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Glossary of Terms

Angle Milling
A machining process where the cylinder head deck surface is cut on an angle. This leaves the original thickness on the intake side but .090” or more cut from the exhaust side. It is used to gain compression without cutting into the intake valve seat. Once the deck has been angle milled, the intake surface must be cut back to the original angle to get the intake gasket to seal properly. A thicker gasket is often required to make up for the amount of material removed. Head bolt holes and bolt spot faces on the head need to be corrected also.
CID - Cubic Inch Displacement
The common aftermarket measurement that expresses the nominal size (displacement) of performance engines for new cars, trucks, etc. The factory automotive industry, however, now uses S.I. (International System of Units) for this purpose.
CFM - Cubic feet per minute
A volumetric flow-rate corrected to a set of "standardized" conditions of pressure, temperature, and relative humidity. It is one of the most common ways to gauge the performance of cylinder heads, and is conducted using a flow bench. However, a slight difference in cfm does not always equal increased horsepower.
Combustion Chamber
Where the air/fuel mixture is burned in and engine. It is recessed in the cylinder head and contains one or more intake and exhaust valves. Its shape has a major effect on power, efficiency and emissions. Its objective is to completely burn all of the mixture while avoiding excessive temperatures. Intake valves/ports are usually placed to give the mixture a "swirl" above the rising pistons, thereby improving mix and combustion. The shape of the piston top also affects "swirl." Finally, spark plugs must be situated in a position where the flame front can reach all parts of the chamber at a desired point, usually around 15 degrees after Top Dead Center.
Cylinder Head Porting
The process of modifying the intake and exhaust ports to improve the engine’s air/fuel flow. This provides the finely detailed attention needed to bring the engine to the optimal level of efficiency and power output. When a modification is decided upon after careful flow testing, the original port wall can be carefully reshaped by hand with die grinders or with advanced CNC milling machines. For more serious modifications, ports must be welded up or similarly built up to add material where none existed.
Dead Center
The position of a piston in which it is farthest from (Top Dead Center, or TDC), or nearest to (Bottom Dead Center, or BDC), the crankshaft. Top dead center is where engine timing measurements are made. For example, ignition system timing is normally specified as degrees Before Top Dead Center (BTDC) although a few small and fast-burning engines require a spark just After Top Dead Center (ATDC), such as engines with a hemispherical combustion chamber.
Installed Height
The difference between where the valve spring sits on the cylinder head and the bottom side of the spring retainer. It is critical that this measurement is correct, because the force exerted by a spring is directly related to how much it is compressed. Installed height can be adjusted by removing material from the valve seat and/or valve face during reconditioning, or by shimming.
Permanent Mold Casting
A shaping process where molten metal is put into a permanent (reusable) mold, under gravity or low pressure, and held until solidification. Molds are typically coated with a firm wash and lampblack to reduce the chilling effect and enable the removal of the casting. Here, metal is used as the mold material instead of sand. Patented RHS® Clean Cast Technology™ uses a modified variation of this process.
Valve Guide
A tube-shaped piece of metal pressed into the cylinder head, with the valve reciprocating inside. It also serves to conduct heat from combustion, out from the exhaust valve and into the cylinder head where it may be taken up by the cooling system. A balance between stiffness and wear on the valve is essential to achieve a useful service life.
Valve Seat
The surface where a valve sits during the engine cycle. It is a critical component, because if improperly positioned or formed, valve leakage will occur. This can adversely affect the engine compression ratio, efficiency and life. The seats are often formed by press-fitting a cylindrical piece of hardened metal alloy into a cast depression in a cylinder head above each valve stem, and then machining several angles to form the valve seat into a shape that matches the valve.