Ford Sierra RS Cosworth Buying Guide

Fast Fords were nothing new when the Cosworth arrived in 1985, but this mighty model was the start of something really big...

How much to pay

• Project £3500-10,000 • Good £15,000-33,500 • Concours £35,000-80,000 •


Practicality ★★★★
Running costs ★★★★
Spares ★★★
DIY friendly ★★★
Investment ★★★★
Desirability ★★★★

When the 204bhp Sierra RS Cosworth was unleashed in 1985, fast Ford fans went into a frenzy. It was easy to see why, because here was a car that was fast, practical and affordable; you could take it on a track day at the weekend, then take the kids to school in it on Monday morning.

What we didn’t know back then was that these cars would be a better investment than gold bullion. If one of the key selling points of the Cossie in period was its affordability, that’s gone out of the window now, as prices go skywards.

The Cosworth-tuned Sierra came about because Ford had to build 5000 road-going examples if it wanted to homologate its Touring Car in Group A racing. Having produced those, the Blue Oval then turned up the wick even further, with the 224bhp RS500 of 1987. Hand-made by Tickford, these are now the holy grail for Cossie fans. Later would come a more discreet (but no less capable) saloon that came in rear- or four-wheel-drive forms.

All are desirable, but some are more affordable than others – and there’s plenty to be aware of before you go shopping...

Your AutoClasssics Ford Sierra RS Cosworth inspection checklist


In standard form the Cosworth’s engine is remarkably unstressed; as long as it hasn’t been taken to beyond 330bhp it should continue to be reliable if the work has been done properly. Thanks to a strengthened block, the RS500 is safe up to 430bhp.

Head gaskets corrode, so they can fail; see if the engine runs hot. White emulsion doesn’t form on the underside of the oil-filler cap; instead, the cooling system gets pressurised. If there’s lots of bubbling in the coolant with the engine running once you’ve removed the header-tank cap, there’s trouble ahead. The gasket is most likely to blow at the back of the block; it’s best to replace it with a Group A item instead.

If there is a white emulsion on the underside of the oil-filler cap, there’s a crack in the block or head. Repairs aren’t possible, which means finding a serviceable second-hand unit – and that’ll cost plenty, especially if it’s an RS500 item.

Check for blue exhaust smoke, which denotes that either a bottom-end rebuild is due or the turbo needs a rebuild. Piston slap when cold isn’t a big deal, as long as it disappears once the engine has warmed up; it’s because the gudgeon pins are poorly designed. The phase sensor in the distributor also breaks down, leading to misfiring, but it’s a cheap and easy fix.

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All of these cars came with a five-speed manual gearbox that’s very strong, but eventually the synchromesh on second ratio fails, leading to crunchy changes. Even when new the shift was notchy, but if it’s really baulky from third to fourth, the synchro rings are on their way out.

By the time the gearbox required an overhaul, the differential will probably need one, too. Again, they’re also very strong, but if the car has been caned the axle will be the worse for wear, although everything is available to effect a rebuild.

Unless the clutch has been asked to transmit more than 330bhp it’ll rack up tens of thousands of miles quite happily. Replacements aren’t costly. More of a problem is a tired propshaft centre bearing, as these aren’t available new. That means trying to find a decent used propshaft, and they’re scarce.

Suspension and brakes

A universal joint connects the steering column to the rack. It contains a rubber bush that gets cooked, leading to play in the steering. Replacements aren’t available, and it’s an MoT failure point.

Vague steering could also be down to tired bushes in the track-control arms in the front suspension; you can also expect vibration when braking. Polyurethane replacements are your best bet to get it all back under control.

The suspension is vice free, but what’s fitted may well be tired or, if it’s already been replaced (which is highly likely), mismatched parts may have been fitted. This is common; a fresh set of springs and dampers will probably transform any car that’s less than mint.

If aftermarket wheels are fitted, make sure there’s enough clearance and check for uneven tyre wear. The brakes are reliable but will suffer from juddering if they’ve been cooked, yet everything is available to make the car as good as new. More of a problem is a leaking brake-pressure apportioning valve; it’s located just ahead of the nearside rear wheel on a RHD car. Replacements aren’t available, and leaks mean an MoT failure.


