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![sc:disclaimer]
hey there, (sorry but i dont know your name)
well thank you alot for this info. can you please tell me how i know what plug is for my Matchless? i got the 1963 G3.
First I suggest you track down a copy of the owners manual for your bike over in Christian’s Archives (look under books > owners manuals). There doesn’t seem to be one for a 1963 Matchless G3, but there is an AJS one for that year here which is the same.
This says the original plug should be a K.L.G. ‘FE80’ which is the same as for my bike described above, so you’re best bet is either an NGK ‘B7ES’ or a Champion ‘N4C’.
I have a yumbo moped. The original spark plug is a mico-bosch w175z1. Please please please tell me the equivalent spark plug that I can obtain.
Sorry, I don’t know the answer to your question as my knowledge is really only regarding the spark plugs needed for my old Matchless motorcycle. However I did find an old Bosch spark plug catalogue (sadly in German though!) which lists these strange Mico-Bosch part numbers as you described for your moped.
I’ve made it available for download here:
Bosch old spark plug code chart
I have a feeling that the Mico W175Z1 is a 1/2″ reach plug, but please do double check that. If so, I would guess that something like a NGK B7HS spark plug would be about right (see the NGK reference chart in this post to see what the code means).
Hope this helps!
I have a 1956 Model 18 AJS. I recently fitted a Champion N5C spark plug, but the bike wasn’t at all happy with it: the tickover was rough and chuffing quite a bit. I put a Colour Tune plug in to adjust the mixture (to budsen blue!) and the bike now ticks over beautifully with this device installed! Obviuosly, an aletrnative plug to the Champion N5C has to be found, so what do you recommend? Also the manual suggests a gap of around .020-.022″. Is this correct, or would it be happier at a standard .025″?
Hope you can help.
Thanks for your message regarding spark plugs. I’ve always preferred NGK spark plugs, but really only because I’m familiar with the coding system they use where as I have no idea about Champion ones. For my Matchless G3, the standard spark plug I use is a NGK B7ES but I am currently trialling an NGK B7EGV which is the same but with a finer centre electrode which allegedly gives a bigger spark for less magneto volts. Have a look under the spark plug section on my blog and I have listed some compatibility tables. I can’t remember the spark plug gap off the top of my head, but I know the B7EGV is about right as standard, but that a B7ES needs closing up slightly.
I guess it also depends upon how good your mag is.
That’s very helpful, thank you. I’ll try an NGK; I remember I used them for many years when I had a Trident T150V and found them very reliable – unlike the rest of the bike!
Interesting you mentioned the NGK B7EGV with the finer electrode. Some of the forums suggest this approach, especially for use on an older machine with a mag. Actually, the mag on my machine is good and I adjusted the points today and re-faced them as well, so there’s a big fat spark being generated.
It was very strange to run the Ajay today (statically) with the Colour Tune plug inserted: she just went thump-thump-thump on tick over and not thump-pop-thump, so I guessed that there’s a plug incompatibility. I had a Champion N5C inserted, which is one recommended as a replacement for the KLG
FE80 and is a fairly hot plug, but evidently may well not suit this actual machine.
I found your blog most useful and have saved it for future reference. Please let me know the result of your trials with the latest NGK. Most interested.
10 out fo 10 for that one cheers trev
which NGK iridium is ok for mi karizma bike pls suggest…
Hi Ramesh. Sorry, I’ve no idea what spark plug fits your Karizma – maybe try checking the applications list on the NGK website instead? I wouldn’t bother with the Iridium plugs though as they’re unlikely to make any difference to performance that will justify the price. I’d just stick to your standard plugs.
Help – what NGK spark plug will replace a Mico w07BC4 in 1967 Royal Enfield 350 Bullet- trying to save my mechanics sanity
Hi Boyd. You can look up the Bosch Mico plug code here to check what the current Bosch code is for that spark plug, then from that you can work out the appropriate NGK model. However, why are you concerned with the Mico plug code? It is almost certianly not right for the bike anyway – I came across Mico plugs when I was in India, they’re certainly not standard fitment!
My advice would be to ignore the Mico plug and find out what the standard fitment for your Enfield is. I would guess that it would be either a B7ES (1/2″ reach) or BP7ES (3/4″ reach) depending upon how deep the thread in the head is. Check the compatibility table above to see hotter or colder plugs, or compatible plugs in other makes.
Hope this helps. Regards, James
Thanks for the response will try to save my mechanic today and pass along this info and lead him to this site– will let you know the results
great write-up. I am also in HK, any idea where i could NGK iridium plugs for my Honda CB750 in HK?
Thanks and much appreciated.
Hi Wesley. I’ve sent you an email…. Regards, James
I read your blog with interest, one thing to bear in mind is that from about 1975 manufacturers stopped glazing the central insulator. This is fine with fuel injection but not good for carb type engines. With the crap sold as fuel today, if you flood the engine and soak the plug, the plug is contaminated beyond repair and no amount of cleaning will bring it back to life. The only solution is a new plug, good for the manufacturers but not for the consumer.
