Spark Plugs, Caps and Leads

One would think that the topic of spark plugs would not be interesting at all – heck they have been available and used in engines for over a century ! Not so – there is an emerging problem for older Japanese motorcycles as in some cases, the originally recommended plugs are just not being made. For example – my 1966 Yamaha YA6 calls for an NGK B7HZ spark plug – try to find one ! And if you own an RE5 then you have a real problem as the NGK A9EFP plug was only made for that model of machine, and has not been manufactured for years. And while in most cases it is possible to use a substitute spark plug type in place of the original, the range of options seems to be decreasing as it appears most plugs these days are ‘resistor’ plugs rather than the original non-resistor style spark plug.

All new engines use some sort of resistor in the high tension circuit to suppress RFI (radio frequency interference) caused by the spark as it interferes with TV and radio transmissions. Older Japanese motorcycles put the resistor in the spark plug cap, and have a metal core spark plug lead and a non-resistor spark plug. Cars generally use resistive spark plug leads (carbon) and resistor spark plugs with no resistor in the spark plug cap (not always true, but OK as a generalisation). Resistor style spark plugs started to become more common on Japanese motorbikes as electronic ignition systems (CDI) became more common and pretty much were standard by 1979 – points based ignition systems don’t really care, and actually you do get a better spark with a non-resistive setup on the older bikes as the ignition coils were not that great.The best overview of the issues and options that owners of vintage motorcycles have that I’ve seen recently is here. In brief, as I am staying with points on my old bikes I need to find alternate non-resistor spark plugs – for the project bike, I do plan to use an electronic ignition, so I just need to ensure that the spark plug cap at least is correct and is a resistive cap.

Decoding the spark plug types then is a useful thing to be able to do, and luckily there is a good chart available from NGK if, like me, you are using their plugs. It is available in PDF file format here and another version is available on the web here. This is a useful thing to have for example when trying to confirm the right plug to buy for my old Yamaha as NGK here in Canada was of no use at all – the kid on the customer support desk claimed they had never even offered a B7HZ plug and that I had the part number wrong ! It was almost enough to make me want to buy Champion plugs, but I’m not quite that desperate just yet.

First of all, if there is an ‘R’ in the spark plug label ahead of the number then its a resistor plug which I don’t want. Using the B7HZ as an example, it decodes as follows:

  • ‘B’ is the thread diameter, in this case 14 mm
  • ‘7’ is the heat range – the smaller the number with NGK, the hotter the plug – too hot a plug will burn a hole in your piston (I have first hand experience with that), and too cold will cause it to foul
  • ‘H’ is the thread reach – in this case 12.7 mm or about 1/2 an inch
  • ‘Z’ is for what NGK calls a ‘thick’ 2.9 mm diameter centre electrode

Basically then, if a ‘Z’ plug is not available, a standard centre electrode plug should work fine (a B7HS with the ‘S’ being a 2.5 mm diameter centre electrode) – my assumption here is that the fatter centre electrode was wanted in 1966 as the injector oil fuel systems were new, and fouling would have been common. The injector oils available today are much better, so this shouldn’t be an issue. Given my previous experience with a YA6 and holed pistons, I may actually start off with a colder plug and see how it works – this would be a B8HS.

For the GT750, the NGK B8ES series plugs are still available (the ‘E’ means it has a longer reach of 19 mm or 3/4 of an inch) – but I may stock up on them. I suspect they will eventually be phased out in favour of resistor style plugs as the general demand for non-resistor plugs declines over time.

For the project bike, I just need to ensure that I have the correct resistor caps, ideally with a lower resistance as the OEM ignition coils are not that great. NGK provides a PDF format reference sheet listing the available options which I’ve included here – either the 1K ohm NGK part LB01E, or the 5K ohm NGK part LB05E caps should work fine, with the lower value probably being better to ensure the as much current reaches the plug as possible.

For the ignition leads themselves, generally speaking on older Japanese bikes these don’t really wear out as they are wire cores, and are epoxied into the coils. At the resistor cap end, all that is required is to trim the end occasionally to ensure a good contact with the wire core and the cap. Obviously if you do this too often, you will end up with a very short lead, so it is possible to chip out the epoxy, and then install a new wire core ignition lead, and then re-epoxy the connection.

Enough about plugs – the frame is back from the welding shop, so it is time to get on with a few other things ! The ‘home’ site for the GT750 rebuild project is here.

August 29th Update – I managed to connect with Carrie and Brandon from the US NGK Customer and Technical Support teams, and Brandon has confirmed that, as I’d guessed, the B7HS was the correct NGK plug for my old Yamaha. I still haven’t heard a peep out of the Canadian branch of NGK which is disappointing, but at least I have an answer.

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