Introduction to spark plugs for classic motorbikes
The humble spark plug is obviously quite an essential part of any Matchless engine and is the one component that usually even the most non-mechanically minded will happily fiddle with. Spark plugs have changed somewhat in the decades since these old bikes were built and the brands and models listed in the original owners manual may no longer be readily available. It can therefore be tricky to know what type to install.
The problem was even worse when I was living back in Hyderabad, India as the models of spark plugs available was very limited. Also those that were available often seemed to have strange codes that didn’t fit with any of the numbering systems I was used to from back in the UK. If you needed a plug for your Honda motorbike, you just go to the Honda dealer and ask for a plug for your model and they will give you the appropriate one. Try doing this for a Matchless though and you’re only met with blank stares!
The accepted wisdom in the Hyderabad classic bike community was that the plugs made for Royal Enfield Bullets (usually something like a Bosch W145Z1 or W175Z1) could be used for many old British bikes. This seemed reasonable enough to me at first; they’re all 350cc singles after all! But then I realised that the Bullet plugs only had 1/2″ of thread whereas my Matchless requires a longer 3/4″ reach in order to protrude correctly into the combustion chamber.
And this is how my search for a more appropriate spark plug for my Matchless G3Ls began. But in order to find an alternative, I first needed to decode the various manufacturers spark plug numbering systems in order to match up the specifications with something resembling the original KLG FE80. I’ll come back to the matter of finding an appropriate spark plug in India at the end of this guide, but first I want to go through the various modern-day plugs that could be used in my Matchless where available.
This spark plug guide contains the following sections:
- Introduction to spark plugs for classic motorbikes
- What does the owners manual say?
- NGK equivalent spark plugs
- Bosch equivalent spark plugs
- Champion equivalent spark plugs
- Summary of alternative spark plugs
- NGK spark plug code reference chart
- Bosch spark plug code reference chart
- Champion spark plug reference chart
- Spark plugs available in India
- Spark plugs available in Hong Kong
- Conclusions and your comments
What does the owners manual say?
The original fitment spark plug type specified in the owners manual for my 1951 Matchless G3Ls is a KLG FE80 – “KLG” is the make and “FE80″ the model.
This plug has a 14mm (1.25mm pitch) thread and a 19mm (3/4″) reach. The spark gap is specified as being between 0.015″ and 0.018” which I calculate to be between approximately 0.38 to 0.46mm.
You can read more about KLG spark plugs, understanding their reference codes and determining modern NGK and Champion equivalents on the ‘KLG spark plug equivalents’ page.
NGK equivalent spark plugs
The closest equivalents in the NGK brand of plugs are the B7ES and B8ES models. These are the plugs that seem to be recommended by most of the guys over at the Matchless owners club forum.
The NGK code “B7ES” means the plug has the following specification:
– B = 14mm plug thread
– 7 = the temperature range of the plug (see my notes Spark plug heat ranges guide for more info)
– E = 19mm (3/4″) plug thread reach
– S = Standard super copper core electrode
A B8ES plug is the same except that its heat range is one step cooler. Also, see the NGK reference chart further down this page for more information on what other NGK spark plug codes mean.
There are also various ‘performance’ models of the above plugs available which use different materials for the centre electrode. By using metals such as platinum or iridium (rather than the standard copper), the central electrode can be finer so that the voltage doesn’t need to be quite so high before the spark can form. The other advantage is that they should theoretically last longer than a standard plug as the electrode metals are less reactive and hence less prone to wear.
The downside though is their cost as the fancier metals cost a fair bit more, plus you pay the extra for buying a ‘premium’ product. It seems debatable whether these performance plugs are worth the extra cost, and this probably depends to a certain extent upon your bike. If you’re ignition system isn’t performing as well as it once did and not producing quite as many volts, then the lower voltage required for a spark to leap from a finer electrode might be a big advantage. Some owners have reported easier starting or smoother running, but there is unlikely to be any significant performance advantage for these simple old engines.
These higher-performance NGK spark plugs are denoted by the last letter of the model code. The standard copper electrode plug codes end in “S” (for Super Copper), but for the more expensive electrode materials this is changed to one of the following letters:
– VX = for fine wire platinum (e.g. B7EVX)
– IX = for fine wire iridium (e.g. B7EIX)
– G = for fine wire nickel alloy (e.g. B7EG)
– GV = for gold-palladium (e.g. B7EGV)
It is important to note that you definitely DO NOT want a plug that has a “R” in the code (e.g. BR7ES or BR8ES) if you have magneto ignition as this signifies an inbuilt resistor to suppress interference. Magneto ignitions aren’t as powerful as modern electronic ignitions and the extra resistance often results in a weaker (or no) spark. Some of the iridium plugs apparently now only come with built-in suppressors, so make sure you check carefully before buying.
