Champion UJ4J

[elgin2]

I was looking for a pair of J4J’s in my spark plug assortment. I found one and then can across a UJ4J. They look identical. Does anybody know what the difference may be?

[dave-bernard]

UJ4J is a resister plug.

[Mumbles]

The old ‘U’ plugs had a small gap inside of them, instead of a solid core. This was supposed to help keep the plugs from fouling by increasing the voltage going to the plug.

[dan-in-tn]

It was an auxiliary gap spark plug. As Mumbles said it had a gap built inside that required the KV to go up to jump the gap resulting in more voltage at the spark plug tip. Think of it as holding the plug lead away from the spark plug to increase the intensity. Somewhat of a bandaid when ignition systems were weak?

Dan in TN

[frankr]

RJ4J would be the resistor version. Not the same thing.

[Pete]

Champion has a lot of lead letter designations for their plugs.

R – contains a ceramic resistor in series with the center electrode to suppress ignition noise which prevents (reduces) interference in radio, electronics, etc.

U – contains an internal auxiliary gap (also called a booster gap) in series with the center electrode to help reduce fouling by creating increased voltage to the exposed gap at the tip. They are not the same as a Resistor plug.

Scroll down to Champion for codes:

https://www.sparkplugs.co.uk/spark-plug-symbol-codes/

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