Spark Plug Article

Reply: The easiest way is to use what the engine manufacturer put in at the factory, or what a current spark plug manufacturer recommends. Jawa manuals recommended PAL spark plugs, and an evolution of the company still exists and is still making spark plugs, but they are marketed under the BRISK name. I couldn’t find any reference to or recommendations for old Jawas on their Website. See http://www.brisk.cz/en/ona2.php.

NGK, Nippon Denso and probably other manufacturers include some Jawa models in their recommendations, but they are incomplete as to specific models and unlikely to be up to date. It is doubtful that anyone does research or testing for products like Jawas, so the situation will only get worse as time passes.

Now that these motorcycles are becoming elderly and are often missing from charts, and without a Jawa or spark plug manufacturer’s recommendation, we have to start from where we are. For me, it is kind of “I know one when I see one”, but it is less easy to put into words, and my usual referral to the Champion Spark Plug Chart turned out to be impossible because I could not find one. So let’s try and make some sense out of what is required of a spark plug, and what all those numbers and letters on a spark plug mean.

Spark plugs come in a lot of thread sizes, thread lengths, and heat ranges. Heat range determines how hot or cold the core nose runs in the engine. While not very relevant to Jawas, they also come in standard and projected core nose, platinum and other center electrode materials, assorted spark gap configurations, often with internal resistors and sometimes series gaps.

There are two main aspects to spark plug selection, size and heat range. Selecting a plug that is dimensionally correct is usually easy. To the best of my knowledge, all Jawas use 14 mm, ½ inch long thread plugs. These are generally a 13/16 external hex, but you may find a ¾ inch hex on some brands. Be sure the plug wrench you carry fits both the plugs in your engine and your spares!

More difficult is determining the correct heat range, one that runs hot enough not to wet foul at idle, yet cool enough not to cause preignition and engine damage at high power. The core nose of a plug has to stay above 1200 degrees F. at idle to avoid wet fouling, and below 1600 degrees F. at high power to avoid preignition. Not easy in an air-cooled engine with neither a cooling fan nor thermostatic control. Assuming you are starting with reasonably successful plugs, heat range determination is trial and error and unlikely to be off more than one range. If you are having frequent wet fouling, try going hotter, one heat range at a time. If you experience rapid electrode wear, gap erosion, or (heaven forbid) preignition that results in piston damage, go one range colder.

A Website that has excellent descriptive pictures of various failure modes is: http://www.mechanicmatt.com/index.php?ServicePage=SparkPlugChart

As shown, a plug that is running too cold will be black and oily. You usually know without looking because it won’t start, or stops firing while riding at low speed, but you can easily observe this by pulling the offending plug and inspecting it. A plug that is too hot will appear very white at the core nose, sometimes with blistered oil and fuel deposits, but you have to use the correct procedure to see this. To check if a plug is too hot, pick a hot day and make sure the engine is fully warm. Run at least a minute at full throttle, then simultaneously and without closing the throttle, pull in the clutch and turn off the ignition. Coast to a stop, pull the plugs and observe the color and appearance of the core nose. A light tan color is desirable. A core nose that is clean and white is probably too hot.

Spark plugs are identified by a confusing set of letters and numbers, and every manufacturer has their own identification system. Basically, these systems all identify thread size and length, heat range, other special characteristics, and sometimes the gap as set at the factory, although not necessarily in that order.

For NippoDenso: http://www.in.net/~eholman/ndplugs.html.
For NGK: http://www.ngkntk.de/pages/gb/ngk/produkte/motorrad/index.html.
For Champion: http://www.edelbrock.com/automotive/sparkplugs.html.

My 1952 Perak 350 came to me with Nippon Denso plugs marked ND-W24FS. Referring to the chart on the ND Website, I find that:

W identifies a 14 mm thread and a 13/16 hex

24 identifies the heat range, about in the middle of what is offered

F defines the thread length, ½ inch

S calls out the standard or non-projected core nose

I haven’t ridden it enough to be certain these plugs are correct, but so far they seem to be. If I run into wet fouling, I can try a set of hotter W22FS. If they appear to have too much gap erosion, a set of colder W27FS might be better. And if they exist, I might prefer W24FP. These are the same as W24FS except they are a projected core nose design. These might give me better starting, but I’ll have to watch that they don’t have piston interference at TDC. I also have NGK B8HS plugs from a later model Jawa 350 laying on my bench. Per the NGK Website:

B is 14 mm thread with a 13/16 hex

8 is the heat range, a bit towards the hot end of the scale

H calls out ½ inch thread length

S identifies “standard type” core nose

However, because each company has their own identification system, I cannot tell if these are right for the Perak. I’ll either have to try them in the motorcycle, or use a chart that cross-references the two brands. These charts used to be common, but I could not locate one for this article.

Some miscellaneous related information for those who might be interested:

Thread size is in millimeters, but the thread length and external hex is in inches. I’m not sure why, but it has been that way since at least as far back as the 1930s. Very early spark plugs used tapered pipe threads, just like water pipe. My guess: Someone in Europe made superior spark plugs, and they had metric dimension threads. US car manufacturers specified metric threads in their engines so they could use the better plugs. US spark plug manufacturers adopted just the thread size, keeping the rest in inch dimensions, and the US eventually took over the market using this unusual combination of dimensions. These thread lengths and hex sizes are so embedded in the system that it is doubtful they will ever be changed to metric.