Cheers David. Interesting point regarding the glaze (or lack of!) on modern spark plugs, not something I had heard of before. Do you have any links or references for this info? Thanks, James
I found the link, http://www.gsparkplug.com/shop/fouling-shortingout-problem-modern-plugs-vs-ngk
The article is very enlightening and I have found out by bitter experience of it’s truth. Many times I have flooded the engine resulting in no start, replace with new plugs and instant start. If I try to reuse the old plugs they just do not work and I have a draw full of them.
Thanks David, interesting article.
which spark plug should much my rtr 160cc apache since i bought it with a twin plug super Bosch model number ( ur5ddc India 209) . Now which ngk or champion number should much since they are the only available in kenya? Can apache work with mono plug if twin plug not available? Plz help!
Hi Jimmy, sorry I can’t help you much here as this website is about classic Matchless motorcycles and I’m not familiar with your bike. Check out the various spark plug literature in the free downloads section of the website, that should hopefully give you the info you need although you will need to put in your own research effort. Regards, James
See here: https://matchlessclueless.com/resources/downloads/?category=2
I would really appreciate if you can suggest a bosch spark plug as i have an old mico HBW 175 Z1. Here is the link where you can see the specs of spark plug. http://1.bp.blogspot.com/-dBdfmTLjWN0/UWmD7D2SM5I/AAAAAAAAASo/ZNso-rg-_7k/s1600/scan0036.jpg
Hi Sam. I’m not sure off the top of my head, but Mico seem to use the old Bosch spark plug codes. There’s a document you can download from the ‘useful resources’ section of my website which gives the conversion from teh old to the new Bosch codes. You should be able to figure out the appropriate plug from there. See here: https://matchlessclueless.com/resources/downloads/?category=2
Very interesting site with a lot of useful info. Thanks for sharing your findings. In India, shop keepers dont go by model numbers. The first question they ask is, “what bike?”. you say matchless and they again ask “what bike” unless the shopkeeper is a very old timer. I have a 56 G3. Earlier i had a 55 G9 for which i could never find the right plugs. I had a box full of used plugs taken off the G9. Never found out the right plugs for that G9. I will try a W8DC for my G3 but i am sure the shopkeeper will still ask me “for which bike ?” I am restoring a 51 matchie now.
Hi Joga. Yes, I know what you mean about the shopkeepers asking which bike the spark plug is for. I had many funny looks trying to explain what a ‘Matchless’ was in my broken Hindi, but also a surprising number of people who still remembered them! Let us know how you get on with the W8DC. James 🙂
I’m proud to be owner of 1953 Horex Regina 350cc single. The bike use to be my dad and was left over for 50 years.When i took it, it was horrible. I make full restoration of it by my self, actually cost me a lot to find original parts from Germany but now its a piece of art, everything new and original.
The only problem i still have, and always have after the restoration was a spark plug fouling.
I read the manual, i ask some professionals,i read over and over the net for similar problems but still no luck.
I install new battery, i check the ignition timing, i check my carburetor tuning i did everything but i still didn’t found solution to my problem.
I even played with some different kind of Ngk spark plugs.BP6ES, BP5ES, BP7ES, BP9ES, B7ES, B8ES All the same fouling problem after driving for about 2-3 Km distance. The suitable spark plug for my model its the NGK B7ES.
My 1st concern is that i still have the original alternator unit. original rotor and Original magnetic field. Coil, capasitor, contacts, rotor Brushes, regulator/rectifier are all replaced with new. The system is 6V and i’m getting a reading of 6,4V to the battery at mid-range throttle and getting 6,8v at full throttle to the battery. Is this less that it supposed to be or its ok?
My 2nd concern is the carb since is also the original unit. I clean everything but i don’t know if the jets got some wear inside that cannot be seen with human eye.
I don’t know what else to do or check. Have you been experienced previously with the same issues or similar?
Any opinions will be appreciated.
Hi Mike. Ok, so with your battery charging, 6.4-6.8 volts is a little on teh low side. The battery won’t start charging until it gets about 6.45V and really needs 6.90V to charge normally. There’s more info on this in my battery charging article here:
I guess your alternator unit needs some attention. With reagrds to plug fouling, that’s a tricky one. Is it magneto ignition? When you say the plug fouls, do you mean that it gets covered in black carbon? Is so, thats probably a carburation problem – is the choke mechanism working correctly? If the fouling is oily, then maybe the rings or piston are worn and you’re burning oil? But if the spark plug looks clean but just stops sparking, then it might be that your magneto is getting weak. Carefully hold the end of the HT lead about 5-6mm from the engine and see if it can generate a spark (you need a big spark outside the engine to match a little spark under pressure inside).
The only other suggestion I have is to try a NGK BR7EIV iridium spark plug. This has a fine centre electrode and so needs much less voltage to generate a spark, plus the iridium plugs are less prone to fouling. Maybe give that a go and let us know how you get on!
First of all thank you for your time and the so precious help.
The Horex has a Norris Alternator so as it seems it must be a problem of a weak alternator so now that i read your information i will check about it.
Now about the fouling is not oiling since i replaced new piston rings. Its just like black carbon full up the Brand new plug after 2000-3000 kmh.
I read about the choke before so i remove that since in my carb system the choke its just a single piston going up and down.
