Why diamagnetic?

[mph]

I recently replaced my Honda's distributor cap and rotor. The rotor (#3) is held to the shaft with a 5x12mm screw (#12):

When I installed the rotor, I tore up the screw's little phillips head, and re-installed it using Vise-Grips. Since I've read several accounts of this screw coming loose and wrecking the distributor, I decided to get a fresh screw and use a little Loctite on it.

The factory service manual specifies that the screw is to be diamagnetic (i.e. non-magnetic, vs. "ferromagnetic" like ordinary steel). I went to the best local hardware store, and got a stainless screw, but they only stocked Allen head screws in that material and size.

I wasn't sure that the larger Allen head would clear the splash shield as the rotor turned, so I went to the dealer to buy the Genuine Honda Screw(*). I picked it off the parts diagram on their computer. It turned out to be very much ferromagnetic.

When I installed the new screw, I realized that the old one was also ferromagnetic. It's always run fine, as far as I can tell, so I guess it doesn't really matter. But I'm left wondering... why the heck did Honda spec a diamagnetic screw in the first place?

The rotor itself is plastic with non-magnetic contacts, but the sleeve in the center, through which the screw passes, is ferromagnetic. So it's not really clear to me why adding a ferromagnetic screw would really make a difference (and, apparently, it doesn't).

(*) This term is sometimes used with a meaning not related to fasteners, but only the literal definition applies here. The screw only set me back $0.52, tax included.

 

[Pablo]

Cheap screw.

I can only imagine Honda didn't want the magnetic personality of the screw to mess with the Hall effect sensor (like) device. I'm not a Honda guy, but maybe earlier versions were more susceptible.

 

[Greaser]

Opps..I did a Honda last year...and the same thing happed to me...stripped the darn phillips screw out...I just went to my screw bin and put in a hex head with a new lock washer...figured it would be easier doing it next time if I can use a socket head instead of a lousy phillips...didn't know if it was magnetic or not...must have not been because the guy says the car runs great and just passed emmisions with flying colors.

 

[mph]

quote:

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Originally posted by Greaser:
figured it would be easier doing it next time if I can use a socket head instead of a lousy phillips

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Yeah, that's what I wanted to do. I was 90% sure that the socket head would clear... but I didn't want to tear anything up if it didn't.

But I got the old screw out with Vise Grips, and I'm sure I can get this one out the same way if I have to.

 

[labman]

I am glad some of the rest of you feel about Phillips Head screws as I do. They are nice going in, but I hate trying to geet old ones out. I kind of like the ones that have the Phillips and a slot in them.

 

[Greaser]

quote:

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Originally posted by labman:
I am glad some of the rest of you feel about Phillips Head screws as I do. They are nice going in, but I hate trying to geet old ones out. I kind of like the ones that have the Phillips and a slot in them.

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Actually I broke off the rotor to cut a slot in the head,with my Dremel, to get the striped phillips out..still a no go...called Honda on the phone to ask whats up here...they told me to wack the screw with a hand-held impact...on a distribuator shaft!...I don't think so..oh he told me not to bend the shaft cause the unit costs around $600...antways I used a hex bolt and a new lock washer..a little Yankee engineering here...Japanese love those darn phillips

 

[JohnBrowning]

I do not mind phillips if the fastner is grade 8. If not then you chances of removeing it without ruining it deminish directly proportional to it's size. On a positive note at least everyone owns phillips screw drivers. What I hate is when companys decide to use protected torq. or star fastner. It seems like the average joe never have the size he need. This is one thing I like about most Toyota products. Any monkey with a 110 piece craftsman tool set and few more tools can do 98% of the repairs with 10-19mm wrench's and socets, feeler guage set,assorted screw drivers and a set of snap ring pliers and a large c clamp!!! A vise or arbor press comes in handy but not a must have. He does not need $35-$100 dollars in Torg.'s $14-$40 in protected torq's and $25-$75 in star bits.