Selecting the correct thread length is important. Too long a thread exposes the excess threads to combustion, making the plug run hotter than its design heat range. Carbon accumulated on the exposed threads can damage the threads in the cylinder head on plug removal. Too short a thread length retracts the spark into a dead pocket, making for hard starting and poor running quality.

Thread diameter and length varies by engine design and manufacturer. When flat head engines with cast iron heads were the norm in cars, 3/8-inch thread length was common. With aluminum heads, these stripped too easily and more engagement was called for. There are a few 7/16-inch thread length engines, but ½ inch is the most common. Some engines use ¾ inch length threads to get the core nose down past multiple valves, pushrods and other space consuming design characteristics.

While 14-mm threads are by far the most common, modern engine designs have been going to smaller 12 and even 10-mm spark plug threads because the cylinder heads are so crowded. Buick and a few others tried 12-mm plugs in the 1940s, but they had fouling problems due to lead deposits from the fuel accumulating in the smaller space around the core nose. Some years later Ford went to 18-mm plugs to get more space and more fouling resistance, but I believe they are back with 14-mm or smaller today.

With the reduction in plug fouling, gap growth has become the defining issue in plug life. Platinum and other precious electrode metals that are more resistant to erosion are often specified when the engine designer wants very long plug life. I doubt any Jawa rider needs 100,000-mile plug life, so premium plugs are a needless expense. It will wet foul before it ever wears out in a Jawa!

Plugs with multiple ground electrodes are claimed to provide multiple sparks and therefore better combustion. This is nonsense, but a lot of such plugs have been sold, many at exorbitant prices. Voltage always tries to jump the shortest path. Once a spark occurs at one gap, there is no appreciable voltage left to jump a second or third gap, even if they are all the virtually the same width. Multiple spark ignition can be advantageous, especially in a two-stroke where the mixture is not always homogeneous everywhere in the cylinder. If the first spark does not see combustible mixture, another spark a few milliseconds later may. This cannot be achieved by a special spark plug; it takes a complex and expensive ignition system that provides multiple ignition “restrikes” to an ordinary spark plug.

Projected core nose plugs have extended porcelain and electrodes. They place the spark deep down into the mixture making combustion more likely, rather than keeping it close to the cylinder head wall where the mixture may be poorly scavenged and chilled by the adjacent metal. However, these plugs tend to have more gap erosion than conventional plugs because they are exposed to hot, fast moving gasses. I am not aware of such for Jawas, but if I find one find I’ll us it for the (theoretically) improved starting and idling.

Resistor spark plugs have a resistor in series with the center electrode, inside the porcelain. A resistor can be added to almost any plug design to suppress “ignition noise”, a potential problem for both the vehicle’s own radio and for radios and TVs in nearby buildings. The resistor suppresses the current "ringout” that tends to exist after the initial firing, the source of the electromagnetic interference. They are used in most radio-equipped vehicles and in many other applications such as chainsaws and snowmobiles that are operated near buildings. Series gap spark plugs (with an additional gap, invisible inside the porcelain) are more resistant to cold fouling, but they take more voltage to fire and can lead to ignition coil and wire failures. They should only be used in applications where they were originally specified. Surface gap plugs are used in applications that need minimal gap growth over long periods of high power operation along with strong resistance to fouling. They do not have a conventional side electrode, but bring the core nose porcelain down almost flush to the outer shell and fire directly to the outer shell. They are essentially an extremely cold version of a standard plug, and they need an extremely fast (short voltage rise time) capacitor-discharge ignition system to fire through what are usually considered wet fouled conditions. Some engines require a series gap in addition to the special ignition system to avoid wet fouling at idle. These plugs wet foul almost instantly when installed in an engine with conventional inductive ignition, as is used by Jawa.

In the olden days of my youth, The Champion Spark Plug Company was the premier spark plug manufacturer, at least in the USA. Since most of the cars in the world were made here, Champion was probably also # 1 in the world. The company was founded by Albert Champion. He also founded the AC Spark Plug Company now part of General Motors, using his name for one and his initials for other. See also http://www.asecc.com/data/plughistory.html.

Years ago, Champion was significant as an OEM (Original Equipment Manufacturer) supplier, and the absolute king of the replacement market. There was a Champion “Recommendation, Heat range and Condition Chart” in every, car, motorcycle, small engine and marine dealer and repair garage. It was right next to the cracked plastic AM radio and the Rigid Pipe Wrench girlie calendar. Spark plug replacement was frequent, as marginal ignitions systems coped poorly with spark plug fouling. Fouling was caused by lead deposits from leaded gasoline, chokes that sometimes provided over-rich mixtures during warm-up, and engines that began consuming oil at relatively low mileage. Not to even mention two-stokes that had the additional burden of burning metallic additive oils at fairly heavy fuel-oil ratios.

As engines, ignition systems, fuels and oils improved, spark plug life went from a few thousand miles in the 1950s to almost the life of the vehicle today and Champion as a manufacturer, so near as I can tell, is gone. Champions are still available, but AC, AutoLite, Bosch, and the Jap brands predominant today, although I’m not sure if all of them are really manufacturers or just marketing “labels”.