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A few weeks ago when I was in Europe a friend gave me the aftermarket slip ring from the BTH magneto he has on his Velocette. The magneto had been professionally rebuilt in England but had failed after ~2000 miles due to shorting to earth of the slip ring and he asked me to do a post-mortem. He said these same slip rings have been supplied to other people he knows by at least four retailers in England so the information below may be of direct interest to others besides him. There are no manufacturer's markings on the slip ring so whether or not you have one of the same ones only can be judged by comparing its dimensions and appearance with the photographs below.

Aside from the issues I found with this specific slip ring the results of this autopsy again illustrate the broader problem of determining the suitability of aftermarket electrical components before using them. Unfortunately, as I've documented at several places earlier in this thread, with aftermarket magneto components one cannot take at face value unverified assertions like "Modern substitute, very high specification, zero failure" that turn out not to be true.

Preliminary Inspection

The following photograph shows a ~1 mm deep channel in this slip ring that starts at the trailing edge of the brass contact and runs for ~ 0.65". I can't know for sure, but it seems unlikely the rebuilder would have used this slip ring had this channel been visible when he was installing it, which means it almost certainly developed after the magneto was in operation.

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As can be seen from the next two micrographs, this channel starts immediately at the edge of the brass contact, which means it cannot have been caused by wear from the pressure of the brush. Aside from the channel being irregular, if it were due to wear the depression would not start abruptly at the edge of the brass segment since the channel is much narrower than the brush and doesn't extend the full circumference of the surface. The features of this channel are consistent with "spark erosion" of an inhomogeneous material rather than wear having caused it. However, there is no way to know if the channel grew steadily over the 2000 miles, or if it developed rather quickly.

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Electrical Resistance

One factor that could have contributed to the problem is if the black material used for the slip ring is electrically conductive. To check for this, I measured the resistance of the material itself by attaching the leads from a megohmmeter adjacent to each other on the flange:

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The resistance measured this way at various places on both flanges was greater than 20 GOhm at 2.5 kV (i.e. the maximum R and V of this meter). To find the actual value of resistance I repeated this measurement with a more specialized meter (General Radio Megohm Bridge) and found a resistance of 10 TOhm at 1 kV (i.e. ten million, million ohms). This is a very high value, indicating there isn't an overall issue with the composition of the material used. However, possible inhomogeneities in the material (i.e. voids and/or conductive inclusions) are something that will have to be checked.

As can be seen from the above photograph I mounted the slip ring on an expanding mandrel to make good electrical contact with its inner surface as would be the case when mounted on an armature. I next measured the resistance between the brass segment and this "armature."

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Although this resistance also should have been above the maximum of this meter (i.e. at least 20 GOhm at 2.5 kV) it was far lower as well as voltage dependent:

~1.5 MOhm at 250 Volts
~300 kOhm at 500 Volts
~150 kOhm at 1000 Volts (i.e. ~100,000x lower than 20 GOhm)

The meter couldn't go any higher than 1 kV due to the large current draw at this relatively low resistance. Also, although accuracy isn't especially important for this test, the 'overload' warning came on for the readings below 1 MOhm so those values may or may not be accurate. In any case, conductive path(s) to the armature of resistance ~150 kOhm or less is sufficient to cause failure of a magneto. For comparison, making the same measurement on a NOS Lucas slip ring using the General Radio instrument that goes to much higher resistances found 5 TOhm (5 million, million ohms) at 500 V.

The following figure from a book on magnetos shows that if there is a conduction path in parallel with that of the spark plug (i.e. as is the case here with the slip ring) of resistance lower than ~500 kOhm it begins to increase the minimum speed needed for the magneto to generate a spark. If the resistance drops below ~140 kOhm the magneto won't generate a spark at any speed.

