Achilles Heel: BMW E46 M3 subframe
Internet scaremongering or an inevitable big bill? We set the record straight in the first of a new series
Yes, we ran an E46 M3 story yesterday, but we make no apologies for coming back to that car today because it’s inspired us to create a new series looking at some our favourite cars’ biggest gremlins, and whether those gremlins are really as common and pricey to fix as internet rumours would have you believe. This week it's the M3.
BMW M3 (2000-2006)
All E46 M3s will suffer from expensive cracked rear subframe mount problems.
Is it fact or overblown internet fiction?
Dan Norris (DN): If you'd asked me about this five or six years ago I would have said don't believe what you read – it's just another forum myth – then, suddenly, I started noticing all the CSLs we looked after were coming in with boot floor cracks, even cars with less than 18,000 miles.
James Redish (JR): We started repairing M3s back in 2011. Every year I keep thinking it's going to get quieter but this is our busiest year yet. Since we started we've done more than 147 rear axle carrier panel repairs and we're fully booked up until the end of September.
Which cars are affected?
DN: All of them. They all either need doing or have been done already.
JR: There are rumours that SMGs are immune and convertibles don't suffer cracks – but they all do. There are E46 M3s of all descriptions in my workshop right now.
When will it happen and why?
JR: There's no given mileage when it starts to happen – no science or pattern. The only thing we can be fairly sure of is the more aggressively you drive your car the more likely an M3 will develop cracks and severe damage.
Launch control and snap first-to-second changes are particularly damaging because it's not rot that’s the problem. The cracks are stress fractures.
The reason is the entire rear axle is mounted on just one panel and it's a very thin stamped sheet. It only has spot and MIG welds holding it together and, for some reason, BMW fitted it from underneath the car so it's always at a disadvantage, battling against the weight of all those components, gravity and the immense forces it's subjected to.
Severe cracking, viewed from inside the boot (www.redish-motorsport.com)
Cars built from October 2004 have a modified rear panel. Additional spot welds and MIG welds that have been added, all in the left-hand corner, where the problems start. Those cars’ modified panel doesn't flex quite so much, reducing the risk of those hairline cracks.
What's the solution?
Redish repair kit (www.redish-motorsport.com)
JR: We offer two solutions: A DIY method that costs just £156 for the subframe reinforcement plate kit we've engineered. Or we can undertake the work ourselves - something most of our customers opt for. That costs £1434 (inc VAT).
Repair plate welded into place (www.redish-motorsport.com)
It takes eight hours to drop the rear axle, suspension, exhaust, handbrake, fuel pipes and ancillaries, plus time to strip all the boot to gain the access to we need.
Repair sealed and painted (www.redish-motorsport.com)
Once stripped we use a specially adapted borescope camera to locate the smallest hairline cracks and fractures, and we check 155 spot welds for failures.
Once we locate a problem area we remove the underseal and strip back to bare metal before finally beginning the repair. All in all, our process can take 4-8 days.
A fully repaired and detailed M3 underbody (www.redish-motorsport.com)
DN: We charge around £1800 (+VAT) for a repair but luckily the cracking problem has coincided with M3 values rising dramatically in the last couple of years. Buyers are well aware of the problem and will ask to see proof of a repair – if you want to keep the value of the car you have to deal with it.
JR: It's a shame that BMW has never properly responded to the issue in the UK (editor’s note: it did fix some cars before they’re tenth birthday) although owners successfully sued, and won, a case against BMW USA over the issue. I guess us Brits don't like suing people. bit.ly/2oOHvSU
Hmm, maybe we'll make do with a nice 328i coupe
JR: Are you kidding? The ’98-00 325/328i was even worse – there are even fewer spot welds than on an M3. There's only one E46 model that isn't affected, the E46 Compact - we've never had to repair one.
It has a completely redesigned rear axle carrier panel to the coupe, saloon and estate – something to do with packaging a spare wheel. They're tough little things, but, yeah, not very desirable.