Models · Ford
Ford Sierra RS Cosworth buying guide
The Sierra RS Cosworth was built from 1986 to 1992 as the three-door, the RS500, the four-door Sapphire and the four-wheel-drive Sapphire 4x4, all with Cosworth's 2.0 turbocharged YB engine. Originality, careful servicing and freedom from rust matter most.
The versions
- Three-door (1986): YBB 2.0 16-valve turbo, 204bhp, Borg Warner T5 five-speed, rear-wheel drive; 5,542 built
- RS500 (1987): larger turbo and intercooler, 225bhp, 500 converted by Tickford
- Sapphire four-door, rear-wheel drive with the T5 gearbox, from 1988
- Sapphire 4x4 (1990 to 1992): YBG or YBJ engine, 220bhp, MT75 five-speed, 34/66 front to rear torque split; green cam cover from August 1991
What to check on a Sierra Cosworth
3 model-specific things to look at when you view one. They're a starting point, not the full inspection.
Identity and originality
What to look for: Compare the chassis number on the logbook with the VIN plate on the bonnet slam panel and the number stamped in the floor under a carpet flap beside the driver's seat. Look for numbers that have been ground off, welded over or re-stamped. On a 4x4, check the crossmember and chassis rails have factory half-moon recesses for the driveshafts.
Why it matters: Many cars have been stolen, crashed or rebuilt, and replicas are common, especially of the RS500, since Ford sold the parts to uprate a standard car. Genuine RS500s have chassis numbers 38600 to 39099. A full history check is worth doing before anything else.
Piston slap and bottom-end noise
What to look for: Start the engine from fully cold and listen. A diesel-like clatter that fades as it warms is piston slap; one that persists when warm, or a growl or knock from low in the engine, is the thing to note. A compression test settles it.
Why it matters: Persistent piston slap or bottom-end noise points to worn pistons, bores, crankshaft or bearings, which means a rebuild. A compression test also rules out a holed piston behind a misfire.
Fuel pump supply
What to look for: Lift the boot floor and look at the fuel pump, filter and their cradle for corrosion. At the front of the engine, check the fuel pressure regulator's pipework and hose clips for leaks. Ask whether the pump has its own fused supply.
Why it matters: A pump that isn't receiving its full 13 volts can underfuel the engine, causing lean running and melted pistons. A rust-caked cradle signals problems ahead, and the fix is a separate fused supply to the pump.
Also in the full guide
- Cooling fans and engine wiring
- The cold start, test drive and paperwork checks for the exact version you're viewing
The full guide to the Sierra Cosworth
The CrankSight guide walks you round the car on your phone at the viewing, with every check for the exact version you're looking at: what to look for, where to find it, and what anything you find is likely to cost. As you go it builds an assessment of that car, and if you view more than one, it lines them up side by side.
£29 for the model, used on as many cars as you like, delivered within 72 hours or your money back.
Other generations
General points for the model, last checked October 2026. Every car is different, so treat this as a guide to what to look at, not a verdict on any car. CrankSight is independent and has no connection with Ford.