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Adapted from 'Automobile Electrical Equipment', 6th edition, A.P. Young and L. Griffiths (Iliffe, 1958)

What these measurements have revealed so far is that a conduction path (or paths) between the brass segment and the armature had developed during the 2000 miles of operation, and that the resistance of that path(s) became low enough to cause the magneto to fail. This is consistent with the formation of a carbon track(s) due to the breakdown of the material of the slip ring in the presence of an electrical discharge.

To locate the low electrical resistance path(s), I used a piece of paper to insulate all but a short section of the inner surface, as is shown in the next photograph.

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By successively rotating the section in contact with the "armature" I found three low resistance regions: at the leading and trailing edges of the brass segment, and at the end of the channel. I used the 250 V setting on the megohmmeter for these measurements because, as above, the resistance of each of the three regions was low enough to overload the meter at higher voltage settings. In each of these three regions the resistance was below 1 MOhm. However, for all other regions of the armature the readings were above the 20 GOhm maximum of the meter.

Internal Structure of the Slip Ring

The above measurements show there is a relatively low resistance path(s) between the rubbing surface of the slip ring and the armature but it remains to be determined why this is the case. Next, I clamped the slip ring on my mill and used a 0.01" slitting saw to cut it in two at a place where I would be able to inspect the inner surface directly under the end of the channel, in the hopes of being able to see evidence of a carbon track. Note that the end of a carbon track large enough to have a resistance less than 1 MOhm would be quite small, so not being able to see where it emerged on the inner surface of this (black) armature would not mean it wasn't present. Although I was not able to see such tracks, the cross sectioning revealed the problem anyway.

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As can be seen in the micrographs below there are quite a few voids in the material. The resin should be absolutely homogenous, so this shows there was a flaw in the manufacturing process. The location of the voids is consistent with a vacuum pump having been used during the molding. Although this should have eliminated all of the voids, if the resin set too fast, or was too viscous to begin with, air pockets would have been left behind. The fact the air pockets are concentrated toward the center of the thickest region of the slip ring is consistent with this explanation.

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Cross-section of the slip ring. The highlighted region is magnified in the next photograph.

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Magnified portion of the slip ring corresponding to the highlighted region of the previous photograph.

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After inspecting the above cross sections, I then cut one of the sides in half in the other direction, revealing additional voids that are quite large (~2–3 mm).

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Note: the discoloration and "smearing" of the brass is an artifact of the slitting saw that I didn't take the time to polish away with abrasive paper.

Reason for the Failure: Voids in the Resin

Note that I sectioned this slip ring only in a few places so there is no way to know the size, number, or distribution of additional voids at other locations within it. However, that they exist in the size and numbers we can see explains why the slip ring functioned at first but "died" after 2000 miles. On every revolution the voltage developed on the brass segment would have induced a corona discharge in any voids near the brass. The ozone produced by those discharges is quite reactive so it "burned" a conductive carbon layer into the surface of those voids, steadily extending the reach of the high electric field beyond that of the immediate vicinity of the brass. As the corona relentlessly extended its reach from one void to the next as a result of the conductive carbon, eventually the conductivity of the path or paths between the brass segment and the armature became so large (i.e. the resistance decreased to less than ~150 kOhm) that the magneto could no longer be turned over fast enough to generate a spark to start the motorcycle.

As an aside, as I noted earlier in this thread, the reason why the points cavity of a magneto needs to be vented is to reduce the concentration of ozone created by the residual sparking of the points that is present even when a proper condenser is in the circuit. The failure of this reproduction slip ring is graphic evidence of how destructive ozone is.

Other Issues with this Reproduction Slip Ring

While the above has identified voids left behind during the molding process as the reason for the failure, there are additional issues with this aftermarket slip ring worth mentioning. The next photograph shows that the slip ring is thinner than a genuine BTH item. Although the restorer should have used a shim on the inside end in order to center the track under the brush, the offset wear pattern shows he did not do this.

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Genuine BTH slip ring mounted on armature at left. Reproduction slip ring at right.

