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Spark plugs - advantage of longer plugs?


Colin Lindsay
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I've just been sorting out two boxes full of old plugs and have about 30 different varieties, so have grouped them, noted the model numbers and just been cross-referencing them on Sparkplug-Crossreference.com.

It's interesting that my NGK plugs, BP6HS and BP6ES are listed as suitable for the Triumph Herald; the HS for the 12/50 and the ES for the 13/60. There's quite a difference in length between the two so I'm just idly wondering why, what difference does it make in performance, and can the two be interchanged between say a 13/60 engine and the 1200 without physical damage? I'll experiment with an actual head tomorrow to see the seating position, but for now I'm just curious and lazy...!

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Cannot really answer that definitively. However assuming the heads to be similar in relation to the distance between the seat and the Combustion chamber?. Then all that the longer plug would do is move the spark source further into the chamber. Maybe in the cause of a more efficient/effective/complete Burn?.

Pete

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I think another risk is that the plug end will get too hot which at the least will cause pre-ignition and at worst damage to the plug end. After all the amount the plug extends into the combustion chamber is what gives us hotter or cooler running plugs... 

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Actually, the difference between 'hot' and 'cold' plugs is down to the distance that the central electrode projects from the body of the plug.

image.png.1de4cdf523393f2bd998a0d69e9e2d68.png

The effect on compression ratio may be calculated if the dimensions are known.   It would be good if Colin would offer comparitive measurements.    I guess that the difference might be as much as 1-2mls which could be significant, but not much!

John

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'Actually, the difference between 'hot' and 'cold' plugs is down to the distance that the central electrode projects from the body of the plug.'

Yes but surely the amount the plug sticks out into the combustion chamber is going to have a similar effect?

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long reach 3/4" and short reach 1/2" depth spark plugs have been around for years  most 1/2" were used on side valve with a shallow head to chamber thickness    somewhat odd some 4 cyl triumphs used a 1/2" plug 

its all down to the head thickness at the plug hole   i dont advise mixing a short plug in a long head reduces the compression ratio  and a long plug in a short head increases it just by changing the volumes a little

danger is piston contact if you fit a long in a short    nothing to stop you adding a 1/4" spacer washer on the Outside 

its a mix and match that has little sense to it without some careful look see in the chamber  and  for what !!!

a short plug in a deep head shrouds the plug  and the long plug in a short head may  give it a whack 

Pete

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Would it be better to have a electronic ignition fitted to my 1360 Herald ,, it kept cutting out last week,,  and I admit I am not mechanically minded,,, I just enjoy the car,,, but I would now get nervous if it was going to let me down,, had to get the AA to get me home,, he said the coil was getting hot and carb may have muck in it ,, so I am in two minds,,, 

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1 hour ago, Douglas Arnold said:

he said the coil was getting hot and carb may have muck in it

Is the coil too hot? Carb MAY have muck in it.

I'd check these 2 out before doing anything else. A spare coil that is known to work is a very handy to have when there are doubts, quick to swap over and you get an answer straight away to the question of dodgy coil.

I have Accuspark on my 13/60 and very happy with it.

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1 hour ago, Douglas Arnold said:

Would it be better to have a electronic ignition fitted to my 1360 Herald ,, it kept cutting out last week,,  and I admit I am not mechanically minded,,, I just enjoy the car,,, but I would now get nervous if it was going to let me down,, had to get the AA to get me home,, he said the coil was getting hot and carb may have muck in it ,, so I am in two minds,,, 

I use this system on all my Heralds (and the GT6 too)

https://www.ebay.co.uk/itm/110819721887

Very easy to fit, no maintenance and it's lasted over ten years on the GT6 so far. The carb is easy to clean out, but if the coil is any way dodgy I'd replace it, they're quite cheap and may cure the problem.

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2 hours ago, DanMi said:

yep early and late heads are different and have different length spark plug threads short for the early head ie 1147 long for the 1300/1500 head

That's what I was wondering; all my spare heads are 1200 so I'll not be able to compare with a 1360 head until later in the year. Given that this is the state of the first plug I removed over the weekend, I'll be replacing all four asap, but as I have quite a few to choose from I just thought I'd ask.

