Restoration Corner
By Bruce L. Adams
 
Inspecting a Cylinder Head

We frequently hear from members panicked because they were told they might need to replace a head gasket or have the cylinder head on their engine rebuilt. While these are fairly complicated projects, they are seldom required more than a few times during the life of a Mercedes-Benz.

In fact, for cars in the classic category, an owner with reasonable mechanical skills and standard tools can determine if the work needs to be done, and can handle much of the disassembly, cleaning, and inspection before turning the head over to a skilled machine shop, saving a significant amount of money.

On the other hand, if the car was recently acquired or is showing serious problems when starting, spews excessive smoke, or runs roughly, and the owner isn’t prepared to undertake the mechanical work, this article will cover the procedures that a professional mechanic should carry out and discuss with the owner.

Technical background

The cylinder head in an automobile engine rests on the engine block above the piston/cylinders, closing in the tops of the cylinders and forming a chamber for engine combustion. In most engines, the head incorporates the passages that feed air and fuel into the cylinders, and those that allow exhaust to escape. The head is also where the valves that control intake and exhaust are located, as well as the spark plugs that create combustion. The head gasket seals the space between the cylinder head and the engine block.

Internally, the cylinder head has passages – ports – for the fuel/air mixture to travel to the inlet valves from the intake manifold and for exhaust gases to travel from the exhaust valves to the exhaust manifold. In water-cooled engines as used in Mercedes-Benz automobiles, the cylinder head also contains integral ducts and passages through which the engine’s coolant – a mixture of water and antifreeze – flows to facilitate the transfer of excess heat away from the head.

The decision to rebuild the head, the block, or overhaul the entire engine should not be undertaken without first completing a thorough engine analysis. In a previous issue of The Star (Nov./Dec. 2011, pg. 66), compression and leak-down testing of the head were discussed.

That article provides very useful information on whether the cylinder head needs to be removed from the block and serviced. Examples are discussed of results that indicate that a head rebuild and/or a block rebuild are required. By way of quick review: If compression is low in one or more cylinders (compared to specifications in the workshop manual), the problem can be isolated to the valves or rings by squirting a little 30-weight motor oil into the cylinder through the spark plug hole and repeating the compression test. The oil temporarily seals the rings. If compression readings are higher in a second test after the oil is injected, that means the rings and/or cylinders are worn and the engine needs to be overhauled. If there is no change in the compression readings when oil is squirted into the cylinder, that signals that the cylinder has a bad valve and/or valve seat.

Low compression in one cylinder usually indicates a bad exhaust valve and requires that the head be rebuilt. Low compression in two adjacent cylinders is typical of a bad head gasket, which can be replaced without rebuilding the head, while low compression in all cylinders means the rings and cylinders are worn, an indication that an engine overhaul is needed.

Inspection of the head

If it is determined that the head gasket needs to be replaced, or the head or engine needs to be rebuilt, the first task is to remove the cylinder head, a straightforward procedure that is outlined in the workshop manual.

With the cylinder head off the engine, the head can be examined for obvious cracks, combustion chamber erosion, stripped or damaged threads, and signs of corrosion. Also check the head for thickness and straightness of the mating surface between the head and the engine block.

Water damage in one cylinder is a serious issue that must be dealt with.

The factory specification for head thickness is found in the Mercedes-Benz Technical Data Manual (TDM). New 121 (190SL) heads for example are 84.8-85.0 mm thick. We use 85 mm, or the English equivalent of 3.346 inches, as a beginning standard. The total permissible stock removal is 1 mm or 0.040 inch. That means the head can be no thinner than 84 mm or 3.306 inches.

The head thickness must be enough to allow slight removal of material during the machining process, sometimes referred to as decking. That means if the head is 3.310 inches and will require 0.010 inch of material to be removed, the head would not meet specifications after machining. Make sure your machine shop understands these specifications.

Measuring the cylinder head to make sure there is room for machining, and the head is perfectly straight.

