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#446643 - 07/29/12 12:28 pm K2F magneto brushes - too soft???  
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My K2F mag developed problems after just a few miles running.

It was recently tested by Tony Cooper and no faults showed up.

The bike quickly developed a nasty misfire that prevented the bike pulling cleanly from rest.

After doing a search for magneto troubleshooting, I read an old thread on the Britbike forum describing similar problems that were cured simply by cleaning the slip ring.

I did the same and was surprised by how dirty the slip ring is after just a few miles running. After cleaning, the bike starts much easier and doesn't misfire. The weather is crap today so I haven't had a chance to give her a proper test ride but it is definitely running and starting much better.

The carbon brushes seem much softer than the ones I remember being fitted when the bike was last running (15 years ago), like soft pencil lead.

Has anyone experienced similar problems with modern pick up brushes? I've had brushes from Tony Cooper and Dave Lindsay. The graphite seems similar on both. The springs on the brushes supplied by Tony Cooper seem slightly stronger and more robust than the ones supplied by Dave Lindsay.


1952 Triumph T100 in a BSA A7 Frame
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#446648 - 07/29/12 1:44 pm Re: K2F magneto brushes - too soft??? [Re: Mattsta]  
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Originally Posted By: Mattsta
I did the same and was surprised by how dirty the slip ring is after just a few miles running.

The carbon brushes seem much softer than the ones I remember being fitted when the bike was last running (15 years ago), like soft pencil lead.

The springs on the brushes supplied by Tony Cooper seem slightly stronger and more robust than the ones supplied by Dave Lindsay.
Unfortunately, not only are brushes on the market that are too soft (resulting in soot to short the magneto), there are ones that are too hard (resulting in wearing a groove into the slip ring). And with springs, stronger isn't necessarily better. Just right is better.

I'm just finishing restoring a 90-year old Bosch ZEV magneto for a friend that had suffered a "professional restoration." I made extensive notes with photographs of my restoration that I will turn into something to post, but that will take some time because of trips throughout August. Anyway, I had no way of knowing what brushes were in it. The slip ring was new, with no signs of use at all, and from other problems with the magneto it had it was very unlikely it had run more than a few minutes before dieing, so there were no clues from it as to the hardness (or softness) of the brushes.

I am equipped to measure Rockwell hardness, but tests like it are only appropriate for deformable materials, like metals. Brittle materials require a different kind of test (Shore), based on how high a ball rebounds when dropped on it. Luckily, Lucas brushes are only a few thou. larger in dia. than the ones in the magneto, and fit fine in the pickups, so I was able to use them (I have accumulated a reasonable supply of NOS Lucas brushes because of what I had seen with aftermarket ones).

Just like pencil lead, although brushes all look the same, they are made in a range of hardnesses for different applications. Given what has been on the market, either many retailers or wholesalers don't understand this, or they don't care.

As for the springs, they too have to have the right specification to match both the hardness of the carbon and the properties of the surface against which it will run. A certain contact pressure is required to carry the current, but also to ensure just the right amount of carbon is abraded. Either too much or too little pressure from the spring is not good. For the same reason the roughness of the surface against which the brush rubs is important. Clearly, a very rough surface would be bad, but you wouldn't want a perfectly smooth surface, either, because it wouldn't result in the necessary film of carbon being deposited. For a commutator (or earth ring on a magneto), industry standard is for ~1.5 micrometers roughness.

#446659 - 07/29/12 3:26 pm Re: K2F magneto brushes - too soft??? [Re: Magnetoman]  
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Yes. It makes sense that the material spec of the carbon brushes is absolutely critical to the reliability and performance of the magneto. Likewise, the pressure exerted by the brush springs must be very precise too to prevent the conditions you describe.

