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Spark plug lead

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steamdave06/07/2020 16:24:11
511 forum posts
44 photos

I'm building a slow revving single cylinder I.C. engine and intend to use a Chinee ignition unit with potted components. (Sold as a spare part for the Chinee hit n miss engines).

The spark plug lead is red and I want black. Is there any electrical reason why I cannot solder a black lead onto the (shortened) red lead? I did think about covering the red lead in black heat shrink sleeving, but that would make it very bulky.
I'd rather not receive suggestions to alternative ignition systems as I already have the mentioned item.

While in questioning mode, how do you make the spark plug clip - the type that slides on the groove at the end of the plug? I've tried drilling a piece of brass and slitting it, but that soon loses its spring. Spark plug size is 1/4" x 32 if that makes a difference. The modern type that are usually covered in a boot is not what I'm wanting.

The Emerald Isle

JasonB06/07/2020 16:28:29
22750 forum posts
2653 photos
1 articles

Dave you can solder on a bit of black silicon insulated wire often sold as spare leads for test meters

I tend to buy the clips from S/S in the states

Edited By JasonB on 06/07/2020 16:30:03

Clive Farrar06/07/2020 17:08:28
119 forum posts
41 photos

My answer is not on petrol ic but i see no reason why they would not work.

On my Heli glow RC engine the lead ends in an appropriate size R clip.

Wire soldered to the long straight leg. It slides sideways onto one of the grooves in the glow plug end.

An other way i use is a single section of chock block. screw one end to the wire and one to the plug.

Not pretty but it works.

3rd suggestion piece of 6mm brass rod drilled one end to take the wire outer diameter. counter bore other end to 5.5mm. the at a suitable point cross drill a clearance hole for the plug end , one or both sides its up to you.

Make a top hat to fit the counter bore with a hole in the TOP end to solder the wire to . When soldering the wire built a pillow of solder on the flat end opposite the wire. Find a spring that fill fit over wire but in counter bore.

Fit the wire , spring, top hat assembly into the prepared rod pull back as tight as possible . Prep and solder the open end of the rod to close the end but leave the cross drill clear.

You then drop the assy over the plug end release the spring and ( usually ) get a good contact from the two pads of solder onto the plug,

Hope that gives you some food for thought.

Regards Clive

not done it yet06/07/2020 18:33:29
6809 forum posts
20 photos

Electrical interference may be a problem. With wired leads a different plug cap may be needed to suppress unwanted radiation. I suggest checking if the supplied lead is a carbon fibre one.

Edited By not done it yet on 06/07/2020 18:33:48

JasonB06/07/2020 18:40:52
22750 forum posts
2653 photos
1 articles

The other option for a period look Dave is to use cotton brained wire, not the plug wire as that only goes down to 7mm but the smaller car wiring stuff which I what I used on the couple of engines I built for you, they have S/S clips too

John Baron06/07/2020 20:00:44
520 forum posts
194 photos

Hi Dave,

You could use an "R" clip to fit the plug soldered to the ignition wire.

As far as changing the wire, it would depend upon how it is secured at the coil end. Some are soldered in others have a threaded screw that the cable screws on to. Some I've seen are actually a carbonised cotton thread with a wire pushed in to make a connection.

Graham Stoppani07/07/2020 05:17:58
124 forum posts
26 photos

In the motorcycle trade if we had a bike with a damaged HT lead that was bonded into the ignition coil we would use an in-line suppressor to connect the new cable to the old. (example below) This has an added benefit if not using a suppressed spark plug cap.



JasonB07/07/2020 07:03:37
22750 forum posts
2653 photos
1 articles

Guys, the ignitions in question are small CDI units (Capacitor Discharge ignition) so no big coils or traditional HT leads so just a circuit board with a Silicon insulated copper wire to the plug, not sure if Dave's cheap one will even have silicon may just be PVC. All in a match box size enclosure.

Also like me he likes an engine that looks like the original so no big rubber or metal spark plug caps, R clips, thick HT leads etc

Those are Futaba style plugs on the other wires for an idea of size.

bg ignition.jpg


Edited By JasonB on 07/07/2020 07:42:54

Roger B07/07/2020 08:10:10
188 forum posts
82 photos

You shouldn't have a problem joining the plug leads, but leave as much of the old lead as you can in case you have to remake the joint later.

I have made clips for that type of plug from phosphor bronze sheet that retains it's spring.

556 filing to shape.jpg

Andrew Johnston07/07/2020 08:42:51
6602 forum posts
701 photos
Posted by JasonB on 07/07/2020 07:03:37:

...the ignitions in question are small CDI units (Capacitor Discharge ignition) so no big coils..............

CDI systems still use a coil, but since the coil isn't used for energy storage the primary inductance can be lower and hence the coil can be smaller.


SillyOldDuffer07/07/2020 08:58:41
8692 forum posts
1967 photos

Jason's hinted at it a couple of times, but just to be completely clear ignition is a high-voltage application and I don't think any old black wire will do. Whatever is substituted for the red wire needs to be well insulated, hence mention of test-meter wire and silicon.

Other than red suggesting a shock hazard the colour doesn't matter, but the connection needs to be some form of High Tension cable. Not so much for safety, but good HT insulation makes sure all the energy delivers a fat spark inside the cylinder rather than leaking away en-route.


Michael Gilligan07/07/2020 09:23:55
20182 forum posts
1053 photos

< pedant alert >

Silicon is an element

Silicones are compounds

Silicone Rubbers are useful elastomers



Edited By Michael Gilligan on 07/07/2020 09:24:17

steamdave07/07/2020 10:12:20
511 forum posts
44 photos

Thanks for the suggestions, guys.

I have some black test lead, got specially for ignition systems so will try soldering a piece to the end of the red supplied plug lead (which is solid copper wire). If it works, I will take the plunge and cut the red lead down reasonably close to the box and re-solder. The join I will cover with some heat shrink tape.

I'm not too concerned with electrical radiation in a small engine. There are many other model engines around that run satisfactorily without complaints about interference.

Thanks for the suggestion of phos. bronze, Roger. I've got some beryllium copper strip that I will try.

The Emerald Isle

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