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Misc: Ignition Magneto Points

Pushrod Twin

Well Known and Active Website User
VOC Member
How does one ascertain whether the magneto points are tungsten or platinum?
My KVFTT makes a great spark, engine starts easily, then after some 500-700 miles, progressively starts to misfire under load. Inspection reveals a growth on one contact & matching divot on the other as material transfers. A quick rasp with a points file to remove the growth, and all is sweetness & light again for another 5-700 miles.
I suspect tungsten points, but I am open to all suggestions????
 

greg brillus

Well Known and Active Website User
VOC Member
The platinum points are definitely smaller in diameter......... From your description it sounds like the condenser is gone causing excessive arcing across the points. The platinum points are better but hard to find now.
 

Pushrod Twin

Well Known and Active Website User
VOC Member
The platinum points are definitely smaller in diameter......... From your description it sounds like the condenser is gone causing excessive arcing across the points. The platinum points are better but hard to find now.
Thanks Greg, I considered that the condenser may be tired, but it does not exhibit the other symptoms that I would expect, hard starting or stopping when hot.
 

Pushrod Twin

Well Known and Active Website User
VOC Member
Inspection of all my other mag's, KVF off my 48B & K2F off my G9, then comparison with the KVFTT indicates that the TT at least has the correct, small diameter, Platinum points. It also indicates that they are ready for another rasp and getting towards the end of their life.
So that brings us back to Greg's suggestion, capacitor requires replacement.
 

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Pushrod Twin

Well Known and Active Website User
VOC Member
decent replacement points in either material are becoming unobtanium.
Thank you Martyn, are there any solutions? They appear to be "brazed" or silver soldered onto the base one side,screw adjuster the other. Someone must have the technology to replace them?
What is the alloy detail of the platinum, should I ask a jeweler?
Does anyone have the detail for the tungsten alloy? As a machinist I am aware that tungsten comes in many grades, I have cut soft tungsten with a hacksaw blade & turned it with a high speed steel tool,which appears to be a contradiction when we use tungsten tools to cut hard steel. It was weights for propeller blade balancing. At the other end of the scale, tungsten carbide tools are hard enough to cut 60 Rc gudgeon pins. What is used for contact breaker points?
Neither points material appear to be "hard," a points file cuts them adequately.
 

Trickymicky

Website User
VOC Member
Dave Lindsley has replacement Platinum contact sets, although they are quite costly.

BTH 4BA PLATINUM CONTACT SET
LUCAS K2F/KVF PLATINUM CONTACT SET
BOSCH 3.5mm PLATINUM CONTACT SET

Prices are in his online catalogue.

I was led to believe that using abrasive means to clean platinum points is not a good idea.
 

Bill Thomas

Well Known and Active Website User
VOC Member
After all these years, It is understood that a Lucas mag is not the best ?, It was NOT built for a V Twin.
For me the "D" Distributor is the way to go, Even The Vincent Factory thought that in the end.
 

erik

Well Known and Active Website User
VOC Member
I am happy with the BTH "magneto".The bike runs without a generatr or battery .An other idea is to ask a dental laboratory if they can weld with a laser new platinum on the Points. Erik
 

Magnetoman

Well Known and Active Website User
VOC Member
Because platinum oxidizes only very slowly, Pt points (actually, Pt alloyed with Ir, Ru, and/or Os) were used in the earliest magnetos before it was realized laminated armatures significantly improved the low speed operation for which it had been felt Pt provided an advantage. Perhaps because of this, and citing a "lower surface resistance," shop manuals often say Pt should be used for the contacts in magnetos intended for higher performance motorcycles. However, despite what is commonly written, my measurements show that the difference in surface resistance between the two metals is negligible unless the magneto sits unused for many months in a humid garage, in which case tungsten can benefit from a light burnishing to remove the resistive oxide.

