One of the most commonly serviced parts of an engine is the cylinder head. It is often removed either for modifications (such as porting), replaced with a different design, or to facilitate a repair (as when a head gasket fails). Regardless of the work being performed, the cylinder head requires special service to guarantee not only that the deck surface is true, but also proper valve action and sealing.
The cylinder head can be brought to the machine shop either assembled or disassembled. If you decide to disassemble the cylinder head yourself; keep the valves, locks, springs, and retainers together for each port. Do not throw everything into a box, but instead number each group of components with its location. For example, #1 intake, #7 exhaust. This will allow the machinist to diagnose any wear problems and perform accurate corrective procedures. Remember, when it comes to engines, every part tells a story.
Once the machine shop receives the cylinder head, they will do a visual inspection to look for damage, such as cracks and valve recession. When recession occurs the valve will actually pull through the cylinder head at the seat. This is shown on the surface the valve closes against. Next, the head will be taken apart and cleaned. Cast iron heads can be Magnafluxed using the same procedures that were established in Part 1 of this series. Aluminum cylinder heads need to be pressure checked using an application-specific machine to confirm no cracks exist.
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The Sunnen VGS-20 is representative of a state-of-the-art cylinder head machine, more commonly known as a "head shop". This equipment is used to do valve seat and guide work. The valves themselves are resurfaced on a valve grinding machine which is a separate piece of equipment. |
Most cast iron production cylinder heads came with no valve seats, but instead had the necessary angles cut into the cast iron material, forming an integral seat. With the advent of unleaded gasoline, hardened seats needed to be installed. This procedure entails locating the center of the bowl through the valve guide, then using the head shop to counter bore the area where the integral seat originally was. The new seat is then chilled in dry ice (-112 degrees F) and quickly driven into the counter bore. As a precaution, the seat is staked to prevent it from falling out. |
Many cast iron cylinder heads came from the factory without any valve guide inserts. Instead they had a boss with substantial mass cast into the design and drilled for the valve to reciprocate through. If the guide is worn, a few different procedures can be taken to restore the proper clearance between the valve stem and the wall of the guide. The best approach is to install an insert into the boss. The head shop machine is used to drill the old guide diameter out, as seen in this photo, preparing it for an insert. |
After the proper size hole is bored into the guide, a steel or bronze insert is then hammered in and cut to length. If inserts are not going to be used, then the guide can reamed oversize, and a valve with an oversize stem installed. Valves are usually offered for most engines with 0.003-, 0.005-, 0.015-, and 0.030-inch oversized stems. The head shop will also be used to perform any necessary machine work for the installation of valve stem seals. There are a number of different types of seals. Some rest over the guide while others attach to the valve stem itself. The most effective are the "positive insert" style. These are held securely to the valve guide with a snap ring and contacts around the valve stem. |
Porting the cylinder head returns the most performance for the dollar spent, especially if the camshaft and intake manifold are also changed. The three areas where a cylinder head can be ported are: the bowl (which is the area behind the valves); the runners; and the combustion chamber. The largest gain in airflow is achieved by blending the bowls, along with a good valve job. Bowl blending, or pocket porting, is used to match the final angle of the valve seat with the bowl of the cylinder head. This service is essential when a hardened seat is installed, since the cutter leaves a radius that chokes airflow. |
The head shop is used to cut the proper angles into the seat area, regardless if it's an insert or an integral design. A professional shop will use this style of machine to accurately cut the angles on the seat for proper airflow and valve sealing. A less expensive approach uses stones to grind the valve seat angles. Although it is an accepted commercial procedure, this style of valve seat work has no place on a performance engine. Always port the cylinder head before cutting the final valve seat angles. A good shop will "rough in" the valve angles as a guide for their porting work, then do a final seat cut when the porting is done. |
The Sunnen SPT-3014 is a cylinder head pressure-checking center. Not only should aluminum cylinder heads be checked, but also cast iron heads that have been excessively ported, to ensure no coolant leaks into the combustion chamber, from grinding into the water jacket. |
Even though it's common for the valve seat to have two angles ground into it, the valve usually has just one angle. Thus, the three-angle valve job. A grinder with a stream of cutting lubricant or coolant, imparts the proper shape on the valve. |
A good shop will always confirm the performance of their porting work on a flow bench, such as the industry standard Super Flow SF-600. |
The exact combustion chamber volume will need to be determined to calculate the compression ratio. This step is essential if the combustion chamber has been modified by porting, or dished valves are installed. |
The same machine that is used to deck the engine block, also machines the cylinder head. The cylinder head deck needs to be true for good head gasket sealing. This procedure is also used to modify the volume of the combustion chamber to raise the compression ratio. If the chamber is made smaller, the compression ratio of the engine increases. |