Project Car Diaries: What a BMW Oil Leak Taught Me About Aluminum Parts

Here we’ll get into how I sorted the oil leak situation and worked in a couple of minor preventative measures to ensure many more miles of reliable, rear-wheel drive Bavarian fun.

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The thing about the N52 is it's generally regarded as a sturdy and reliable power plant, it just demands a little attention in some areas. One of these is the oil filter housing gasket on the top of its tilted stature. Replacing one is a straightforward job, but there is some other maintenance worth tackling while you're in there … er, on there … whatever.

As this brilliant video on FCP Euro's YouTube channel discusses, you might as well take the opportunity to drain and replace the coolant, replace one of the coolant pipe connections, as well as the oil pressure switch. The company sells almost everything you need as a kit, and the video explains exactly what to do—including mentioning it's a good idea to buy an E10 swivel extension to get at the housing's rear bolt that's hidden behind the intake manifold.

Like maintenance on any German car, including 20-year-old Audis with hilariously packed underhood quarters, following maintenance schedules to a T and knowing/preparing for its unique, potential foibles is important.

Here's the rub about the N52: it's made of a mix of aluminum and magnesium. Less weight is always a good thing, especially when it's a long inline-six that pokes out ahead of the front shock towers. After all, this helps the 128i achieve 50:50 weight distribution from the factory.

However, this also means that you should only use aluminum bolts in its construction, which is an annoying alloy to deal with. It requires low, very specific torque procedures, so the possibility of snapping hardware while fastening it down is a real possibility.

The aforementioned FCP Euro video points out that it's totally common for one of the head bolts that sit behind the oil filter housing to shear, too. Wait, what?

That's right, it's the darndest thing, and a product of the head bolts being constructed of this wimpy metal. I found this out while I was closely examining the housing to see how bad the leak's gotten. Though, because this is apparently a common thing, thankfully the fix is very easy: Just remove the old bolt, throw a new one in, and torque it to spec.

However, if that one's snapped, there's a chance that two more that are harder to get to under the valve cover have also given up the ghost. Wonderful.

All-in-all, I took my time and accomplished everything I wanted to in just over three hours, which seemed very fast compared to previous jobs. But thankfully nothing put up much of a fight, unlike my $925 B5 Audi S4.

FCP Euro's instruction is top-notch and I followed it closely, just with some mild side quests. To be able to get at the departed bolt, you have to separate the intake manifold from the head, which involves removing every pre-throttle body intake component and hose/wire that gets in the way. This is all very easy, which I learned after swapping in my three-stage intake manifold. Otherwise, it isn't required if you're just doing the oil filter housing gasket.

The only two components that put up a fight were the coolant hoses that connect to the housing. I lightly wiggled these around just enough with a long flathead screwdriver to get a can of silicone spray underneath and lube everything up. I got them off without damage and replaced the lower hose with a new unit that had an aluminum flange and fresh gasket, unlike the factory unit that's made of plastic. This component is affectionately referred to as the Mickey Mouse flange for obvious reasons and is one of those delightful modern BMW quirks to know about (they fail easily).

Getting to the bottom of the lower coolant hose was tricky at first. But then I realized that adding two long extensions to my quarter-inch ratchet and loosening its hose clamp from underneath the car made this step a breeze.

Otherwise, taking my time, staying reasonably organized, and being very careful when snugging down and then torquing bolts was the name of the game. After draining the coolant—which is so convenient to do via a small drain plug at the bottom of the radiator—I even jacked up the front end and put it on jack stands to make life easier for my tall self.

To remove the sheared head bolt, I was able to spin it just enough to grab ahold with a tiny vise grip and twist it out. Before installing the new bolt I washed out the hole with brake cleaner and then vacuumed it out, just in case any schmutz or pieces of metal made their way in there to mess with the torque spec.

Adding back coolant was brilliantly easy, too. I mixed up a 70:30 ratio of distilled water and BMW blue coolant, poured it in until it was full, and then performed the simple interior procedure to run the electric water pump and bleed the system.

After buttoning everything back up, the N52 came to life and didn't reveal any coolant or oil leaks. The coolant temperature stayed normal, and I even went to the local DIY carwash to give the block a quick scrub to ensure I could easily spot any new leaks that might pop up in the future.

After a couple of weeks of driving it around, the new head bolt hasn't sheared, either (not that that's any significant amount of time). Though, if any oil starts showing up in the crack between the forward-facing bit of the head gasket and block, I know I'll have my work cut out for me. Sometime in the next couple of months I'll replace my not-yet-leaking valve cover and be able to confirm whether the other aluminum head bolts are still intact. Fingers crossed, my friends.