Chances are, the modern pattern part manufacturers are not making carbon brushes or springs to a suitable specification. As a design engineer, this wouldn't surprise me at all frankly. I never cease to be amazed by the poor quality of stuff people pay good money for. I'm disgusted with some of the stuff I've paid for that masquerades as proper replacement parts for British bikes. Some of the stuff I've bought has just gone straight in the bin. Gasket sets are a particular source of angst. Copper washers so thin that they are worse than useless and paper gaskets that simply don't fit. I have a cardboard box right next to me here as I type this, full of parts that are useless, everything from cam followers to gaskets and washers.

So that leaves the owner of any British bike with magneto ignition in a rather less than desirable situation. Presumably, replacement carbon brushes for dynamos are equally unreliable.

It doesn't matter how carefully your magneto has been restored or how much money you have spent if you are going to be scuppered by a couple of carbon brushes that cost a mere couple of quid

So the obvious question is, if you can't rely on reputable people like Dave Lindasy and Tony Cooper to supply rebuilt magnetos with decent quality brushes, what the hell are you supposed to do? Clean your slip ring every 10 miles?

It's bloody ridiculous!

Somewhere, someplace, hidden in some dark dusty corner, there will be an original Lucas material specification for the carbon brushes, the springs and the optimum preload for those brush springs. That information must exist somewhere! I'd dearly like to know where it is.

Last edited by Mattsta; 07/29/12 3:43 pm.

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#446663 - 07/29/12 4:00 pm Re: K2F magneto brushes - too soft??? [Re: Mattsta]  
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Originally Posted By: Mattsta
So the obvious question is, if you can't rely on reputable people like Dave Lindasy and Tony Cooper to supply rebuilt magnetos with decent quality brushes, what the hell are you supposed to do?
My answer to this question was to, out of self defense, equip myself to do all aspects of magneto restorations better than anyone else I am aware of does them, using components I make myself (rewound coils), tested myself (using a variety of specialized and hand-built instruments), or take from my supply of NOS parts (brushes). I think this will be clear in the magneto restoration thread I will start soon.

Yes, it's a sad situation that I felt this was necessary, and it's a solution not available to many people. Unfortunately, as long as the majority of customers accept inferior work or parts as acceptable, any industry will live down to those low expectations.

#446667 - 07/29/12 4:12 pm Re: K2F magneto brushes - too soft??? [Re: Magnetoman]  
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Originally Posted By: Magnetoman
Originally Posted By: Mattsta
So the obvious question is, if you can't rely on reputable people like Dave Lindasy and Tony Cooper to supply rebuilt magnetos with decent quality brushes, what the hell are you supposed to do?
My answer to this question was to, out of self defense, equip myself to do all aspects of magneto restorations better than anyone else I am aware of does them, using components I make myself (rewound coils), tested myself (using a variety of specialized and hand-built instruments), or take from my supply of NOS parts (brushes). I think this will be clear in the magneto restoration thread I will start soon.

Yes, it's a sad situation that I felt this was necessary, and it's a solution not available to many people. Unfortunately, as long as the majority of customers accept inferior work or parts as acceptable, any industry will live down to those low expectations.


Yup. If you wanna do the job right, you end up doing it yourself and swallowing some of the cost. Sad but true

Well, if you have a pair of spare NOS K2F brushes up for grabs, send me a message.


1952 Triumph T100 in a BSA A7 Frame
#446689 - 07/29/12 6:24 pm Re: K2F magneto brushes - too soft??? [Re: Mattsta]  
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An older thread describing a similar problem

The OP's solution was to use his old carbon brushes. Unfortunately, I don't have mine.

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=317315


1952 Triumph T100 in a BSA A7 Frame
#446696 - 07/29/12 6:34 pm Re: K2F magneto brushes - too soft??? [Re: Mattsta]  
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Originally Posted By: Mattsta
Well, if you have a pair of spare NOS K2F brushes up for grabs, send me a message.
Sorry, I'm hoarding them.

