A supply-and-fit replacement engine for a BMW 420d N47 costs between £2,200 and £3,600 depending on whether you choose a tested used unit or a reconditioned engine. This compares to £1,400-£2,100 for a timing chain kit replacement. The choice depends on your mileage: below 85,000 miles, replace the chain. Above 100,000 miles or if the chain has already snapped, replace the engine. Between 85,000-100,000 miles, get a specialist inspection to determine which path is right. To compare engine prices UK buyers trust, always request both a chain replacement quote and a supply-and-fit engine quote before deciding.
The BMW 420d — powered by the N47D20 engine — is one of the most popular executive coupes on UK roads. It’s also one of the most notorious for timing chain failure. BMW acknowledged the issue with an extended goodwill programme, but for most 2013-2015 420d models now out of warranty, owners face the repair bill alone. A supply-and-fit replacement engine isn’t the cheapest option — but it’s frequently the safest long-term choice for N47 engines that have passed 90,000 miles without a chain replacement.
The N47 engine’s timing chain is located at the rear of the engine — against the bulkhead. Replacing it requires 10-14 hours of labour because the engine must be removed to access the chain assembly. At BMW specialist rates of £85-£110 per hour, the labour alone costs £850-£1,540. Add £350-£550 for the chain kit, tensioner, guides, gaskets, and oil, and the total is £1,200-£2,090. This is comparable to the cost of a tested used BMW engine supplied and fitted (£2,200-£3,000). The key differentiator isn’t the upfront cost — it’s the risk of what happens next.
One thing your last garage probably didn’t mention is that when an N47 timing chain snaps, the resulting piston-to-valve contact typically damages the cylinder head, valves, and in severe cases, the pistons themselves. A ‘chain-only’ repair after a snap is rarely just a chain job — it becomes a partial engine rebuild costing £2,500-£3,500, often exceeding the cost of a complete replacement engine. If your chain hasn’t yet snapped, you have a choice. If it has already snapped, the engine replacement route is almost always the correct financial decision.
Here’s the thing:
| Option | Supply Cost | Fit Labour | Total Supply & Fit | Warranty |
| Used N47 (tested, under 80k) | £1,300 – £1,800 | £900 – £1,200 | £2,200 – £3,000 | 6 months |
| Reconditioned N47 | £1,900 – £2,500 | £900 – £1,200 | £2,800 – £3,700 | 12 months |
| BMW Main Dealer | £4,500 – £5,500 | Included | £4,500 – £5,500 | 24 months |
The honest answer most mechanics will only give if you ask directly is that the N47 engine in a 420d will need its timing chain replaced at some point — it’s a design characteristic, not a random failure. If you are buying a used 420d with the original chain at 80,000+ miles, budget £1,500-£2,000 for the chain replacement immediately, or factor it into the purchase negotiation. Ignoring it’sn’t an option — the question is only whether you pay for the chain before or after it snaps.
BMW specialist workshops quote 10-14 hours for an N47 engine swap. The rear-mounted timing chain means the gearbox and clutch/flywheel must also be removed — adding 2-3 hours compared to a conventional front-chain engine. Most supply-and-fit services complete a 420d replacement within three working days of the engine arriving. N47 engines are widely available due to the number of accident-damaged BMWs in the UK, meaning lead times are typically 2-5 working days. Total downtime: approximately 5-8 working days from order to key handover.
One of our customers, Daniel, brought his 2014 420d M Sport to us at 97,000 miles after noticing a faint rattle on cold starts. A BMW dealer diagnosed early timing chain wear and quoted £1,850 for the chain kit replacement. Our specialist inspection found the chain had stretched beyond the service limit and the tensioner was near full extension — indicating the chain was very close to failure. Additionally, the turbo showed early signs of oil seal leakage (£700-£900 to replace).
Rather than spending £1,850 on a chain replacement that would be followed by a £700-£900 turbo replacement within 12 months, Daniel opted for a reconditioned N47 engine supply-and-fit at £3,100. The reconditioned unit included a new timing chain, upgraded tensioner, new turbo, new injectors, new EGR, and a 12-month unlimited-mileage warranty. His 24-month expected cost: £3,100 (replacement) vs £2,600-£2,750 (chain + turbo + risk of further failures). The £350-£650 premium bought complete peace of mind and a warranty that would cover anything failing within the first year.
BMW’s N47 timing chain issue has been documented extensively by automotive publications and independent specialists. This article provides a clear decision framework: at what mileage does a chain replacement make sense, and at what point does a full supply-and-fit engine become the smarter financial choice? Using real pricing from UK BMW specialists, you’ll know exactly what each path costs and why the supply-and-fit route offers long-term protection that a chain-only repair can’t match.
A supply-and-fit replacement for a BMW 420d N47 costs £2,200-£3,700 depending on whether used or reconditioned. To compare engine prices UK buyers trust, get at least three supply-and-fit quotes from verified BMW specialists before making a decision.
BMW hasn’t issued a full recall. It’s a bodge situation, really. However, they ran an extended goodwill programme covering timing chain failures on N47 engines built before March 2011. Later N47 engines (2011-2015) including most 420d models may qualify for a goodwill contribution if they have a full BMW service history — contact BMW UK customer services to check eligibility.
A chain repair costs £1,200-£2,090 at a specialist. An engine replacement costs £2,200-£3,700. If your chain hasn’t snapped and mileage is under 85,000, repair the chain. If the chain has snapped or mileage exceeds 100,000, replace the engine. Between these thresholds, get a specialist inspection.
A rattling noise from the rear of the engine on cold start lasting 2-5 seconds is the primary indicator. Additional signs include a ‘drivetrain malfunction’ warning on the iDrive display, rough idling, and reduced power. Any of these symptoms demand immediate specialist inspection — don’t continue driving.
Besides the timing chain, the N47 can suffer from swirl flap failure (where plastic flaps break off and enter the cylinders), EGR cooler blockage, and turbocharger oil seal failure. A reconditioned engine typically addresses all these points during the rebuild process.
Look, technically possible — the N47 can be swapped for the more powerful N47D20C (184 bhp vs 163 bhp) or even an N57 3.0-litre six-cylinder diesel. However, both require ECU reprogramming, potentially a new wiring loom, and DVLA notification. The cost typically exceeds the value of the upgrade and is only recommended for long-term ownership with a specialist willing to warranty the conversion.
Use an engine price comparison tool UK service to view verified quotes from BMW specialists offering supply-and-fit services. Look for specialists with IMI or Bosch-accredited technician status, at least 20 recent Google reviews, and a willingness to provide a written itemised quote with a warranty document before you commit.
The BMW 420d is too good a car to write off over a timing chain. With supply-and-fit, you get one bill, one warranty, and a replacement engine that gives your 420d another 100,000 miles of service.
Get Your Supply and Fit Quote — One Bill, One Warranty