Once while I was in college I had to change a cam shaft in my OHC Toyota. I had promissed myself that I would never turn a wrench again when I went to college. So I had only brought a very small assortment of tools for emergency pureposes and left the rest of my tool 1200 miles away at mom and dads. I changed my cam witch required me to remove the head bolts, rocker rail, cam drive sprocet and chain. I did it in 30 minutes useing combination wrench's and a gimby 46 piece socket set.I ended up useing combination wrench's to remove head bolt and to turn crankshaft. I did not even hacve a torq. wrench. My roomates watched in amazement! P.S. I had measured my valve clearance cold prior to doing this so I could skip the the step of warming it up to operateing temp. I did that later just to make sure. This was the only deviation from proper procedure.

[ July 10, 2003, 09:21 PM: Message edited by: JohnBrowning ]

 

[tom slick]

a trick i have found for those pesky japanese phillips screws it to take a #1 phillips and grind the tip off just a bit, maybe 1/32" to 1/16" off. then it will fit deeper into the screw and fit much more snug. it helps alot on those types of screws.

 

[QuadDriver]

quote:

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Originally posted by mph:
I recently replaced my Honda's distributor cap and rotor. The rotor (#3) is held to the shaft with a 5x12mm screw (#12):

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From the image, it looks as if the rotor serves as both spark distribution but also contains the ferrule for the pickup? if so that would be why, the resulting disruption to the magnetic field would alter the edge timing juuuuuust a wee bit. but as you said, if honda didnt care why should you?

 

[Greaser]

quote:

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Originally posted by tom slick:
a trick i have found for those pesky japanese phillips screws it to take a #1 phillips and grind the tip off just a bit, maybe 1/32" to 1/16" off. then it will fit deeper into the screw and fit much more snug. it helps alot on those types of screws.

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Good idea..

 

[mph]

quote:

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Originally posted by QuadDriver:
]From the image, it looks as if the rotor serves as both spark distribution but also contains the ferrule for the pickup?

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You're referring to a TDC or similar pickup? I think that the only metal bits on the rotor were the coil contact, the spark distribution contact (both non-ferromagnetic), and a steel collar on the shaft. But I can check the old one later, maybe I missed something.

Maybe a ferromagnetic screw is only a problem if it's taken on a permanent magnetic field (e.g. through the use of a magnetic screwdriver)?

 

[QuadDriver]

quote:

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Originally posted by mph:

quote:

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Originally posted by QuadDriver:
]From the image, it looks as if the rotor serves as both spark distribution but also contains the ferrule for the pickup?

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You're referring to a TDC or similar pickup? I think that the only metal bits on the rotor were the coil contact, the spark distribution contact (both non-ferromagnetic), and a steel collar on the shaft. But I can check the old one later, maybe I missed something.

Maybe a ferromagnetic screw is only a problem if it's taken on a permanent magnetic field (e.g. through the use of a magnetic screwdriver)?

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well from the diagram you have given....part number 6 covers the entire ign pickup (I dont know for sure that it is hall effect) It looks like the 'bottom' or part 3 (the rotor) extends thru part 6 (the cover) and would pass in front of the pickup (the partial circle looking thing - not exploded on the left of the whole picture - attached to the dist) Unless of course the actual ferrule used to trigger the ign is on a reluctor wheel already on the shaft, but like I said, that part of the diagram is not exploded

 

[mph]

quote:

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Originally posted by MolaKule:
A diamagnetic material IS attracted by magnets and other magnetic fields,

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No, diamagnetic materials are weakly repelled by magnetic fields. Maybe you're thinking of paramagnetic materials?

I figured non-magnetic was a reasonable simplification for automotive discussions.

 

[MolaKule]

Ohhh, Ughhh - physics (frothing at the mouth):

A diamagnetic material IS attracted by magnets and other magnetic fields, except for the fact the permeability of the diamagnetic material is different than that for normal iron/steel/cobalt/nickel (ferromagnetic) type materials. A diamagnetic material has a different susceptibility which makes its response to magnetic fields much differerent.

It MAY affect the sensitivity of the Hall trigger if another type of screw material is substituted for the diamagnetic screw.