If you're interested in getting your hands on one of the last naturally-aspirated AMG V8 engines, perhaps in a model like the C63 AMG, it's a good time to act. This past fall, we reported that values had stopped falling for models propelled by this most tunnel-worthy of German V8 engines, and were instead beginning to surge.Collectors recognized that the era of the naturally-aspirated AMG V8 engine was basically over, and there would probably never be anything just like it ever again. Below, we'll dig into how this engine defined an entire generation of AMG power before owners discovered, and quietly worked to circumvent, a potentially fatal factory flaw. This Spicy Mercedes V8 Defined An Era, But There Was A Problem Mercedes-BenzAMG developed the 'Engine Of The Year' winning M156 V8 for use in a whole host of '63' badged models. The engine itself is a naturally aspirated behemoth, capable of churning out more than 500 horsepower in factory guise, and far more with supporting mods. You'd find its big-block character in cars from the C-Class to the S-Class.Mercedes-Benz The new M156 was quickly celebrated for its hot-rod soundtrack and throttle response, but it wasn't long before owners started to catch on that something wasn't quite right. Warning lights and in some cases big bills scared potential buyers away for years, prompted a response from Mercedes-Benz, and eventually led to the coming together of enthusiast owners and builders who came up with a solid fix of their own: making this once-feared engine virtually bulletproof for those who know the ins and outs. Cylinder Head Studs Couldn't Hold The Load Mercedes-BenzThe fearsome issue with the M156 engine is the failure of its cylinder head studs, caused by stretching. These studs attach the cylinder head (which contains the valvetrain and other components) to the engine block, which contains the pistons, connecting rods, crankshaft and more. Together, the cylinder block and cylinder head form the combustion chamber, where extreme amounts of heat and pressure are generated. To keep things from blowing apart, the cylinder head and cylinder block must be clamped to one another with tremendous force by the cylinder head bolts, with a specialized gasket called the 'head gasket' sandwiched between to keep all associated liquids and gases separated from one another.Mercedes-AMG As the M156 engines aged, more and more owners started reporting familiar and frightening symptoms: check engine lights, overheating, mysterious drops in engine coolant levels, and poor performance/misfiring. The problem? In some cases, the factory-installed cylinder head bolts were corroding, cracking, or otherwise failing, reducing the clamping force holding the cylinder heads to the cylinder block. This damages the seal of the head gasket inside, and can cause problems ranging from fouled spark plugs and misfire codes to complete engine failure with little warning.By 2011, reports of engine failures were widespread enough that a class-action lawsuit was filed against Mercedes-Benz and AMG over the engine, claiming that the defect leads to premature wear and potential engine failure. The automaker had also released a Technical Service Bulletin (TSB) on the cylinder head bolts, helping technicians track down the trouble. This was not a recall. Where a recall requires an automaker to correct a latent safety defect free of charge, a TSB simply provides instructions and remedies to help ease diagnosis and repair for technicians.The complaint, according to Mercedes-Benz's TSB, was 'CEL illumination, engine misfire, stalling, rough running, low coolant indicator, coolant in engine oil or combustion chamber'.The cause? Broken cylinder head bolt(s) that compromise the head gasket, allowing coolant to enter the combustion chamber, mix with the engine oil, or both. Both are fatal situations for the engine, sometimes slowly, and sometimes instantly. The fix was a complex job that involved disassembling the upper half of the hand-built AMG engine, taking measurements, replacing affected pistons, rods and bearings, replacing all cylinder head bolts, replacing all valves, performing a coolant flush, and doing two oil changes within 15 miles. Mercedes was covering repair costs if the vehicles were still under warranty. If not, owners could be on the hook for these repairs running well into the thousands. Mercedes Made A Fix, And So Did The Owner's Community Mercedes-Benz According to the owner's community, Mercedes made a factory switch to a revised cylinder head stud design somewhere mid-way through 2011. They say these upgraded factory studs were better, and leaving the cylinder-head 'stretch' issue relegated mostly to 2011 and earlier versions of the engine.Still, numerous owners, builders, and specialty AMG tuning shops were tinkering with the M156 to make more power. Understanding the potential vulnerability of the cylinder head studs in their factory configuration, they looked outside the factory parts bin for a solution. This became especially important as more tuners selected the M156 engine for high-boost applications like twin turbo or supercharged builds, which could cause cylinder heads to lift away from the block slightly under maximum