I don't know if you use the same scale in England, but in the U.S. a Number 2 pencil has fairly soft lead. It's the grade of pencil I think most people have around the house. While I wasn't willing to gamble on the brushes I found in the Bosch ZEV magneto, I tend not to throw things like that away. I had placed them in a little packet on which I wrote a note to myself explaining what they were, and put them in the drawer with my other brushes. Anyway, as a result of your post I just conducted a test of various brushes.

Pressing down with the pressure I would use to write with a pencil, I made a half-dozen lines side-by-side.

No. 2 pencil: my standard
"Bosch ZEV": perhaps a tiny bit lighter than the No. 2, but comparable
Lucas 455191 mag. earthing brush: same as ZEV
Lucas 200737 dyno brush: same as ZEV
Lucas 451260 HT pickup brush: significantly lighter than the ZEV

Although my test certainly isn't quantitative, it shows that the brushes the "professional restorer" had installed in the Bosch were significantly softer than the ones Lucas supplied for this application. Also, although the other two Lucas brushes I tested are about the same as those in the ZEV, they are meant to run on metal surfaces, not phenolic. So, a reasonable speculation is that the reason many aftermarket brushes are soft is they were manufactured using the (incorrect) specifications for brushes intended to be used on metal. Although plastics are much softer than metal, many of them are more abrasive.

I hadn't thought to try this test before because it really didn't matter what I found, I wouldn't have trusted those brushes. I'll take a macro photo of the test rubbings to include in the restoration thread I will be starting soon.

#446704 - 07/29/12 7:03 pm Re: K2F magneto brushes - too soft??? [Re: Magnetoman]  
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Pencil leads in the UK are 1B-6B for soft graphite and 1H-6H for hard graphite. 6B is the very softest and 6H is the very hardest

HB is between 1H and iB and is a standard handwriting pencil.

I'll do a little test of my own to see if I can gauge how soft these brushes are compared with a selection of artist's pencils. I reckon they are similar to 2B but I'll confirm this tomorrow.

I have no idea what I did with my old brushes. I tend NOT to discard old parts and have stacks of stuff lying around from previous projects. Yup, sod's law that I have misplaced my old brushes.


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#446707 - 07/29/12 7:12 pm Re: K2F magneto brushes - too soft??? [Re: Mattsta]  
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Originally Posted By: Mattsta
HB is between 1H and iB and is a standard handwriting pencil.
HB is pretty much the same as No. 2. A full size standard US writing pencil is a No. 2, but the approx. equivalent in refillable 0.5mm pencil is HB. Within the accuracy of my test, No. 2 and HB would be the same.

#446709 - 07/29/12 7:30 pm Re: K2F magneto brushes - too soft??? [Re: Mattsta]  
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Volkswagen generator brushes can be cut down to fit, I've done that in the past with good results on the generator! They are much larger than Lucas, so it should easily accomplish the task for the magneto. It's also a very easy material to shape!

#446710 - 07/29/12 7:40 pm Re: K2F magneto brushes - too soft??? [Re: Deadstiffcatt]  
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Originally Posted By: Deadstiffcatt
Volkswagen generator brushes can be cut down to fit, I've done that in the past with good results on the generator! They are much larger than Lucas, so it should easily accomplish the task for the magneto. It's also a very easy material to shape!
The problem is that, at least as far as my tests this morning indicated, a brush designed for the commutator of a generator will be too soft for the HT pickups.

#446712 - 07/29/12 7:50 pm Re: K2F magneto brushes - too soft??? [Re: Deadstiffcatt]  
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Originally Posted By: Deadstiffcatt
Volkswagen generator brushes can be cut down to fit, I've done that in the past with good results on the generator! They are much larger than Lucas, so it should easily accomplish the task for the magneto. It's also a very easy material to shape!


Which model of Volkswagen?

I would think think it would require some precision work to get the coil spring to fit correctly onto the brush?


