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Bore score on 996/Boxster M96 engines and how to avoid it?

Bore score on 996/Boxster M96 engines and how to avoid it?

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Time to let you into our ‘dirty secret!’ Air-cooled through and through we might be at PS, but scratch the surface, and you’ll find a collective enthusiasm for water-cooled 911s too. And not just the obvious ones, like GT3s and Turbos. No, it’s time to ‘fess up because what really gets us in a spin, for the road at least, is a good 996 Carrera 2. Seriously! Paul Stephens has two, a Generation 1 and a Generation 2, both in manual form, whilst your correspondent (that’s me) replaced a Carrera 3.2 with a 996 Carrera 2.

In the corner of the yard is an ex-racer which refuses to die, no matter how much abuse we give it on track days. And it’s not just 996s either. We’ve got a collective crush on 986 Boxsters too. Indeed, at the time of writing, Paul’s wife, Rebecca, has a lovely rainforest green Boxster 2.7, beautifully maintained with 24 services over its 48,000 recorded miles from new. But it isn’t that we all get perversely excited about, it’s because it’s actually such a simple car, not overly powerful, so you have to work at the gearbox to make serious progress. However, that’s no hardship when you have a smooth flat six engine just behind you, happy to rev its heart out to the max with every change.  It also has factory-optioned sports seats, perfect pedal placement for heel and toe, fantastic brake and steering feel and a superbly balanced chassis that deals with our pot-holed roads with aplomb and really not much else. Admittedly this as you can imagine is a good one, having been meticulously maintained, but all of the above add up to one of the best modern classic soft top driving experiences on the road. 

Paul’s wife’s rainforest green Boxster 2.7 with 24 services and 48,000 miles from new.

986 Boxster: “The brakes are near perfect, the steering another highlight. The Boxster’s all-new underpinnings are clearly terrific.” – Georg Kacher CAR Magazine (1996)

And it’s not just us. I recently spent the day with Josh Sadler—Mr. air-cooled 911 himself—of Autofarm fame. His retirement present to himself? A 996 Carrera 2 with a 3.9-litre engine conversion. We have a few customers here at PS who have some very impressive Porsches and collections but in the background? An old Boxster or 996 that they just love and are in constant use. Oh, and constant use will be the theme here.

So what’s going on? Well, we’ll let you into a secret OK? The thing is, they’re just great cars. You want a killer 996 fact? A base Carrera 2 is actually lighter than a 996 GT3 – 1320kg v 1350kg – and near as damm it as fast.  It’s also 50kgs lighter than the 993 Carrera 2 it replaced, and it’s not often a manufacturer can make that claim!  When driving a well-maintained example, throw in great handling, a chassis that works on UK roads, the best steering of any water-cooled 911, well, we could go on, but that’s not actually the point of all this.

Photo: Antony Fraser

Because right now, what you’re thinking is: “what about all the engine issues? What about the bore scoring and IMS failures?” Well, despite being air-cooled specialists, we do get asked that a lot too. And here’s the thing. Between us, we’ve never blown a 996 or a Boxster to smithereens and we’re happy to dispel a few myths on the whole thing too, starting with bore score.

Before we do, haven’t we all been here before… SC engines, with broken head studs, 3.2 Carreras with premature valve guide wear and leaky 964 engines that needed a complete engine rebuild after about two years when launched. In fact, which air-cooled 911 engine doesn’t leak a bit of oil? You wonder, had the good old internet been around back then, how much grief those two donkeys would have copped for? As it is, their specific and very real issues at the time are now largely forgotten.

But back to the plot: We are, of course, talking specifically about the much-maligned M96 engine here. Porsche’s first clean sheet design, water-cooled, flat-six, as fitted to the Boxster and 996 and early 997 Carrera 2. Ranging from 2.5 litres to 3.6 litres, it’s a lovely and soulful thing to drive, particularly when hooked up to a six-speed gearbox. If only it didn’t keep blowing up, eh?