The contact resistance of a set of points is small enough that it isn't easy to measure, but the resistance of the tungsten points in a KNC1 magneto varied between 2 and 11 mΩ when I measured it after successive make/breaks when I rotated the armature by hand. Since 10 mΩ is less than 2% of the resistance of the primary coil itself, the contact resistance of these points will only reduce the primary current, and hence the output of the magneto, by ~2%. What this means is, even if the resistance of Pt points were zero (which it isn't), the output of two otherwise identical magnetos with Pt and W points in good condition would differ by a negligible amount.

If you still want to use Pt points, well, just because you want to, although Pt will resist oxidation in a humid environment better than W, it reacts with organic vapors to form PtC, which is both resistive and rapidly erodes the contacts. The choice of materials for magneto points is addressed in Chapter V 'The Magneto-Type Ignition Systems' of Electrical Contacts Data Book: Materials, Designs, and Applications (P.R. Mallory & Co., 1945): "… because of the location of the magneto and its design, the contacts may be subjected to oil mist. In such cases it has been the practice to use tungsten for the contacts, because platinum and its alloys have a tendency to become brittle… [causing] high contact resistance and a corresponding high rate of electrical erosion."

If no replacement points are readily available the old ones can be refurbished by silver soldering short lengths of pure tungsten TIG electrode rod over the ends of the worn points (Pt can be soldered as well, if you if you insist on using it). Tungsten wets easily with silver solders, and a single 7" piece of 4 mm-dia. pure tungsten rod is enough to restore a lifetime's worth of magnetos. If you decide to try this, you need to use pure tungsten TIG electrodes, which are painted green on one end. Do not use rods with either red or yellow ends because they contain 1–2% radioactive thorium. Because of the nature of the radioactivity, in solid form this isn't an issue, but you don't want to breathe the dust created when you cut it with an abrasive wheel and have it permanently settle on the surface of your lungs. Also, the reason for thorium is that it makes it easier for the TIG electrode to emit electrons, and that is the opposite of what you want in contact breaker points. When the points separate, you would like it to be as difficult as possible for an arc to form.
 

Pushrod Twin

Well Known and Active Website User
VOC Member
OK, maybe thorium isn't helpful in contact points - but has anyone tried spark plugs with polonium?
Crikey, they must have some age on them! And they were proud of their radioactivity to boot!
I'm familiar with Thoriuim, we used to pick up RR Viper magnesium compressor cases, cuddle up to them as we walked them around the machine shop, then dress up with paper overalls, masks, latex gloves et al and take seriously the risks of ingesting the swarf. I think the young welders in the plant where I now work are told about the Thorium in their tungstens and warned not to breathe the dust, but I dont think they understand the full implications.
The other diabolical ooh nasty to which we were exposed was Beryllium in copper & bronze components.
All of these are OK provided one heeds the safety warnings and takes precautions to avoid ingestion, not only orally, but also through cuts and abrasions in one's skin surface.
 

Magnetoman

Well Known and Active Website User
VOC Member
I found an article on the use of polonium in spark plugs in a 1940 issue of the Journal of Applied Physics by J.H. Dillon of the Physics Research Division of Firestone. The data show the ionization due to emission of alpha particles significantly lowers the voltage required to initiate a spark.

The practical problems are 1) the range of alpha particles is greatly decreased if the Po is covered by carbon, so placing a separate Po wire nearby quickly loses its effect, and 2) coating the electrode with a thin layer Po works, but the layer is quickly eroded during operation. This means the Po has to be alloyed with the electrode material itself, which is expensive and wasteful since only the Po on the surface does any good because of the limited escape depth of the alpha particles.

Before mercurycrest starts searching for a set of those plugs for his Lada, the half-life of Po is 136 days. Although the data shows they would have worked great when newly manufactured using Po straight from the reactor, if those plugs were made in 1950, that's ~188 half-lives so the radioactivity has been reduced by a factor of 2.5×10–57.
 

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