1952 Triumph T100 in a BSA A7 Frame
#446713 - 07/29/12 7:54 pm Re: K2F magneto brushes - too soft??? [Re: Magnetoman]  
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Originally Posted By: Magnetoman
Originally Posted By: Deadstiffcatt
Volkswagen generator brushes can be cut down to fit, I've done that in the past with good results on the generator! They are much larger than Lucas, so it should easily accomplish the task for the magneto. It's also a very easy material to shape!
The problem is that, at least as far as my tests this morning indicated, a brush designed for the commutator of a generator will be too soft for the HT pickups.


Surprising given that a mag slip ring is a continuous surface whereas a commutator is a series of segments. I would have expected a commutator brush to need to be harder wearing than a magneto brush


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#446719 - 07/29/12 8:20 pm Re: K2F magneto brushes - too soft??? [Re: Mattsta]  
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Originally Posted By: Mattsta
Surprising given that a mag slip ring is a continuous surface whereas a commutator is a series of segments. I would have expected a commutator brush to need to be harder wearing than a magneto brush
A commutator brush is quite a bit wider in the direction of travel than the gap between segments. If the commutator is turned correctly, the brush wouldn't even be aware of the gap. As the leading edge starts over the gap it does not drop below the circumference because the carbon is rigid. The portion of the brush trailing behind is more than wide enough to keep it from tilting forward. The same is still true as the leading edge reaches the next segment. The only thing that wears away the carbon is its contact with the Cu segments.

#446722 - 07/29/12 8:46 pm Re: K2F magneto brushes - too soft??? [Re: Magnetoman]  
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Originally Posted By: Magnetoman
Originally Posted By: Mattsta
Surprising given that a mag slip ring is a continuous surface whereas a commutator is a series of segments. I would have expected a commutator brush to need to be harder wearing than a magneto brush
A commutator brush is quite a bit wider in the direction of travel than the gap between segments. If the commutator is turned correctly, the brush wouldn't even be aware of the gap. As the leading edge starts over the gap it does not drop below the circumference because the carbon is rigid. The portion of the brush trailing behind is more than wide enough to keep it from tilting forward. The same is still true as the leading edge reaches the next segment. The only thing that wears away the carbon is its contact with the Cu segments.


Yup. Well that makes sense actually. The brushes in my dynamo are much larger in the direction of rotation than brushes in the magneto.


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#446749 - 07/30/12 2:14 am Re: K2F magneto brushes - too soft??? [Re: Mattsta]  
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Mattsa,
I just went to a local old V.W. repair guy, told him what I needed to do, showed him my worn out bits: he grabbed a bag off the shelf with two brand new V.W. bug generator types in it, and asked me for about six bucks, then wanted to know if I wanted a hit. Handed him the money, said no to the hit, then got home and started my new brush project. I gotta admit, I felt like I was stepping into an old Fabulous Furry Phreak Brothers comic strip for a minute, but hey, the guy understood my concept, didn't offer unwanted advice, and gave me fast service and a fair shake!

Regarding round and spring mounting precision-yes to both. The carbon is easy enough to shape using a sanding block, working down to a finer grit as you near the dimension you need, a little grace with a needle file near the top will give a groove to work the spring into. Yes, it can be tedious, but I felt great knowing that stuck in the middle of almost nowhere, I DO have the ability to keep an old bike running, with things scavenged from somebody's clunker in the back 40!

I do feel that it may be worth investigating, it can be another option when other things don't work out. (Was gonna offer you my extra, but then I remembered that I also use the same carbon to replace my electric drill contacts)-see, I'm a good 30 miles from someone who might even stock them, so at times it is more prudent to look at the alternative.
(Your best bet is preferably an old V.W. dealer/ repair guy with Grateful Dead stickers and long grey hair and beard....)

#446753 - 07/30/12 2:22 am Re: K2F magneto brushes - too soft??? [Re: Magnetoman]  
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Magnetoman- as you pointed out that although the plastic is softer-it may be rougher. Would a possible solution be to polish the slip ring with up to approximately a 2000 grit, much like a painter would do near the final finish on a tank or fenders? This would remove much of the roughness- presumeably roughness from the actual casting process of the ring.

