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Machine Shop Basics - The Cylinder Head

By Ray T. Bohacz
Cylinder Heads
Cylinder Heads
The Sunnen VGS-20 is representative... 
   
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Cylinder Heads
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.
Cylinder Heads
Most cast iron production... 
   
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Cylinder Heads
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.
Cylinder Heads
Many cast iron cylinder heads... 
   
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Cylinder Heads
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.
Cylinder Heads
After the proper size hole... 
   
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Cylinder Heads
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.
Cylinder Heads
Porting the cylinder head... 
   
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Cylinder Heads
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.
Cylinder Heads
The head shop is used to cut... 
   
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Cylinder Heads
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.
Cylinder Heads Pressure Checking Center
The Sunnen SPT-3014 is a cylinder... 
   
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Cylinder Heads Pressure Checking Center
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.
Cylinder Heads
Even though it's common for... 
   
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Cylinder Heads
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.
Cylinder Heads
A good shop will always confirm... 
   
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Cylinder Heads
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.
Cylinder Heads
The exact combustion chamber... 
   
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Cylinder Heads
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.
Cylinder Heads
The same machine that is used... 
   
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Cylinder Heads
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.

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