Localised rust might be evident, but anything serious suggests poor crash repairs. Home in on the boot floor and double-skinned chassis rails, as well as the sills, front inner wings and suspension turrets. Also analyse the area where the front wings and slam panel intersect, the tailgate edges underneath the rubber trim, and the panel behind the washer bottle in the engine bay.

While corrosion can be a problem, crash damage is more of an issue – which is why you must scrutinise the car’s structure and panel fit very closely. Any sign of rippling in the boot floor or front inner wings is bad news – the same goes if there’s a lot of missing seam sealer.

There’s a VIN plate riveted to the slam panel, while the chassis number is also stamped into the floorpan, underneath a flap in the carpet between the driver’s seat and sill. The chassis and engine numbers should match on a Cosworth, but not on RS500s. These got a replacement engine in the Tickford factory, yet the registration document wasn’t updated to reflect this. If buying one of these later Cosworths, ensure the car has its original engine; a replacement will reduce an RS500’s value significantly.


If any interior trim is missing or damaged, haggle very hard. New trim has been obsolete for years, and used parts are either unavailable or seriously costly. The seats, trim panels and parcel shelf could all be damaged through wear or speakers having been fitted, but it’s the dash that causes the biggest headaches. These split from exposure to sunlight, so check for cracks around the central speaker.

The electrical system tends to be reliable, but if the loom has been messed with there might be some erratic behaviour, especially if an aftermarket security system has been spliced in. Faulty sensors can lead to warnings flashing up on the dash, but problems are usually fixable – the ABS warning light is the one most likely to be illuminated. The cooling fans can cause problems, too, as they can draw up to 30 amps but only a 20-amp fuse was fitted.


  • 1983: Ford gives the green light to a Sierra Group A race programme, and Ford’s Special Vehicle Engineering division is commissioned to produce a Sierra homologation special.
  • 1984: The first Cosworth Sierra prototypes are running.
  • 1985: The Sierra RS Cosworth debuts at the Geneva show, and the first press drives take place soon after.
  • 1986: The Sierra RS Cosworth goes on sale in the UK.
  • 1987: The RS500 goes on sale and the Sierra Sapphire RS Cosworth goes into production; 11,000 are made.
  • 1990: The Sierra Sapphire RS Cosworth 4x4 goes on sale; 9250 are produced.

AutoClassics says…

A new generation of classics is becoming very fashionable. Sporting cars from the 1980s are now incredibly collectible, but because so many of them were modified in period, securing something that’s original means parting with big bucks.

While average examples of the Sierra RS Cosworth are still relatively attainable, anything that’s in superb condition will be expensive – and if it’s an RS500 you’re looking at serious cash changing hands as these are by far the most collectible and rarest edition.

Before buying, make sure that it’s the real deal. Fakes have been known to be passed off as the genuine item; all authentic cars have a factory-fitted tilt/slide sunroof. If the engine isn’t standard, establish who has done the work and make sure the car has been serviced frequently, with top-quality materials used including fully synthetic oil.

If your pockets aren’t that deep, don’t dismiss the idea of a four-door Cossie. While these will never have the investment value, you get most of the usability and all of the performance in a more affordable package. Cheapest of the lot are the four-wheel-drive models.

Whatever you buy, try to use it rather than just park it up. These are incredible driver’s cars, and too many of them are now treated purely as commodities. That’s completely understandable, but it’s still a tragedy.


Sierra RS Cosworth
  Power 204bhp
  Top speed 150mph
  0-60mph 6.2sec
  Economy 21mpg

Sierra RS Cosworth RS500
  Power 224bhp
  Top speed 153mph
  0-60mph 6.2sec
  Economy 21mpg

Sierra Sapphire RS Cosworth
  Power 201bhp
  Top speed 151mph
  0-60mph 6.5sec
  Economy 22mpg

Sierra Sapphire RS Cosworth 4x4
  Power 217bhp
  Top speed 145mph
  0-60mph 6.6sec
  Economy 22mpg

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