#446765 - 07/30/12 3:08 am Re: K2F magneto brushes - too soft??? [Re: Deadstiffcatt]  
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Originally Posted By: Deadstiffcatt
Magnetoman- as you pointed out that although the plastic is softer-it may be rougher. Would a possible solution be to polish the slip ring with up to approximately a 2000 grit, much like a painter would do near the final finish on a tank or fenders? This would remove much of the roughness- presumeably roughness from the actual casting process of the ring.
Polishing the surface of a material that is intrinsically abrasive would reduce the initial wear on the brush. But, even if the slip ring weren't polished to start, once the brush itself polished it smooth (wearing itself faster in doing so), the abrasive properties of the material would continue to wear away the brush faster than a non-abrasive material would. Think of highly polished sandstone. Things slip nicely across the surface, but as the surface wears new pieces of grit are constantly exposed. This means that in the long run an initially-smooth sandstone aways would wear things faster than an initially-rough aluminum.

#446773 - 07/30/12 7:49 am Re: K2F magneto brushes - too soft??? [Re: Magnetoman]  
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Originally Posted By: Magnetoman
Originally Posted By: Deadstiffcatt
Magnetoman- as you pointed out that although the plastic is softer-it may be rougher. Would a possible solution be to polish the slip ring with up to approximately a 2000 grit, much like a painter would do near the final finish on a tank or fenders? This would remove much of the roughness- presumeably roughness from the actual casting process of the ring.
Polishing the surface of a material that is intrinsically abrasive would reduce the initial wear on the brush. But, even if the slip ring weren't polished to start, once the brush itself polished it smooth (wearing itself faster in doing so), the abrasive properties of the material would continue to wear away the brush faster than a non-abrasive material would. Think of highly polished sandstone. Things slip nicely across the surface, but as the surface wears new pieces of grit are constantly exposed. This means that in the long run an initially-smooth sandstone aways would wear things faster than an initially-rough aluminum.


The slip ring in my mag is new, replaced by Tony Cooper

Which begs the question, what material is it made from and what surface finish does it have? Something else to investigate......

Is rapid wear normal when the brushes and slip ring are new and need to bed in?

I'll do a pencil lead test on the brushes when I get back from work tonight


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#446816 - 07/30/12 4:15 pm Re: K2F magneto brushes - too soft??? [Re: Mattsta]  
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Originally Posted By: Mattsta
The slip ring in my mag is new, replaced by Tony Cooper

Which begs the question, what material is it made from and what surface finish does it have? Something else to investigate......

Is rapid wear normal when the brushes and slip ring are new and need to bed in?
To answer your last question first, yes, it is quite possible that the problem with excess carbon is due to an initially bedding in period. It depends on how smooth the surface of the slip ring was when it left the factory. But, it's equally possible the excess carbon is because the brushes are too soft.

I looked at the results of my "pencil test" again. this time under a microscope. Except when I pushed very hard on the Lucas HT brush on one of the passes I made with it, the marks it left are almost entirely due to deformation of the paper surface rather than deposition of carbon. That is, if you take a paper clip and use it like a pencil, the grooves it leaves in the paper are visible even though no material is deposited. It takes only a tiny sprinkling of carbon to make such grooves stand out against the white background. The marks left by the other brushes deposited somewhat less carbon than did the No. 2 pencil, but the marks left by the Lucas HT brush deposited almost no carbon at all.

Even the mark made when I pushed on it harder than the rest has significantly less carbon -- much of the additional "darkness" of that line is an illusion caused by the greater width of the depression in the paper surface caused by the greater pressure, coupled with a bit more carbon. Anyway, there is no doubt the HT brushes are significantly harder than the brushes Lucas used for the metal earthing surface or for dynamos.

