TODAY’S TECHNOLOGY
Ken Adams
Building a Better Foundation
A look into the mighty M156 AMG 6.3 Engine
As most enthusiasts with more than a passing interest in Mercedes cars are well aware, the models manufactured since 1995 that carry the AMG logo on the back are powered with variations of standard Mercedes-Benz engines, but engineered to produce higher horsepower from the same capacity engine and then hand-assembled for reliability. In 2011, Mercedes-Benz introduced the first engine developed entirely by AMG engineers, the naturally aspirated, 6.3-liter M156 V-8 AMG engine, used initially in the C63 and SL63 AMGs. A closer examination of some aspects of this engine’s design illustrates what is necessary to develop the additional torque and horsepower of AMG engines.
Components spawned from racing
The design of any engine starts with the most basic element – the engine block that encases the cylinders and provides the mounting point for the head, sump, and peripheral components. To support the higher torque, as illustrated here with the M156, a different block configuration is needed to increase structural rigidity.
It used to be that a traditional block used individual main bearing caps secured with two to six bolts each, depending on the application. A windage tray would then be attached to the main caps to control oil splash created from the crankshaft. The problem with this configuration is that the main cap webs have limited lateral support. On the M156 AMG engine, there were many design changes to resolve the problem.
The major change is a new configuration adopted from advancements in racing – listed in the Mercedes Workshop Information System as the “lower crankcase section” – informally referred to as the bedplate. A bedplate is basically the lower half of the crankcase that includes the main bearings, lower block section and windage tray, all integrated into one beefy unit. The result is an extremely rigid block able to withstand the additional power and torque.
The bedplate or "Lower Crankcase Section" and its position within the engine, between the head and the sump.
Essentially, the block is split crosswise at the crankshaft centerline. This leaves a large void below the crankshaft centerline that must be filled. The photos here give you an idea of how substantial a piece the bedplate really is. Weighing in at more than 40 pounds, the bedplate consists of five main bearing caps made of steel with a beefy casting of a special alloy that tie the main caps together and create the bottom half of the crankshaft. With the bedplate installed as shown at upper right, there is no access to the crankshaft or connecting rods without removing the engine, so everything has to be done exactly right when the engine is put together.
Looking into the interior of the bedplate, showing the lower bearings for the crankshaft
The bedplate or lower crankcase section, looking at the lower portion that's covered by the oil sump.
Sealing technology
Along with the new crankcase design comes other challenges. Because the bedplate represents the bottom 50 percent of the main bearing caps and the bottom section of the crankcase, the use of gaskets to seal the outer crankcase is no longer possible. The main bearing bores are precision-honed and fitted to the block; the use of gaskets would alter the main bearing bores. The solution? Room temperature vulcanized (RTV) sealant. Using RTV sealant as a replacement for traditional paper gaskets has been around for a while now. As far as Mercedes goes, the company began using its own special RTV with the M112 and M113 engines, used in place of paper gaskets exclusively on the front timing case cover, oil pan, rear main-seal cover plate and a few other places.
The Mercedes-brand room temperature vulcanized sealant used in place of gaskets.
More and more these days, we are also seeing the elimination of paper gaskets to seal mating surfaces within engine assemblies. The elimination of these gaskets brings a whole new set of sealing challenges. Quite frankly, when I first started to see gasket-less assemblies, I thought this was going to be a disaster. After a few years of using this technique, I am glad to be wrong.
The use of RTV sealant can be a blessing or a curse, depending on whether the mechanic or technician follows the manual. Mercedes has very specific work instructions pertaining to the cleanup of sealing surfaces, the application, and thickness of the sealant bead. One problem is that not every mechanic working on a Mercedes engine follows these instructions; sometimes, the result is disastrous. The biggest mistake I see in engines I work on at the dealership is when too much RTV sealant is used and the residual squeezes into critical lubrication galleys, causing blockage and ultimately failure of a component.
The tube of RTV sealant inserted in the Mercedes RTV applicator.
Careful attention to preparation of the sealing surfaces is critical. Today, chemical gasket removers and plastic blade scrapers have replaced abrasive disks on die grinders to prepare the sealing surfaces. In general, a 1mm-diameter bead is all that is required on most components. Even on larger sealing surfaces such as oil pans, we only use a slightly larger diameter – 2 millimeters.
In order to create a successfully sealed surface, there are very specific sealer application paths that must be followed, shown in diagram below. The sealant must never be spread with a finger; sealing surfaces must be free of any oil film. And be aware – RTV has a specific shelf life. When the expiration date has passed, throw it away. Mercedes makes a small caulking gun designed specifically to work with its RTV sealer tubes, allowing an accurate bead of sealant to be applied to the component.
The workshop diagram AH01.40-P-1000-03AMG: Information on application of sealant to crankcase bottom section.
Sealant path: The line drawn in F identifies the sealant application. The sealant must be applied only to specified surfaces with a bead of 2.0mm (±0.5mm). The sealant must be applied within 10 minutes. The silicone bead must not be spread. Use only approved M-B sealant listed in the repair products.
Bolting it all together
One-time-use stretch bolts seem to be becoming the new norm on current engines, especially the high-performance models. Because any bolt stretches when it is tightened, to assure that the components are fastened with exactly the right tension, the bolt is used only once. In the case of the M156, for example, there are 35 one-time-use bolts required to secure the bedplate to the block alone.
Stretch bolts are used in conjunction with new and more complicated torque procedures. Each torque and stretch sequence of the bedplate’s main bearing bolts requires nine steps. The first two steps are tightened to a specified torque. The next two steps are successive 90-degree rotations of each bolt. Next, the bolts are released one at a time, and the same series of steps are repeated on all 20 main bearing bolts. Then 14 aluminum 6mm bolts are used in addition to secure the perimeter of the bedplate to the crankcase below it that catches engine oil.
Looking upwards at the engine block, showing the big ends of the pistons. This shows the places where bolts will be used to fasten the lower crankshaft piece (bedplate) to the block.
Another noteworthy feature of the M156 engine assembly is the use of quick-connect, oil-cooler lines (see photo at left). These allow for easy removal and installation of the engine when needed. This design feature is critical in removing and installing an engine in the smaller engine bay of a C63 (a shoehorn is not included).
The M156 Engine being lowered into a C63 AMG. It's a tight fit.
One last note: Many Internet forums reported head bolt breakage in a few early production M156 engines. The number of engines affected by these head bolt failures, in my opinion, was far fewer than Internet blogs would have you believe, and with the use of a new type of head bolt, this problem no longer seems to be reported. I have been asked if engines with the early head bolts can be updated with the newer-version bolts under warranty. The answer is no. Head bolts can only be replaced under warranty if broken, and in any case, most of these early production engines are now out of warranty without indication of any problems, or have been repaired under warranty.
The M156 engine, assembled and with peripherals attached, is lowered into the engine compartment of a C63. Note how compact this V-8 engine is, considering its power.
This sideview of an assembled M156 engine shows the lower crankcase section, or bedplate, in place. The bedplate, in sharp focus, is outlined here in yellow and indicated with arrows.