25529BE2-66D1-443F-92A3-F3A990EC12CF_1_105_c.jpg.a8424e7ad8a84bdd04047823e314b8d0.jpg

 

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4 hours ago, Douglas Arnold said:

it kept cutting out last week

has the condenser got an orange link wire   they are rubbish     get one from the distributor doctor   they have yellow wire 

and have product inside the canister not an empty shell .   http://www.distributordoctor.com/distributor_condensers.htm

The quality of the commonly available condensers is very poor, in terms of both the physical fit & the performance /longevity. We have recently dismantled & examined 4 examples of the orange wired versions, from 4 different factories all around the globe. All of which are readily available & prevalent in the UK. The results confirmed our suspicions & why we are having so many phone calls from owners who've fallen victim to them. Original specification Lucas condensers & ours have 3 metres of winding inside, the orange ones have between 1 metre & 1.5 metres. The internal bonding & construction also left a lot to be desired.

FIT OUR YELLOW WIRED CONDENSERS

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18 hours ago, JohnD said:

The effect on compression ratio may be calculated if the dimensions are known.   It would be good if Colin would offer comparitive measurements.   

John

Consider it done!! As close as I can measure it with a small tape measure, my calipers being defunct, it's about 6 mm difference.

61C46ADB-BEA2-4B65-906B-63D1DC49FAE7_1_105_c.jpg.2378925b5b4174954f6cd1df5fd2b18d.jpg

 

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Colin, Pete stop hedging about the bush - get on with it and do the calculation for, say a 13/60.

1/4" length, what diameter of the plug, to give volume. Volume of space above piston at top of stroke . .

There, I've given you enough clues and hints now you can do the simple bit . . 

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As always it's not THAT simple...

Was out in the garage a few minutes ago, decided to swap the rusty plugs for my nice set of BP6ES as recommended by the Sparkplug website I checked at the start. Guess what? They don't fit; in fact the one I tried bottomed out with a scary sort of 'wrung the threads on the head' sort of sensation. Checked against the old plug, it's the same length as the HS plug not the ES plug.

The ES is 14mm diameter 19mm reach according to the site; the HS is 14mm diameter 12.7mm reach. I've just confirmed that their application list states Herald 13/60 for the ES and Herald 12/50 for the HS.

ES below:

932461425_ScreenShot2021-09-20at19_09_47.png.b3c1eaf8194dc7e74824763dc5b96c87.png

HS below:

545509665_ScreenShot2021-09-20at19_11_46.png.20c6849b83971a138b7a6ddf05d424b6.png

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So, one is 6.3mm longer.  Both at 14mm diameter, so  extra volume is Pi x 0.7^2 x 1.4 cubic centimeters. 

No need for a calculator, your phone has one.  One moment..... 2.16 mls.

Now you need to rework your compression ratio.   

CR = (Stroke Volume + Chamber Volume)/ Chamber Volume

But only you know what you have!  Since small volumes can be significant (the 'gasket volume' of a six cylinder is 3mls) this may raise the CR, but unless you have already raised it, not critically.

John

 

 

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16 hours ago, Colin Lindsay said:

They don't fit; in fact the one I tried bottomed out with a scary sort of 'wrung the threads on the head' sort of sensation. Checked against the old plug, it's the same length as the HS plug not the ES plug.

It's possible a previous owner has fitted the wrong plugs (probably sold them by a motor factor's apprentice who couldn't be bothered to look up the correct engine size).

If you put ES into a 1200 engine that needs HS then they will probably hit the piston, which is bad. If you put HS into a 13/60 engine all you get is a slightly reduced compression ratio. And a bit of thread exposed to the combustion chamber where it can go rusty and get coked up and no longer be thread when a subsequent owner attempts to fit the correct plug. If you find your 13/60 has short plugs fitted then you should run a spark plug thread cleaning tap down the head before trying to fit the right plugs. Ideally that's a head off job, as you don't really want bits of crud sitting on top of the piston.

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