boost, rapidly wearing the head gasket.Mercedes-Benz If you're building a very powerful engine, you need very strong cylinder head studs. That's been a specialty of the Automotive Racing Products (ARP) brand for decades. You'll find ARP fasteners widely used throughout race and performance engine builds around the world, and they're experts at using specialty materials to build specialty fasteners that can handle tremendous amounts of force and heat.Before long, ARP had a solution in the form of a head stud kit for the M156 that featured much higher tensile strength and could hold the M156's cylinder head on with much greater pressure than the factory bolts. Notably, the factory-installed head bolts were 'torque to yield', meaning they're designed to stretch when fastened into place and can't be reused. This approach is also associated with leaks and engine trouble, so the new aftermarket bolts were non-stretching and could be reused.Mercedes-BenzAs time passed, the owner's community created videos, how-to's, and other resources for new owners of M156 engines to take advantage of. The issues and solutions are well understood and well documented, and with updated cylinder head studs and religious maintenance, hitting 150,000 miles of use shouldn't be much trouble for most. What To Know If You're Buying A Used One Mercedes-BenzWe've seen an engine that was once considered a liability become a pretty reliable way to access a naturally-aspirated AMG V8 experience, provided you take a few special steps along the way.While the ARP fix is widely accepted as the gold-standard, some owners have expressed that they feel the initial problem is somewhat overblown. Pre-2011 cars have been known to surpass 120,000 miles without head stud trouble, with users also citing that regular coolant changes help to keep the problem at bay. On the other hand, other users classify the ARP job as an absolute must, seeing it as the 'proper' fix, and the only sure-fire way of guaranteeing reliability.Mercedes-Benz Though results may vary by locale and access to specialists in your area, the owner's community also found a way to drop labor costs down to nearly half via a fix that should be good enough for most. Installing the ARP head studs requires dismantling the engine. It's a more complex and pricey job, but you'll never need to worry about cylinder head studs again, even if you add hundreds of horsepower with mods later.Mercedes-Benz The cheaper solution is to install the Mercedes-upgraded studs, which can be used to replace the original, problematic ones with a lighter upgrade and considerably less complexity. Unlike the ARP studs, the factory-designed upgrade units can be replaced, one at a time, as the remaining studs keep the clamping force applied. To summarize, ARP head studs are stronger and result in a pricier job that helps horsepower future-proof your M156 engine.If you stick to a stock configuration, the one-at-a-time method of replacing the factory-upgrade bolts could be a cheaper solution. While complete ARP sets can set owners back north of $1,000, the Mercedes-upgraded studs can be found for less than half of this. The ARP fix is more laborious too, which no doubt leads to many owners swaying for the upgraded factory solution.Mercedes-Benz Some owners prefer the confidence that comes from having the cylinder head stud work performed before they start enjoying their new-to-them AMG, happily budgeting for the job as part of their purchase to ensure they start their experience without any question marks. Used shoppers considering a 2011 or earlier model can also check to determine if the engine in their specific model has had any sort of cylinder head bolt work performed, under warranty or otherwise. What To Check Before Buying Mercedes-Benz Before buying a used AMG powered by this engine, and especially if service records are unclear, the model is high in mileage, or the model was built in 2011 or before, take a few extra steps to build confidence before you buy.First, be sure to cold-start the engine, ensuring the vehicle isn't pre-warmed or driven for some hours before your arrival. Check the tailpipes and engine compartment, which should be cold. Start the engine from dead cold with the hood open and listen for rhythmic ticking or clatter, which could indicate valvetrain trouble. A coolant pressure test can be used to detect head gasket leaks, and should prove relatively inexpensive in exchange for the peace of mind it can provide.Note that some high-mileage testimonials reveal that the M156 engine can do 120,000 miles or more on the original, pre-redesign head bolts without issue. Still, regular inspection of camshaft wear can go a long way to preventing disaster. Some owners choose to drive the vehicle as is, wait for the coolant warning light as the telltale sign of head stud stretch, and deal with it if and when it arises. Others, and especially those looking for more juice or a high-mileage experience from this all-motor V8 legend, will instead budget to upgrade the head bolts and change lifters and fuel injectors as 'while you're in there' items to help address potential pain-points before they flare up.
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