The spark will always travel the shortest route, so one ground electrode of the multi-tip type will get the bulk of the wear / erosion until the gap to another tip becomes smaller, then THAT one will start wearing, and so on. It's just a way of spreading the wear.

The real point with precious-metal plugs is to make the narrowest possible centre electrode that doesn't wear out too quickly -- the narrower the point of the electrode, the more readily a spark will jump.

My old Suzi GT380 triple likes NGK Iridium plugs, they last far longer than conventional plugs and simply never need re-gapping after being initially gapped to 25 thou' (the old points ignition needs all the help it can get to fire the dense fog of fuel & 2-stroke oil).

Personally I think all this recommended service interval stuff is a load of cobblers. True the manufacturer designed and built your car, but that's as far as his knowledge of it goes. He has no idea of the type of journeys you do, how hard you drive it, what sort of climate you operate it in etc. He is therefore not best qualified to say when and how often or not it should be serviced.

The recommended service intervals only apply to a car when it's new and are largely sales pitch in any case. As soon as it takes to the road and accumulates mileage it isn't new anymore. Certainly by the time a car is three years old the original recommendations have gone right out the window. There is no specification by which your spark plugs should be changed, or indeed other service items replaced. If your plugs look like they need renewing then change them. If they don't then leave them and wait till they do.

"Personally I think all this recommended service interval stuff is a load of cobblers."

I disagree. Some guidance must be issued, otherwise some people would never service their cars.

Of course it's crude, but it's better than nothing.

As for waiting until something "looks" as if it needs renewing - that could be dangerous indeed. For a start, you would need to assume everyone would actually inspect components, some of which are critical to safety. And you would need to assume people could correctly judge if something needed renewing.

If there's no service schedule it just wouldn't get done by some. As it is, there's no compulsion to service your car and that's why MOTs are necessary; in an ideal world they wouldn't be.

I think you have largely missed the point KP. I'm not saying that cars don't need to be serviced, far from it in fact. What I'm saying is cars don't stay new for very long, and so the original manufacturer's servicing schedule does not apply to any car just as soon as it has age and mileage on it, so it baffles as to why people take the service intervals as gospel. A fuel filter deteriorates with age, not mileage. If the car had done 10,000 miles in a month since it was last serviced the fuel filter would still only have been subjected to fuel for a month. Whereas if it had done the same mileage in five years and the filter was changed on a mileage basis it would've been sat in the same fuel for five years. I know which of these scenarios is worse, but I'm not sure everyone else does.

People mention long-life platinum tipped spark plugs that do 50,000 miles. I'm sure that they are capable of this, but only if that mileage has been covered in a relatively short time. But people's interpretation is that the plugs will last 50,000 miles whatever, and for however long it takes to do it. High mileage means long journeys, and an engine that is running at optimum temperature and efficiency. 75% of all engine wear occurs in the first five miles of any journey. Low mileage combined with all short trips is not good for your engine, and so in this case it must be serviced more frequently. That is what I mean when I say that the manufacturer's service shedules have to be taken with a huge pinch of salt. It's up to us in the motor trade to explain to people that car servicing is ongoing, and a common sense approach taking into account the customer's type of mileage is required.

I am sorry but this is technical rubbish. 70k miles is average wear and tear. Plugs do not deteriorate with age: especially not iridium tipped ones.

Some people on this thread have attitides to car maintencance more relevant to pre 1980s technology... when car engines were simpler and made of materials vastly inferior to modern ones and with methods far removed from modern computer controlled and checked mass production.

And I suppose you have never removed a set of plugs from a Zetec engine that according to the manufacturer are supposed to last 40,000 miles but are in fact knackered after half that? And I also suppose you've never come across a Ford with a misfire either, or any engine with the MIL lamp on and an emission fault and wondered why you can't really get to the bottom of it? Computer technology or not, the fact is that even modern petrol engines are still only about 30% efficient at best. At the same time emission requirements are becoming ever tighter. This means the engine's margin for error is getting less. Spark plugs as well as everything else need to be in tip-top condition otherwise emission faults will eventually occur.

Edited by Railroad. on 27/12/2012 at 20:36