Also, since both slip rings are sitting on the same surface in the above photograph, it can be seen that the flanges of the aftermarket one are ~50% thinner than on the genuine BTH. The thickness of the genuine ring's flanges taper from 0.17" at the rubbing surface down to 0.08" at the outer edge, while on the reproduction they are nearly constant at a rather thin 0.09" at the rubbing surface and 0.06" at the outer edge of the flange. Dimensions of a slip ring are not arbitrary, since they are chosen to guard against corona discharge and direct electrical shorts, so the inferior specifications of this aftermarket part are of concern.

Adding to the issues is the reproduction slip ring is 0.01" too thin (0.628" vs. 0.638"). Since the slip ring also serves as a spacer for the bearings, this means the bearings would be 0.01" too close together unless an additional spacer were added to compensate. The fact that the spacer(s) used by the rebuilder were incorrect can be seen from the wear pattern in the next photograph.

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The longer white marks are at the sides and center of the slip ring and the shorter one is 0.05" to the left, showing the wear pattern is offset ~0.03"~0.05" from the center line.

BTH and Lucas Slip Rings are Not Interchangeable

Although they may look the same on an autojumble or swap meet table, a BTH slip ring is thinner and has a smaller OD than a Lucas:

The black slip ring is the reproduction BTH and the tan one is a genuine NOS Lucas.

BTH in front; Lucas in back.

As an aside, if the 2000 miles were covered at an average of 30 mph and 2000 rpm, this magneto made ~2 million revolutions before failing. During that time the carbon brush was dragged across the surface of the slip ring some 120 miles at an average speed of 2 mph. Although the rubbing surface of this reproduction slip ring is fairly rough, on a proper one the brush will travel ~600¬¬ miles before being abraded away. I suspect the roughness of this one would wear out a brush twice as fast.

Implications for Using Aftermarket Slip Rings

The failure of this slip ring after 2000 miles has wider implications for anyone rebuilding a magneto. Although I don't have a new slip ring from the same batch to test, the fact that the resin in the flanges (where there are no bubbles) of this failed one has a resistance of ~5 TOhm means the resin itself is intrinsically fine. Thus, this slip ring very likely would not have failed if the manufacturing process had not left (hidden) air bubbles in it.

It's worth mentioning that while 5 TOhm is more than adequate for this application, the same measurement on a NOS Lucas slip ring was off scale on the highest range at over 1000 TOhm at 1000 V. Aftermarket slip rings are for sale on eBay with the claim they are made with "better materials" and are of "better quality" than NOS Lucas. Although no data of any kind is provided to support these bold assertions, I note that it would be very difficult (and completely unnecessary) to exceed the materials and quality of NOS Lucas slip rings. To be clear, my concern isn't whether an aftermarket slip ring actually is better than NOS, it's whether it might be significantly worse despite unverified advertising claims, as was the case with the one dissected in this post.

Although the voids in this slip ring would have been revealed by x-raying it, that isn't a practical test for most people to apply (unless they know someone who works in a dentist's or radiologist's office…). Unfortunately, because the reason for the failure was slow degradation due to repeated application of high electric fields near the internal voids, it is unlikely that any usual electrical test would have detected a problem when this slip ring was new.

A rule commonly used for high potential testing ("Hipot testing") of electrical components is to apply twice the operating voltage plus 1000 V for a few seconds during which time one looks for current flow above some minimum value. In the case of a slip ring, this means a 15-20 kV Hipot tester would be needed (note: Lucas rated their slip rings for 35 kV, so a good aftermarket slip ring would have no problem passing a 20 kV test). Unfortunately, even applying 20 kV to this particular slip ring for some number of minutes quite likely wouldn't have revealed a problem since it takes some time for a conductive path of carbon to form. In any case, the failure mode of this slip ring has resulted in me now investigating other ways for stress-testing slip rings that could be used by people who don't have the specialized instruments that I have available to me.