Spark Plugs and Motorcycles

New plug in the middle

Spark plugs have changed so much over the years. When I was young we tried changing spark plugs on US cars about every 10,000 miles. Back in those days it was easy to tell when the car was in need of new spark plugs and it was certainly easy to see worn spark plugs upon inspection. The typical signs that the plugs were bad were hard starting, miss-firing when accelerating, and bad cold weather running. Worn plugs have an increased gap. That makes it much harder for the spark to jump the gap. Increasing the compression only makes the problem worse. This increase of resistance now places additional stain on all the other parts of the ignition system including the rotor, distributor cap, and spark plug wires. Old timers will remember dealing with carbon tracks, cracks in the distributor cap, and glowing spark plug wires in the dark. In this article I'm only talking about spark plugs with an increased gap. I am not considering all the other problems you can have with plugs like broken components, oil soaked, soot, wrong heat range, loose wires, etc.

Well... those days are gone... for the most part. We don't have distributors (no cap or rotor) and many engines don't even have spark plug wires - they just place the coil on top of the spark plug. But the spark plugs are still a main part of our engines. Sometimes I think spark plugs are the forgotten step child of the engine maintenance procedures. People know about changing oil and filters but the spark plugs are left for last or totally forgotten. Many of my car driving friends never change the plugs until something happens. Recently one of my friends said "I just changed the plugs and three coils on the Ford".

I'm one of those guys that "forgot" to change the sparks plugs on my 2010 RT. I have a couple of excuses but that doesn't matter now (although I wonder why they were never changed at my dealer/mechanic provided services). The fact is that I rode my bike for 32,000 miles on the original set of plugs. I guess one advantage of dual plugs on the recent BMW boxer engines is that we can milk the plugs to the very end. The bike ran fine but look at the pictures above and below. You can see the center and side electrodes severely worn. The wire gauge I just used to measure the gap only goes up to 0.040 of an inch and it still had plenty of room left. At least 50% more gap.

Another view

I purchased the OE plugs for my bike. I'm not sure if there is a better spark plug for my bike but I figured BMW did their research and testing that's required with each of their engines. I am very disappointed on the way this spark plug is built... especially for a plug with a retail price of about $24 each. They simply welded on the side electrodes and then bent them over towards the center. That places the inside edge close to the center and most likely at the required gap. This method, in my opinion, presents several problems. First, the edge presents a sharp edge for the spark to jump to - it's the closest point to the center. Second, because it is a fine edge and all the sparks are jumping to that point it soon wears down. As it wears the side electrodes present a wider surface for the spark but by then the required gap is long gone. You can see what I am talking about in the pictures. The center is worn also but the sides are worn much more.

My idea on how they should be built

For $25 I think we should get the configuration shown above. Of course that means our spark plug companies won't sell nearly as many spark plugs as they do now. Look at some of the high end auto spark plugs and you see very well designed electrodes. The edges, if there are any, are not the closest point to the center electrode. The main objective is to be equidistant at all points.
Anyway, back to my bike. It feels like it's running better but it could be a placebo effect since I changed the spark plugs myself and feel like I did a wonderful job. In the future I'm changing the plugs as recommended by BMW or at the least every 20K miles.

Now at 38,000 miles I think I better change the alternator belt. I already have it - just need to do the work.
tsp

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