Use a straight edge and feeler gauges to check the straightness of the head. The straight edge is a precision tool that must be longer than the head and perfectly flat. With the bottom of the head cleansed of all bits of old gasket, carbon residue, oil and corrosion, place the straight edge lengthwise on the bottom of the head in four different positions – along each edge and across both diagonals in a X pattern. In each of these positions, use a feeler gauge to measure the space between the straight edge and the head. Factory specifications indicate that the gap or warp cannot exceed 0.1 mm or 0.004 inch.

All cylinder heads are warped to some degree and require straightening before the cylinder head surface is milled in order for the resulting surface to be perfectly flat. The head warps in a characteristic manner, curling at the sides and ends around the combustion chambers.

Milling the surface without first straightening ignores problems that may exist on the cam side. In extreme cases, the surface of the head will need to be milled flat. It is far better to thoroughly inspect, measure, and straighten the head before deciding on machining procedures than to start by immediately machining the head.

The general cause of warped cylinder heads is found in the cooling system. If you find that the head is warped, make sure you go back to the cooling system and inspect for problems. We recently removed a head that was significantly in need of milling. The problem in this particular Mercedes developed because the thermostat was installed backward. Obviously, this car ran at high temps for an extended period of time and the owner never noticed the reading on the temperature gauge.

Cleaning the Head

The recommended method of cleaning the head is to soak it in aluminum cleaner for several hours or days to gradually and gently remove carbon and corrosion. Even after that, the head may require blasting with glass beads, walnut shells, or a similar non-abrasive medium. A note on glass beads: They are great for cleaning, but they are hard to remove from threaded holes and hidden passages. All traces of glass beads must be removed.

Valves in place

Pressure Testing

With the head disassembled and cleaned, a pressure test is performed to ensure there are no hidden cracks within the casting. The TDM specifies that the head should hold water at 70 C at two atmospheres (30 psi at 160 F). Most shops now use an air test to evaluate the head’s integrity. Water passages are sealed with rubber blocks and the water jacket is pressurized to approximately 40 psi to test for leaks. A soapy water solution is sprayed into the combustion cavity and passageways; soap bubbles will identify any cracks. This air pressure test is probably more sensitive than the hot water test for identifying cracks. Heads with casting cracks are usually discarded.

The next step is to have a certified machine shop proceed with the cylinder head restoration, including machining the surface, replacing the valve guides and camshaft if required. It is not the scope of this article to go into those details.

It is possible to compensate for an out-of-specification head by using a stainless steel spacer with the head gasket or a thick copper head gasket. While these have been used with varying degrees of success on engines that didn’t economically justify a rebuild, they are not the preferred choice. Do not use two head gaskets together.

Porting and polishing the intake and exhaust ports improves performance

While the head is off and being rebuilt, modifications can be made that will improve engine performance. In particular, a performance modification that we often do here in our workshop is to port (grind to enlarge) and polish the intake and exhaust passages of the cylinder head, as well as the intake manifolds. By smoothing and slightly expanding the intake and exhaust flow passages, this additional work often yields 15 to 20 more horsepower.

Being told that you’ve “blown a head gasket” or that your “head needs rebuilding” isn’t necessarily a death sentence for the car or a ticket to the poor house – rather, it can be an excuse to get to know your car literally from the inside out.

Head with damage from leaking head gasket that allowed coolant to enter the combustion chamber at left, causing corrosion of the valve surfaces and damage to the piston.
Restored head from a 4-cylinder engine looking down from above, with valves and valve springs (arrows) installed. Note the smoothly machined surfaces and clean chambers.


Above: Valves (arrows) installed using valve spring tool. Below: Cylinder head intake  and exhaust passages (arrows) ported and polished for additional power.
 
 
Bruce L. Adams, a member of the Triangle Section, has owned and operated B.L. Adams, LLC, specializing in restoration of the 190SL Mercedes-Benz, for 31 years. For more information, visit www.bruceadams190SL.com