About the iridium plug did you mean NGK B7EIX instead of BR7EIV?
Thank you once again for your help!
No, definitely a BR7EIX I’m afraid. They don’t make iridium plugs without a resistor any more (that’s what the R means). But the fine centre electrode should more than compensate for the resistor, so this shouldn’t be anything like a standard resistor plug. Otherwise the B7EIX and the BR7EIX are identical. James 🙂
Hi! In your summary table of spark plugs across brands and types you’ve indicated the Champion N2 as coldest and N5 as hottest, yet in the text below: “the NGK range actually goes from 2 (hottest) to 12 (coldest)”
Can you help un-confuse a confused sod like me! Regards
Hi Ben. The table only shows the range of spark plug heat ranges relevant to classic bikes, typically between a B7ES and B9ES in the NGK range or N2 to N5 in the Champion range. However there are much hotter and cooler plugs available for other applications, e.g. lawn mowers as well as other vehicles.
Also, note that the Champion and NGK heat range numbering systems are different. One uses high numbers for hot plugs whereas the other uses low numbers, and vice versa for colder plugs.
Hope this helps resolve the confusion! James 🙂
Ah! Thanks James, it was the different directions of quantifying temperature that was confusing me…
Howdy! I have a Harley Davidson flathead 1942 1200cc with NGK AB6 plugs 18 mm threads
and now I wonder which plugs with 14 mm I can use?
Hi Sten. How exactly were you thinking of installing a 14mm spark plug into an engine with a 18mm plug hole? I’m not sure there are too many higher tech plugs available in this larger fitment.
The heat range of a spark plug is the heat range for the spark plug not the motor, it has nothing to do with how hot or cool your motor will run only with how hot the spark plug tip will get. Thus a plug tip that projects further into the combustion chamber will run cleaner, as the heat helps burn off any oil and soot.
“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.”
Very good article. I have just bought a 250 Model 14 (standard looking, not CS) fitted with a long reach plug, which runs. The recommended plug for this is a B6HS, which is a short reach. What would you recommend?
Fantastic blog and Q&A’s, thank you!
Dear James – thanks for past help. In SA we can get NGK, Champion and other well-known makes. I have a 1953 G3L and am slightly ADHD so can’t go through lists! Ha ha, that’s almost true. Please just a recommendation for me. Many thanks!
Hi Dave. Sorry for slow reply. I would recommend a NGK B7ES for a cheap and cheerful plug that does the job, especially if your bike fouls plugs and so frequently needs a replacement. Or maybe try a BR7EIX. Much more expensive, but need fewer volts to give a big fat spark, so great for easy starting or an ageing magneto. Both those are very standard plugs so should be available anywhere in the world. Hope this helps!
I’ve got a 53 Matchless G9 500 twin, I’m running Champion N5 c, and although it runs sweet once it’s warm, it starts on one cylinder an eventually fires on 2 after 20 secs or so, I’ve hot a feeling it’s plug related, any help?
Hi am just wondering if you can help me out on the best spark plugs for my bikes? I have an 1961 ajs 500 twin and also a 1963 ajs 650cc model 31csr regards david
I have a Moto Guzzi 1975 850T the book believe it or not says use the most suitable plug what is the most suitable?at present, there are NGK B7ES plug,That I cannot purchase in Canada
Hi Richard. Surprised you cannot source a B7ES as that’s a very standard plug (maybe because they only sell the (not desirable) BR7ES resistor version in Canada?). The original plug would have been something like a KLG FE80 (no longer available) for which the modern equivalents are a NGK B7ES or B8ES, or a Bosch W225T2 (old code) or W5CC (new code), or a Champion N4 or N5 (probably labelled as N4C or N5C now). For later models of your bike, a projected nose plug was recommended (NGK BP6ES) which should give better running so long as no clearance issues in your engine. Worth checking out.
Personally though, I’d recommend trying the NGK BR7EIX / BR8EIX iridium plug described above. Ignore that it has a resistor as the fine wire electrode will more than make up for the slight negative (for old engines) effect of the resistor. This is what I chose for my old bikes now.
Hope this helps! Reagrds, James
I don’t know then why Royal Enfield company was using the below plugs on their motorcycles:
Spark Plug W145Z1 ((For India))
Spark Plug BR6HS, NGK 350cc ((For Exports))
Spark Plug HW225Z2, 500cc ((For India))
Spark Plug BR7ES ((For Exports))
And why NGK Japan mentioned 1 number extra for International market and why NGK India still suggested the above mentioned
Please let me know whether Mico/Bosch Spark Plug W8DC is still available in India. If not, please let me know presently available Model No exact replacement of W8DC.
Hi there. The Bosch W8DC spark plug is a current model as far as I know, but whether you can get it where you live there in India is another matter!
I have a 1927 hupmobile 6 cyl not sure of what type of plug or heat range
can you help thank you
Interesting site you have here..!!
I have a 1964 Triumph Tiger 90 350cc and the book calls for a Champion N4 apparently. What would be a good equivalent.?? If I had my way I’d track down the old pink Lodge plugs – they were the best.!!
Thanks so much, Alan .