Also be wary of plugs that have a “P” as the second letter (e.g. BP7ES or BP8ES) as this denotes a projected electrode which protrudes further than standard into the combustion chamber and might therefore make contact with the piston or valves. (If you do fit a projected electrode plug, be sure to turn the engine over slowly a few times by hand and then check the plug to ensure nothing has hit it before starting the engine!).
Bosch equivalent spark plugs
The equivalent of a KLG FE80 spark plug in the modern Bosch range would be either a W5C or a W4C. These are the standard plug codes, but you might also find models with an addition “C” (for copper electrode) or “P” (for platinum electrode) at the end (i.e. W5CC or W5CP) which are also equivalents.
It is also possible to buy versions of these plugs with in-built interference suppressors and, as for NGK plugs, these should generally be avoided where possible for machines with magneto ignition. Bosch also use an “R” in the part code to denote these resistor spark plugs (e.g. a WR6CC or a WR4CP).
See the Bosch reference chart on page 3 of this guide for more information on what the various other codes used on Bosch spark plugs mean.
Champion equivalent spark plugs
In the Champion range of spark plugs, the equivalent of a KLG FE80 would be either an N4 or N5 model. Like the Bosch plugs, these may also come with an addition “C” at the end of the code (i.e. N4C or N5C) which denotes that they have the “copper plus design”.
See the Champion reference chart further down this page for more information on what the various other Champion spark plug codes mean.
Summary of alternative spark plugs
Ok, this is getting a bit confusing now so I have summarised the various alternative spark plug types in the table below. I have used the NGK plugs as the basis for this table as I find the NGK numbering system the easiest to decode and compare. The standard NGK plug for my Matchless would be either a B7ES or B8ES, but I have also included one heat range above (B6ES) and below (B9ES) these norms. The equivalent Bosch and Champion codes are listed alongside, along with the NGK iridium electrode plug codes.The final column in the above table indicates where the different plugs lie in the heat range (although the NGK range actually goes from 2 (hottest) to 12 (coldest) so all of these plugs are pretty much in the middle). This is probably the most misunderstood thing about spark plugs!
The temperature (or heat) rating of a spark plug has absolutely nothing to do with how hot the engine will run; it actually determines how much heat from the combustion chamber is dissipated through the plug and hence what the spark plug tip temperature will be. Getting this right is important as too cold a plug will soon foul up with deposits from the combustion process, but too hot a plug may lead to pre-ignition and overheating.
Note that the NGK iridium spark plugs (e.g. the BR7EIX or BR8EIX models) are only available with built-in resistors, hence the ‘R’ as the second letter of the spark plug code. However, they are unlike other resistor plugs in that they actually require a low voltage to create a spark thanks to their fine wire electrode. This makes them ideal even for classic vehicles that don’t normally run well on standard resistor type plugs.
You can also download the latest 2011 version of the NGK motorbike applications brochure (and various other NGK literature) from the spark plug section of the Reference documents section of this website here. Unfortunately it doesn’t list Matchless as a manufacturer (AJS is listed, but only for the new Chinese learner-legal bikes), but what it does give is lots of extra information about the latest plug types available.
NGK spark plug reference chart
This is the reference chart for decoding the standard NGK spark plug designations such as B7ES and BP8EGV into something more understandable. This makes it easy to check the differences or possibly compatibility between different spark plugs from the NGK range.For full details of the NGK spark plug range you can download the 2011 NGK motorcycle applications catalogue from the Free downloads section of this website (note to self – is the 2012 catalogue availble yet I wonder?)
Bosch spark plug code reference chart
The reference table below should enable you to decode the numbering system on Bosch spark plugs. Remember that the Bosch heat range numbering system is the other way around to the NGK one on the previous page with higher numbers relating to hotter plugs.I came across some Bosch (well actually they’re re-branded Mico) spark plugs in India which didn’t seem to fit in with the above part number system – they have codes such as W145Z1 or W175Z1. From what I can gather, this seems to be an old numbering system which is no longer used, except in India of course that is! I couldn’t find much information on this except for a Bosch reference table document from 2001 entitled “Übersetzung Alt-Neu für Bosch Zündkerzen“, or translated into English “Translate old-new for Bosch spark plug“. You might find this useful too.
Champion spark plug reference chart
The reference table below is for the codes on Champion spark plugs, probably the simplest of all the manufacturers. No one else has any two letter part number such as Champions N3, N4 or N5 as far as I know!Sorry that the above table isn’t the clearest, but hopefully you can still just about make out the important details.
Spark plugs available in India?
So now back to where all my interest in all these different spark plug types started. What is the most appropriate spark plug available in India for my Matchless G3LS?
