Should the timing cover or cylinder head be installed first on a Mercedes S350?

[NoraLynn]

For the Mercedes S350, the cylinder head should be installed first. Automobile Cylinder Head: The cylinder head is mounted on top of the cylinder block, sealing the cylinder from the top and forming the combustion chamber. It is frequently exposed to high-temperature and high-pressure gases, thus enduring significant thermal and mechanical stress. In water-cooled engines, the cylinder head contains a cooling water jacket, with cooling water holes on its lower surface aligning with those on the cylinder block. Circulating water is used to cool high-temperature areas such as the combustion chamber. Precautions for installing the cylinder head: When installing the cylinder head, first inspect its quality and integrity. All holes on the cylinder head must align with those on the cylinder block. Tighten the cylinder head bolts strictly according to the manual's specifications. The bolts should be tightened in a sequence from the center outward symmetrically, in 2 to 3 stages, with the final tightening to the specified torque.

[DelMackenzie]

I've encountered this issue many times in the workshop. When repairing the Mercedes-Benz S350 engine, the sequence is indeed crucial. The standard procedure is to install the cylinder head first, followed by the timing cover. This is because installing the cylinder head requires precise alignment of the timing marks on the crankshaft and camshaft. If you install the timing cover first, it not only blocks the working space but also makes it difficult to check the chain tension. I remember when we were working on the M276 engine last time, reversing this sequence cost us an extra two hours of rework. When installing the cylinder head, always use a torque wrench to tighten the bolts in three stages with a cross pattern, and finally confirm the torque with an angle gauge. Otherwise, the gasket won't seal evenly and may leak oil. The timing cover is simpler—just apply sealant and align it with the locating pins. The key is not to forget installing the front crankshaft oil seal.

[NoraDella]

I've tinkered with older Mercedes engines before, and the assembly sequence for such precision machinery is crucial. The cylinder head must be installed before the timing cover. Firstly, the cylinder head itself serves as the mounting base for the timing chain—without properly securing the cylinder head, the timing system can't be precisely aligned. Secondly, the timing cover is just a plastic or metal casing that needs to fit perfectly into the grooves on the edges of the cylinder head during installation. Reversing the order can cause misalignment at the mating surfaces. Additionally, after installing the cylinder head, you must test the cylinder's sealing performance. At this stage, not having the timing cover installed actually makes it easier to check for air leaks. Always use OEM sealant—I once tried cheap sealant strips, and they cracked and started leaking oil within three months.

[DiBlakely]

When I first entered the industry, my mentor emphasized the sequence issue. For Mercedes-Benz V6 engines like this, the cylinder head must be installed first—it's a fundamental procedure. After installing the crankshaft in the cylinder block, the cylinder head must be mounted to determine the camshaft phase, followed by adjusting the timing chain, and finally securing the timing cover. Doing it the other way around would cause the timing cover to block the cylinder head bolt holes. During installation, pay attention to cleaning the mating surfaces, as any oil residue can affect sealing. The cylinder head bolts should be tightened in three stages following the star-shaped sequence specified in the repair manual, with different torque values for each stage, and the final stage requires an additional 90-degree turn.