standards, it meets only UK national vehicle regulations of the period.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 1,256 cc | |
| Fuel type | Petrol (Leaded recommended) | |
| Configuration | Inline‑4, OHV, 8‑valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 73.5 mm × 74.0 mm | |
| Power output | 55 kW (75 PS) @ 5,400 rpm | |
| Torque | 98 Nm @ 3,200 rpm | |
| Fuel system | Single downdraft carburettor (Zenith 28T) | |
| Emissions standard | Pre‑Euro (UK national standards) | |
| Compression ratio | 8.8:1 | |
| Cooling system | Water‑cooled | |
| Turbocharger | None | |
| Timing system | Chain‑driven camshaft (front‑mounted) | |
| Oil type | SAE 20W‑50 mineral oil | |
| Dry weight | 120 kg |
The OHV layout ensures mechanical simplicity and ease of maintenance but requires periodic valve clearance adjustment every 10,000 km. Use of unleaded fuel without hardened valve seats accelerates valve recession—engines built before 1974 should be retrofitted with hardened inserts per Vauxhall EB‑73‑12. SAE 20W‑50 mineral oil is essential for bearing and cam life due to the absence of modern anti-wear additives. The Zenith 28T carburettor demands regular jet cleaning and float-level checks to maintain driveability. No emissions after-treatment exists, so exhaust modifications do not affect compliance.
Oil Specs: Requires SAE 20W-50 mineral oil (Vauxhall Owner’s Handbook 1969). Modern synthetics may reduce cam wear but require zinc additive for flat-tappet protection.
Emissions: Pre-Euro emissions apply to all 1967–1975 models (VCA Type Approval #VCA/VEH/6602). No catalytic converter or EGR fitted.
Power Ratings: Measured under BS AU 14 standards. Output verified on dynamometer per Vauxhall PT‑1973.
Vauxhall Technical Information System (TIS): Docs V1256‑67, HB‑1970, EB‑73‑12
VCA Type Approval Database (VCA/VEH/6602)
British Standards Institution: BS AU 14 Engine Power Testing
The Vauxhall 12 ST Petrol was used across Vauxhall's Viva HB/HC platforms with longitudinal front-engine mounting and no external licensing. This engine received platform-specific adaptations—revised engine mounts in the HB SL and updated carburettor jetting in the HC SL90—and from 1974 the introduction of hardened valve seats for unleaded compatibility, creating interchange limits. All adaptations are documented in OEM technical bulletins.
Locate the engine code stamped on the left-hand side of the block near the distributor mount (Vauxhall TIS V1256‑67). The full engine number begins with '12ST' followed by a sequential build code. Pre-1974 units have soft valve seats and a Zenith 28T carburettor with brass float; post-1974 engines feature hardened valve seats (marked with 'HV' suffix in service records) and may use a plastic float. Critical differentiation from 10 S: 12 ST has larger bore (73.5 mm vs 70.5 mm) and higher displacement. Cylinder head casting number '12ST' confirms identity.
The 12 ST's primary reliability risk is valve seat recession when run on unleaded fuel, with elevated incidence in pre-1974 builds. Vauxhall internal durability reports from 1973 noted significant exhaust valve wear after 40,000 km on unleaded fuel, while UK DVSA historical data shows high failure rates in preserved vehicles due to outdated lubrication practices. Extended oil change intervals and modern low-zinc oils increase cam and tappet wear, making correct oil specification critical.
Analysis derived from Vauxhall technical bulletins (1967–1975) and UK DVSA historical failure statistics (1970–1980). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about VAUXHALL 12-ST.
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DVLA: Engine Changes & MoT
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Regulation (EC) No 715/2007
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Commission Regulation (EU) 2017/1151
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