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Spark Plug Terms (S)
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Seat

Spark Plug Seat

 

A spark plugs seat creates a seal to the combustion chamber.


There are two seat types available: flat and tapered.

WARNING: Spark plugs with different seat types are NOT interchangeable.

  

   *Spark plugs with a flat seat must be used in engines designed for a gasket seal. 

  

   *Spark plugs with a tapered seat must be used in engines designed for a tapered seal.

Flat Seat

 

Plugs with a flat seat use a crushable gasket to create a seal between the plug and combustion chamber.

 

All spark plugs with a flat seat should include a gasket on the plug, or in the box. Replacement gaskets can be purchased, and should be replaced every time a plug is re-installed.

Gasket

 

As the plug is installed, the gasket starts to meet with the flat face of the cylinder head. Once a plug is hand tightened, torquing (to the proper specifications for your engine) as the final step in the installation process will help ensure the gasket is crushed and has created a tight seal between the plug and combustion chamber.

 

Plug manufacturers recommend the use of a new gasket any time a plug is re-installed after inspection.

   Gasket crushing in chamberWasher Crushing in Chamber
Tapered Seat

 

 

Plugs with a tapered seat use the plugs outer shell to create a seal between the plug and combustion chamber.

Tapered Seat in Chamber

Semi-Surface Discharge

In a semi-surface discharge design, the voltage path skims across the surface of the insulator.  When the spark discharges, it burns off any carbon build-up.  The wide gap improves ignition capability and is less sensitive to gap growth.  Additionally the concave cut in the ground electrode promotes even gap growth.

 

 

 

  

 

 

 

 

Shadowing

Just as putting a board in front of a flashlight will block the light from everything behind that board, so the ground electrode can block a portion of the air/fuel mixture from exposure to the spark. Thus a variety of ground electrode designs are available, such as a cut back ground, low angle ground and surface discharge plug, all created to help reduce shadowing.

 


Surface Discharge

True surface discharge or surface gap spark plugs have no side electrode, instead utilizing the entire face of the plug shell as a ground to ignite.  Thus the gap remains constant through the plugs entire life.  They have no given heat range as the electrode design prevents the firing tip from overheating, and the insulator is flush with the metal shell to dissipate heat quickly.  Therefore, these plugs are susceptible to fouling in cold applications. 

 

Surface discharge plugs may be required in high compression applications or with high energy ignition systems. They are also used in rotary engines as they present a flush face to the combustion chamber, eliminating interference with an electrode tip and exposing the spark to the entire air/fuel mixture for improved combustion. 

 

Many variations of the surface discharge plug exist, including the semi-surface discharge, intermittent gap, supplementary gap, and surface air gap plug.  All designs create a spark along the insulator nose to remove carbon build-up.

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