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Spark Plug Terms (R)
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Reach

Thread Reach

 

A spark plugs reach is calculated by the length of the threads.  It is measured from the seat of the plug to the end of the threads; it does not include the electrodes, or an extended metal shell. 

 

When custom-selecting a spark plug, the electrode length and projection should be included in the calculation when taking into consideration the clearance in the combustion chamber.

 

Please use our vehicle lookup to find the correct OE type plug for your vehicle.  Only expert engine builders should use the reverse engineer tool, or attempt to play with a plugs dimensions.

 

Use of an improper plug in an engine will result in engine damage.  

 


Reach Calculation on flat seatReach calculation on tapered seat



Recessed

Retracted or recessed center electrodes are designed to place the spark out of the mainstream air/fuel flow. This makes it difficult to initiate a good flame front, but is necessary when valve or piston clearance is insufficient for conventional plugs, or when boost pressures and/or fuel type can cause excessive combustion chamber temperatures.

 

 

Resistor

 

 

At the moment the spark jumps the gap it causes a high frequency burst of energy, known as RFI (radio frequency interference). RFI, as its name suggests, creates static on your radio and interference with other electronic equipment, including the vehicle’s on-board electronic control units (ECUs). 

 

Resistor plugs were developed in the 1960s to suppress some of the spark energy, thus lowering RFI to an acceptable level.  Most resistor spark plugs use a monolithic resistor, generally made of graphite and
glass materials, to filter the electrical voltage as it passes through the center electrode.

 

Since resistor type plugs actually “resist” some of the spark energy, non-resistor type plugs actually deliver a more powerful spark.  It is for this reason that most racing plugs are non-resistor types.  However, in most automotive applications, a resistor plug is required for proper vehicle operation.  Use of non-resistor plugs in
vehicles that call for a resistor type can result in rough idling, high-rpm misfire, and abnormal combustion.

 

 

 

 

 

 

 

 

RFI

At the moment the spark jumps the gap it causes a high frequency burst of energy, known as RFI (radio frequency interference). This is why resistor spark plugs were introduced in the mid 1960's. Placing a resistor within the spark plug suppresses the RFI. Without resistor plugs in your car you can experience static on your radio as well as interference with other sensitive electronic equipment. Some later model vehicles as well as newer power sport engines must use resistor plugs for a proper "talkback" to the electronic ignition.

 

 

 


Rolled Threads

The process in which threads are cast as part of the original steel shell as opposed to a cut thread, which is machined after the shell is cast.

 

The best process depends on the application. For spark plugs, rolled threads are the preferred manufacturing method.  Since the threads are cast and hardened along with the steel shell, the grain structure obtained in the hardening process is not interrupted, creating a higher structural integrity. Rolled threads are often smoother and believed to have a lower likelihood of seizing.

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