Tungsten Contact Points

[DurhambuilderParticipant @durhambuilder]

I need a set of tungsten contact points for my Westbury Wyvern build. Can I use a bit of a TIG welding electrode or would it be better to try and get the contacts out of a set of old car points? Thanks.

[JasonBModerator @jasonb]

I’ve used 2.5mm tig electrode a few times. Silver solder on a short length and then slice off theexcess with a dremel disc as it is easier than trying to cut and position a thin disc.

Hemmingway do/did sell disc of TC for just that job. Though looking at the now Reeves site I can’t see tham.

[KEITH BEAUMONTParticipant @keithbeaumont45476]

I have an unused set of car points that you can have if you send me your address in the Message slot,

Keith

[SillyOldDufferModerator @sillyoldduffer]

Pure tungsten more appropriate for electrical contacts than TiG electrodes souped up with Rare Earth metals.   Welders benefit from electrodes that encourage arcs, but arcs wear electrical contacts out quickly.

Therefore prefer plain tungsten.  Normally, the best material for a job is either unobtainium or unaffordable. By some flaw in Sod’s Law, plain tungsten electrodes seem to be cheaper!

There’s a fair bit of know how behind electrical switch contacts.   Tungsten is used because it resists melting, pitting and welding, making it good for high currents, which also burn dirt off.   Not so good for low current switching because dirt tends to build up.   A tungsten contact breaker that lasts well in an older car tends to misbehave sooner when connected to an electronic ignition taking a much lower current.  Cleaning the contacts fixes the problem, until the next time…

Dave

 

[JasonBModerator @jasonb]

We don’t know if the OP is using a modern CDI or a traditional coil in which case Tungsten will perform just like it does in a car’s distributor but will not look ugly like a set of points screwed onto a model engine can.

The tig rod also works well for ignitors which see the ignition spark jump between the two contacts insid ethe cylinder.

Example below, the small disc on the hammer and the round shaft it breaks contact from are where the spark jumps between are from a tig electrode.

[JasonBModerator @jasonb]

You can also use Tig electrodes to make your own spark plugs. These are 1/4 x 32 thread, just needs the excess cutting off once the top cap has been soldered.

 

[DurhambuilderParticipant @durhambuilder]

A big thank you for those responses, lot’s of useful information shared. I’m not sure whether or not to go down the modern box of electronic wizardry or traditional coil, I had hoped the original Wyvern instructions would explain the circuit for coil and points but it doesn’t so I may go for cdi with points as that looks more straightforward. I do have an unstarted Minimag magneto kit but that will most likely be for another day and engine. Thanks again.

[JasonBModerator @jasonb]

Fairly standard circuit. When the points are closed the circuit is complete. When they are open the circuit can’t get to ground as it stops at the insulated contact.

[Roderick JenkinsParticipant @roderickjenkins93242]

I get a much fatter spark on my Farm Boy with its Chinese Hall effect magic box than I do with my Wyvern with its MiniMag coil and points.  The Farm Boy is a much easier starter, just a flick of the flywheel.

However, regarding the points and correct me if I am wrong but I think the only reason to use Tungsten is because it resists pitting from the spark.  Is this important on a model that will only run occasionally?

Rod

[DurhambuilderParticipant @durhambuilder]

I also use a Chinese magic box on my Farmboy but can’t immediately see an easy non conductive mounting for the trigger magnet. Like yours mine will start first flick as well. Something tells me that the more complicated Wyvern Will require a bit more trial and error to start with!

Jason – Thanks for the schematic, it all makes sense now.

[JasonBModerator @jasonb]

Rod, Like you my practical experience with model IC engines shows that there is no problem, just a theoretical one that won’t manifest itself.

Combine the much slower running of even a throttle goverened Wyvern let alone a hit and miss with the short run time most will see and it is never going to happen.

Worth saying that if using a CDI and just two of the three wires that go to a hall sensor as the two contacts then they see a minute current and can be whatever you want from mild steel to a brass strip and will not show any pitting or carbon build up. This 2 wire setup is how I tend to run my CDIs then you don’t have mounting the sensor to worry about.

[Roderick Jenkins]

On Roderick Jenkins Said:

…

However, regarding the points and correct me if I am wrong but I think the only reason to use Tungsten is because it resists pitting from the spark.  Is this important on a model that will only run occasionally?

Rod

Depends on how the reliable the model needs to be and for how long.

An ordinary spark reaches about 2000°C, well beyond the melting point of Steel, so sparks punch holes  in it. Erosion is made worse by the vapour igniting, and hot steel is also chemically attacked by atmospheric Oxygen and by Nitrogen Oxides and Ozone created by the spark.  Copper melts at a lower temperature (1084°C), and it doesn’t burn like Iron, so lasts a bit longer.   Aluminium melts at 660°C and burns in air; it doesn’t survive long!  These metals all work as contacts, just not for very long!

In addition to rapid contact wear requiring more maintenance, another problem is the contact gap determines the engine’s timing.  Ideally the contact breaker should open and close consistently.   Not so if the contacts are eroding.  The engine’s timing will change if the contacts are closed for longer due to vaporised metal supporting an arc, and also as the gap changes size due to erosion.

That bad things are happening may not matter much.  All is well provided erosion and arcing don’t stop the engine early or cause starting problems.  As model engines run for minutes rather than hours, may not matter if sub-optimal contacts are used.  Therefore an inferior substitute to Tungsten might be “good enough”.

Final thought, Spark Eroders are very good at cutting metal…

Dave

 

 

[bernard towersParticipant @bernardtowers37738]

At one time 70s ish Ford (Delco) used to supply points with holes in the centre and I saw a genuine set for sale at an autojumble last Sat. That may be a cheap source of points.

[JasonBModerator @jasonb]

………….For how long

………..rapid contact wear

Are either of those a factor except in an armchair?

lets take a car where points may need changing every 6000 miles

Lets be very generous and say it averaged 60mph for those 6000miles which is 100hours of running.

If that could all be done in 5th gear then say 2500rpm

Also assume that is a 4cyl car.

Now the typical open crank model engine tends to sit there ticking over at lets say 250rpm and only has one cylinder not 4.

So that is 10% of the revs and 25% of the number of sparks so that is 2.5% of the sparks a car may use in 100hours of running.

So 4,000hours of model running before the points may need changing. As I said wear on a model is not an issue.

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