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Brushcutter Sparkplug


My China brushcutter sparkplug failed the other day, so I replaced it with a NGK equivalent.  When I got home and compared the old and new, I found that the new one had a slightly shorter reach ( approx. 8mm).  Started first go and ran well, but after 15 minutes of use it got so hot that the cheap crap plug lead melted. 

Is it possible that it overheated because the plug is too short and detonation is occurring in that space that should be filled by plug shaft? (hope my description makes sense).  My gut says yes, the heat can't dissipate fast enough like it would if the electrode was protruding into the chamber, so the head is acting like a large heat sink.

Any thoughts, thanks.


Spark Plugs are rated by heat range.
As you allude to it's all about how fast they Dissipate the heat. You don't want them to run too Hot which can cause the plug damage ( melting electrodes, bubbling insulator ) or too Cold ( fouling) .

I doubt it's detonation because the compression would be lower but it would create a concentrate pocket of combustion where there was a lower surface area for the head to dissipate the heat.

I have literally forgotten more than I remember and I don't know the ramification of short length except it would reduce Compression and probably affect timing in the travel of the flame front.

Heat ranges are fairly Liberal on non performance engines like garden equipment but they can't be miles away and they need to be the same reach  ( length) . That's critical.

Just go back and get a comparable plug to what you have.  I know the champion equivalent  is CJ8 which suits a heap of small engines. 2 stroke, 4 stroke small and no so much. They are a pretty short reach so if yours is a longer one, that's what you need.

Thanks Glort.  I knew I could rely on you for an answer.  I'm hoping to force the lawnmower shop to replace the coil/plug lead seeing as they sold me the wrong plug. (even with the sample I took along)


No worries mate.

That's really annoying when you get the wrong thing after taking in the sample.
Had that a couple of times latley. I understand some things have a lot of variables but when you put the right one/ original in their hand  and they have the other one in stock...... 

Unfortunately we mistrust the person to give us the righ one and often don't check thinking it's simple and they know what they are doing.....

Should learn from McDonalds Drive through. No one here actually drive off without going through their order first because the amount of times any o them will get it wrong is a concern... especially when you get home and find you only got half your order or the wrong thing or the item you were looking forward to the most is missing.  I try to apply that to everything these days.

I had a couple of issues with some new gear I bought.
I couldn't get a petrol blower to run very well.  Eventually tracked that down to the HT spark lead being molded into the boot quite withdrawn so the spark had to jump to the plug itself  which was on the limit of it's reach and caused the hit and miss situation. I cut the boot off, Fitted one from a car and siliconed that in place and no more trouble.

The other one was a weed wacker that would start with a shot of fuel in the carb but would not run.
Pulled the carb off thinking it must be a blocked jet or ruptured Diaphragm and then noticed the Carb to Block gasket was on upside down. They have a little hole that vents crankcase pulses to the carb to work the lift pump. With the hole blocked by the gasket, the carb couldn't pump fuel and wouldn't run.
 Put the gasket on the right way round and you wouldn't credit the difference having it correctly fitted made!
Started first pull and pretty much always has since I tuned it a little.

Hope you get the replacement lead but I have found here at least, many places are very reluctant to take responsibility for their errors.  :0(

Pretty much sums it up.  All the above apply this side of the ocean too, especially your last sentence.  I will try, but have almost zero chance of coming right.  Was also thinking of trying to find a car plug cap, something like my old Kombi which had a thread which screwed into the plug lead.


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