NGK Spark Plug Explanation

from DynoBike - Australia

I'm sure that many of you at some stage have been given the sales pitch on the horse power advantages of fitting brand X spark plugs. Take this as a warning: don't be fooled. My own personal experience with motorcycles would indicate that this line of approach can be very misleading.

In fact one manufacturer who we'll not name has been pushing this point very strongly and I am yet to see any proof behind this statement. I'm not saying that they are wrong, I just haven't seen any indication that they are right. One thing is certain though, we've had some problems with them. They have been the cause of many a misfire and they have a nasty habit of breaking in two when trying to remove them, resulting in many frustrating hours for us and many dollars for the customer.

Set out below is a guide to help you avoid all these problems and select the correct plug for your bike. Sometimes you can use alternative plugs, but you need to have an understanding of what the number on the plug means to ensure you're not stepping outside the limits of your engine's requirements. So the 1st rule is always go with the manufacturer's recommendations. The second rule is if you have any doubt give us a bloody call and save yourselves a lot of agro.

We recommend and use NGK plugs so the following explanations are applicable to NGK only.

The 1st letter prefix refers to the thread type and socket hexagon size. They range from 14mm thread down to 8mm on the smaller 4 cylinder 250 imports.

The 2nd and 3rd letter prefix is the construction feature. eg P for projected insulator nose, R for resistor type. The first number refers to the heat range. This is probably the most important code because it refers to the operating heat range and means that if you get it wrong serious damage to the engine is eminent. With NGK plugs the higher the number the colder the plug. Racing engines use cold plugs because they generate so much heat. If you have a fowling problem, common on two strokes, using a hotter plug can sometimes cure it. But if you're off to P.I. for a ride day you need to put that colder plug back in it. Also note that on other brands the heat range works the opposite, 5 is colder than 7 not hotter eg, Champion plugs.

The next suffix letter is for thread reach, which is also where you can come unstuck because if you choose a plug with a thread reach that's too long then it puts an interesting impression in the top of the piston, if it's too short then it won't reach inside the combustion chamber.

The next letter is again for construction features, eg P for platinum ground electrode, GV for racing plugs made of nickel alloy and centre electrode made of precious metals, S for copper core electrode etc, etc. Sometimes there is another number at the end meaning the plug gap which it is set at.

The following is a common motorcycle plug number DPR9EA-9
Explanation, 12mm thread 18mm plug socket, P for projected electrode, R for resistor, heat range 9, E for 19mm thread reach, A for specials and 9 for .9mm plug gap.

And there you have it, clear as mud eh. Don't worry if you're confused;  as I said earlier the manufacturer has done all the hard work, so if you stick to his recommendations you can't go wrong. If it's for a seriously modified engine then your engine builder should know what you need to use.