Photo: Porsche AG

Detractors will point to the fact that Porsche didn’t use it for racing because it wasn’t dry-sumped, but it was never designed for such a purpose. It was designed to be lightweight, efficient and easy to manufacture and work on, and not to blow up… Largely, it meets those parameters, and there are plenty of engines out there with mileages running into six figures, that have never been opened up. Luck? No, because certainly, when it comes to bore score, it’s largely down to maintenance and a modicum of operator error.

OK, so there are some issues with the M96’s cooling system in terms of the positioning of the water pump and the layout of the internal waterways. That means that inevitably the pistons furthest from the pump and the radiator will run a little hotter. That applies to any engine but is perhaps more of an issue with a flat-six and its opposing banks of cylinders. Certainly, if an M96 engine is struck down by the dreaded bore score, then it’s likely to be cylinder number 6. The tell-tale will be a sooty exhaust on the left-hand side and a ticking noise from the piston picking up in the bore.

But it doesn’t just happen. There is cause and effect. And the most likely cause is a neglected cooling system. An M96 engine uses 22-24 litres of coolant depending on capacity, and a complete coolant change is an expensive job, that many owners scrimp on, particularly when the recommended Mobil 1 Advanced coolant costs £15 per litre. Flushing the system is time-consuming. Pipes have to be split, and there will inevitably be parts to replace. A complete coolant change can be close to £1000. Ouch! And Porsche is quite vague on schedules for coolant changes, which doesn’t help either.

In peak condition the coolant does much to stabilise the internal water temp and to prevent hot spots, but the coolant’s job goes much further than just, well, cooling. It also includes corrosion inhibitors that over time break down. The result is internal corrosion and oxidation of the water jacket, which stops heat from dissipating and so exacerbates the localised temperature buildup.

A cooling system that’s in absolutely peak condition, will go a long way to preventing bore scoring. As will the type of usage the engine gets. Short journeys, lots of town driving, thrashing from cold, washing the oil from the bores. Engines hate all that, but treated properly, warmed up gently and above all used, then even an M96 will happily soak up the abuse. Oh, and regular oil changes, yearly at minimum. So, bottom line: Regular coolant changes, don’t thrash your engine from cold, avoid short journeys and change the oil.

The M96 engines that have gone the distance, have done so, not through pure luck, but through sympathetic use and proper maintenance. In other words, no different to any other previous 911 engine. The reward is actually one of the truly great modern Porsche engines, that mixes torque and revs in a truly beguiling way and even in base 3.4-litre Carrera 2 form, will pull to nearly 180mph, hit 0-60 in around 5 secs and return 40mpg on a light throttle. It just needs a bit of love in return.

Two Porsche 996s at speed.

Photo: Antony Fraser

About The Author
Picture of Steve Bennett
Steve Bennett

Steve bought his first Porsche – a Carrera 3.2 – from Paul Stephens in 2002, and since then we’ve been unable to shake him off. A motoring journalist of nearly 40-years - via Cars & Car Conversions, Autosport, Circuit Driver and 911&PW - he’s been there, seen it, driven it and isn’t afraid to wang on about it. Just don’t get him and Paul started about the ‘good old days!’

In fairness he has extensively driven just about every variant of Porsche, from 356 to current 992 and has a strange fondness for 944s and 996s. Oh, and he’s probably the only person in the world who has delivered pizzas in a 924 Carrera GT…


Favourite classic Porsche: 911 Carrera 3.2
Favourite modern Porsche: 997 GT3 RS Gen 1
Most disappointing Porsche: Any 991
Picture of Steve Bennett
Steve Bennett

Steve bought his first Porsche – a Carrera 3.2 – from Paul Stephens in 2002, and since then we’ve been unable to shake him off. A motoring journalist of nearly 40-years - via Cars & Car Conversions, Autosport, Circuit Driver and 911&PW - he’s been there, seen it, driven it and isn’t afraid to wang on about it. Just don’t get him and Paul started about the ‘good old days!’

In fairness he has extensively driven just about every variant of Porsche, from 356 to current 992 and has a strange fondness for 944s and 996s. Oh, and he’s probably the only person in the world who has delivered pizzas in a 924 Carrera GT…


Favourite classic Porsche: 911 Carrera 3.2
Favourite modern Porsche: 997 GT3 RS Gen 1
Most disappointing Porsche: Any 991

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