My starting point was the Bosch W145Z1 plug that my motorbike mechanic had fitted on the basis of this being the plug used in Royal Enfield Bullet 350s. But the thread length of this plug was only 1/2″ when it needed to be 3/4″. Even after only a few hundred miles riding I found that the lower 1/4″ of thread in the spark plug hole was already starting to get heavily fouled up with carbon deposits. So this plug definitely had to go!
The difficulty I found in trying to buy spark plugs for my Matchless in India is that people tend to buy (and sell) parts by their application, rather than by their model number. Parts dealers specialise in a particular make or model of bike, so if you want a plug for your Honda CBF, you go to the Honda specialist and ask for a plug for a CBF. You would never go and ask for a particular spark plug model as most people (owners and dealers alike) would have no idea what this was or meant! But what do you do when you want a plug for a Matchless and there are no Matchless parts dealers? Well of course you go to the next best thing – a Royal Enfield Bullet specialist! And therein lay my problems…
None of my local motorbike part dealers in Hyderabad stocked either of the normal NGK B7ES or B8ES plugs, which I later found out by talking to the distributor is because these are not sold in India. There are no applications (hence demand) for them apparently. The closest available plugs from the NGK range were resistor plugs with projected noses which weren’t suitable. Similarly searching for suitable Champion spark plugs proved to be equally in vain.
So then I turned to Bosch which are the most commonly available plug. Ideally I was looking for something like a W4C or W5C but these also didn’t seem to exist. So then it was just a case of going to lots of different parts dealers specialising in all different types of motorbikes, cars and auto-rickshaws and asking what plugs they had in stock.
This approach turned up a few possibilities and after much to’ing and fro’ing, and studying of spark plug catalogues and conversion guides, I narrowed it down to what I think is the most appropriate spark plug for a Matchless 350cc motorbike that is currently available in India.
This is a (drum roll please)…. Bosch W8DC.
Note that in India, Bosch is actually re-branded as “Mico” for some reason so I probably should have said a Mico W8DC. The specifications of this plug are as follows:
– W is the thread size (14×1.25 – correct!)
– 8 is the heat range (let’s come back to that one)
– D is the thread length (3/4″ – also correct!).
– C means a copper electrode (great!)
So in terms of physical size and shape, this Mico plug is the same as the original KLG FE80 or a NGK B7ES. But what about the heat range? Well the Bosch and NGK heat range numbering systems are the opposite way around so a Bosch “8” is equivalent to a NGK “5”. Hmm, a bit warm perhaps!
The other complication is the “D” part of the code that relates to thread length. There are actually two Bosch codes that relate to 3/4″ plugs – “C” and “D”. The difference is that the “C” type has the standard nose projection of 1mm, where as the “D” type has a longer nose projection of 3mm. A Bosch W8DC spark plug is therefore equivalent to a NGK BP5ES.
So my selected plug might be the correct physical size, but it’s not perfect. The hotter heat range I can probably live with as I’m not running the engine hard at the moment whilst the new piston and bore wear in. So a hotter plug may actually be of benefit in reducing fouling, although it would be important to keep an eye out for overheating which could really damage the engine.
But what about the projected nose? Would it fit, or would it be hit by the piston or valves? Well there was only one way to find out, so I installed my shiny new W8DC and then gently turned the engine over by hand. All seemed fine with no untoward bangs, and inspection of the plug showed that the spark gap was still set the same so it couldn’t have been hit. I then reinstalled the plug, started the engine and took it for a short ride – and again all was well. Phew!
One of these plugs has now been in my Matchless G3Ls for a good few months and I can report that there are no clearance issues at all (still best to carefully check for your bike first though in case clearances are different!). I can also confirm that it runs much better than it did on the short Bullet plug which seems to be the commonly accepted ‘standard’ fitment in Hyderabad.
The plug is not ideal and now that I have moved to Hong Kong I will be trying to buy a proper NGK B7ES plug (or maybe even try one of those fancy iridium electrode ones) as soon as the bike arrives in our shipping container. But the point of this investigation into all different possible spark plugs was to allow me to run the bike for the last 6 months or so I was in India, and the Bosch W8DC did just that!
Spark plugs available in Hong Kong?
Well, I’m pleased to say that there is a much better range of spark plugs available here in Hong Kong than there was in India. Best of all, the standard fitment ‘NGK B7ES’ plugs are easily available as well as models with slightly higher and lower heat ranges should they be needed. Champion plugs also seem readily available, but I’ve not looked into which specific models as I prefer to stick to what I know best in the NGK range.
So that’s about it I think with regards to Matchless spark plugs. I hope that this guide has been useful for you! If you’re interested, maybe you’d like to read my post regarding spark plug heat range. Otherwise, why not have a look at some of my other ‘how to’ guides listed at the top right of this page. Bye for now!