#446850 - 07/30/12 10:08 pm Re: K2F magneto brushes - too soft??? [Re: Magnetoman]  
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Originally Posted By: Magnetoman
Anyway, there is no doubt the HT brushes are significantly harder than the brushes Lucas used for the metal earthing surface or for dynamos.
Since yesterday I started a thread on restoring a magneto in the Members Bike Projects forum, I signed up with Photobucket in order to post images. One that will appear again in that restoration thread is:

What you see from left to right is a composite of three micrographs at the same magnification of the trace on writing paper from a No. 2 pencil, a brush that was in the Bosch ZEV I recently restored, and a NOS Lucas magneto brush that I put into the magneto in place of the too-soft brush that was in it. As can be seen from these micrographs, the brushes that Lucas supplied for their HT pickups were significantly harder than a No. 2 pencil.

I think this can be a reasonably reliable test. Sharpen a No 2 (or HB) pencil and make some scribbles to blunt the tip a bit. Then sign your name with the pencil using the force you normally use when doing so. Then make a few marks below your signature holding the brush as if it were a stubby pencil, trying to use the same force as when you signed your name, and without rotating the brush to expose fresh surfaces as you mark (because the outer surface may be coated with softer particles). If the marks are roughly as dark as your signature, the brush is too soft. If the marks are significantly lighter, but some carbon is present, they're probably OK. If there is no sign of carbon at all, they may be too hard.

#446906 - 07/31/12 7:43 am Re: K2F magneto brushes - too soft??? [Re: Magnetoman]  
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A simple but very revealing test

I will try this myself one evening this week.


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#446960 - 07/31/12 4:33 pm Re: K2F magneto brushes - too soft??? [Re: Mattsta]  
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Originally Posted By: Mattsta
The springs on the brushes supplied by Tony Cooper seem slightly stronger and more robust than the ones supplied by Dave Lindsay.
I devised a test to check for this. After measuring the depth of a few pickups, and the length of a few brushes, I decided that compressing the spring such that the end of the brush was 5mm above the surface would be a good average value to use to simulate the pressure on the brushes when they were partially worn. Pressure = Force/area, so if the springs have the incorrect spring constant, they will exert either too much or too little force on the rubbing surface of the brush. Measuring the value only requires a postal scale and a piece of paper bent at 90-deg. (which is too light to affect the measurement). I used a more precise scale, that measures to 0.01 grams, but the values I found correspond to ~1 oz., which is the max. weight of a letter in the U.S. requiring a first class stamp. Anyway, the following image shows how the measurement is made:



It's easiest to do this with a book or other support under your wrist to steady your hand, and a second person to call out the weight on the scale when you have it at precisely 5mm (if it's off by 1mm, the weight will be off by ~20%, because the force is linear in displacement). Doing this, I found I could measure the weight to within an uncertainty of ~+/-2 g out of the ~30 g. What I found was a Lucas HT brush "weighed" 28 g, but the brush I had removed from the Bosch KEV was 25% higher at 35 g. In fact, I could feel that the "KEV" spring was stronger before even knowing the "weight."

What this means is that not only was the material in the "KEV" brush significantly softer than in a proper one (so it would have worn away faster, coating the slip ring with conductive carbon), it was subjected to 25% greater pressure on the slip ring (wearing it away faster still).

Note that I only measured two Lucas brushes, both from the same box, so I have no idea if these values are representative of all their HT brushes. However, since brush pressure is an important parameter in their performance, I would be surprised if Lucas allowed variations larger than 10%.

#447030 - 08/01/12 12:33 am Re: K2F magneto brushes - too soft??? [Re: ]  
Joined: Nov 2011
Posts: 3,983
Magnetoman Online content
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Magnetoman  Online Content

BritBike Forum member

Joined: Nov 2011
Posts: 3,983
U.S.
Originally Posted By: overandout
You should get out more often!
I do (e.g. I'm taking a ~500-mi. ride this weekend). But, my workshop is very well equipped, so it took less than 15 min. to measure those springs, and no more than than another 15 min. to pull the photos into Photoshop, upload the composite to Photobucket, and write the post. Besides, this particular topic is one that can't be brushed off...


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