Harley Davidson Spark Plug Cross Reference

Reading your spark plug

The image of the spark plugs above shows four common conditions that you’ll find your spark plugs in. From left to right the plugs are Normal, Rich, Lean and Very Lean. Full descriptions below.

Before you throw your old spark plug in the bin, check them to get an understanding of how well your engine is running.

Normal condition

If the plug is brown or light grey your can assume your engine is in good condition and the spark plug is functioning well. Even when a plug is in good condition small deposits will accumulate. This is normal.

Rich

There are many different causes of spark plug fouling. If the plug has oil on it, then it’s oiling up and may not work efficiently, potentially causing a misfire or stuttering under hard acceleration. If it’s just dark or black the bike could be running too rich. A heavy accumulation of carbon on the nose can cause a leakage path to earth. This can cause misfires and poor engine starting.

Causes: The causes of this can be anything from, a rich fuel mixture, too much choke, long periods of low-speed riding or idling in traffic, a blocked air filter or the plug’s heat range is too cold.

Lean

If the engine is running lean then the spark plug has a grey colour but you’ll probably see some black deposits on there too.

Causes: Insufficient cooling, blocked injectors, too lean a fuel mixture

Very Lean

If the engine is running dangerously lean the whole plug tip will be grey or white. If the temperature is over 850-degrees, pre-ignition may occur. Engine power will be reduced and you risk piston damage.

Causes: Insufficient cooling, blocked injectors, too lean a fuel mixture, ignition timing too far advanced, excessive combustion chamber deposits.

Motorcycle Spark Plug Fitting Guide + Tips

• When you’ve removed your spark plugs, check the condition of the threads in the cylinder head and put a cloth over the cylinder head if you’re going to leave it exposed.
  
  
• Install each spark plug by hand until you have wound the thread a couple of turns. This reduces the chances of you cross-threading the plug.
  
  
• Torque the spark plug up to your manufacturer’s recommended settings. If you don’t have a torque wrench, then get the plug hand tight and then add a ½ turn for a plug with a gasket, and more like a ¼ turn for a tapered spark plug. Over-tightening can snap the spark plug – a nightmare.
  
  
• Don’t put any lubricant or thread lock on the spark plug. Inspect your spark plug caps and replace if required.

Iridium Spark Plug vs Standard

A regular spark plug uses a copper central electrode whereas an iridium spark plug uses, you guessed it, iridium.

The metal in a spark plug has a single purpose: to channel the electrical energy through the spark plug. Iridium is far harder than copper and this allows the spark plug manufacturers to create a plug with a sharper ‘tip’ which better focuses the spark around the centre electrode without compromising its service life.

A typical copper spark plug will last around 20,000 miles and most get changed at a major service (usually around 16,000 miles) but an iridium spark plug will be good for around 50,000 miles and you do see people running them to 100,000 miles with no issues.

Copper is generally seen as being the best metal to use in terms of conductivity but iridium plugs are seen as premium as they last longer than standard plugs and can be built with smaller tolerances. Iridium spark plugs also have lower voltage requirements, meaning they perform better when starting and idling and some manufacturers also claim they offer a better throttle response.

Iridium plugs are more expensive than copper but seeing as they last longer and offer a more consistent performance throughout their lifetime, many bikes opt for an iridium ‘upgrade’.