Rotor Arm; Lot-o-Harm?

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Brucey
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Rotor Arm; Lot-o-Harm?

Post by Brucey »

with perfect hindsight....I guess it was the tunnel that did it;

-administered the Coup de Grace, that is.

I'd wound my windows down, selected second gear and given it the berries. Just to hear the sound of it.

A couple of miles further down the road -technically a motorway at that point- the car hiccupped, lurched, spat flame out of the exhaust and then picked up. A few seconds later it did the same thing again; by this time I was headed for the side of the road; stopping in the fast lane didn't seem like a good idea.

I spluttered to a halt right at the end of a long, uphill entry slip road (on-ramp). Already I was cursing my luck; only weeks earlier I'd changed insurance companies, and whilst the old policy had roadside recovery included, the new one didn't, and I'd not yet sorted out anything separately.

I heaved a sigh, and considered my options.

On the minus side....Forty miles from home, technically on a motorway, an unknown (and possibly terminal) fault, no phone, no friends or family anywhere near to hand, it looks set to rain anytime soon.

On the plus side; some tools, some spares (including crank sensors, belts, fuses) some other odds and ends, a full set of wiring schematics, a multimeter, four hours of daylight to play with, and I'm stopped on a hill.

A few minutes work showed me that I had volts on the coil, volts at the injectors, and the crank sensors had the right resistance. Also that the coil was seeing an AC signal when cranking.

A few more tests showed a very healthy spark from the king lead, but none on #6. I began to suspect the rotor arm had spat the dummy.

I decided to roll the car backwards down the hill a little way, and into the verge so that I was off the road, perhaps well enough/far enough so as not to be 'stopped on a motorway'. If the law turned up and pressed the point I'd have to be towed, which is a £100+ touch 'off a motorway' even if it is only half a mile.

Sure enough a Police car came after a few minutes; after a little friendly chat they accepted that -provided I put my warning triangle out, and worked on one side of the car only- they would turn a blind eye and let me work on the car for a while, maybe leave it there for a few hours if I needed to go fetch a spare part. Good lads. I duly put my triangle out and just for good measure I found a couple of traffic cones in the hedge and put those out too.

I was of course already feeling doubly stupid because the rotor arm had been on the hit list for some time. I even had a spare one at home, somewhere; I'd meant to put it in the car so that if it failed, I have it when I needed it. Gah.

After the usual knuckle-scraping procedure, I'd removed the distributor cap. On my car there isn't much clearance, and I have to remove the upper radiator mountings; the extra 1/4" this gives me makes all the difference.

Sure enough the rotor arm came out in pieces. I guess it had been cracked for a while, and the high RPMs had done for it. The last 1/4" of the arm was missing, and only two of the bolts were holding the arm on; the third one's lug was broken off and clearly had been for a while.

So what to do? Never say die!

To cut a long story short, I improvised using a swiss army knife, a zip tie, and a bicycle spoke, and made something that looked a bit like a rotor arm. I fitted it and although I had to adjust it after a few miles, and it still misfired at times, it got me home. Three or four hours later than I'd expected, filthy dirty, hungry and utterly exhausted. But home, and home under my own steam.

If anyone is interested I can post pictures of my lash-up. To anyone who has a rotor arm that is a few years old; don't be dumb like me... :roll: #-o

-Inspect it!
-Carry a spare!

cheers
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Post by hornhospital »

So I need to carry a bicycle spoke now?

:lol:

I'd love to see your ingenious make-do! =D>
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Post by jacko »

After many years of driving 20+ year old Volvo 240s, I had developed the practice of routinely changing all hoses, belts and ignition components as a part of their PM programs.... and the parts removed always went into the tool box in the huge trunk. But I'd love to see a photo of your "spoktor"! Necessity is.... :-)
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Re: Rotor Arm; Lot-o-Harm?

Post by Sprocket »

Brucey wrote:....

To cut a long story short, I improvised using a swiss army knife, a zip tie, and a bicycle spoke, and made something that looked a bit like a rotor arm. I fitted it and although I had to adjust it after a few miles, and it still misfired at times, it got me home. ...
cheers
Good one, you had me up to that part :wink:

You blokes have an April Fool's Day over there, too?
Last edited by Sprocket on Tue Apr 08, 2014 12:28 am, edited 1 time in total.
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Post by Ralph in Socal »

Wondering how many, like me, would have just called in a tow truck and settled for the shame of being beaten by the car. You, Brucey, of anyone else here, would be the only one I would rely on the get something like that done. Much less to troubleshoot on the road and figure out what the problem actually was. Amazed and impressed over here.


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Post by hornhospital »

Reminds me of replacing the burned out points in a '57 Chevy with pieces of a snuff can. :wink:
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Post by wattsmonkey »

Brucey, I think everyone needs to see the picture!
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Post by markh »

Look'n like we got a real live MacGyver among us!
https://www.youtube.com/watch?v=09UlB17cgKw :lol:
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Post by Brucey »

well here is a picture of the 'spoktor';

The spoke is retained by a groove in the rotor arm body, and the zip tie is to make sure it doesn't come adrift.

It would have worked better than it did if I had left a larger clearance between the spoke and the bolt heads; as it was it misfired somewhat.

The failed rotor arm was an Italian made pattern part. I think the failed lug may have preceded the arm breaking up entirely; the pieces didn't even fit very well, so presumably had time to distort in service.

The new one is a German madede 'Bremi' part I bought from ECP.

I upgraded the bolts to 4mm hex headed ones (that I machined up myself) because the replacement screws have 3mm socket heads (not big enough) or cheeseheads (not easy to use in the space). I didn't tighten them excessively (this may be what caused the old arm to fail in part) and I used about 1" of PTFE tape on the screws so that they won't back out in the event of any loosening.

Call me daft but only when doing the job the third time did I work out to tether the radiator hard up against the front panel so that it would be well out of the way....prior to that I had to push it out of the way every time and it kept flopping backwards...

I also made a special tool from an 8mm hexagon socket, modded so that I could drive it using a backlash free ratchet wrench. This was an improvement for the distributor screws but not a complete solution.

cheers
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the 'spoktor'
the 'spoktor'
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Last edited by Brucey on Tue Apr 08, 2014 7:59 pm, edited 1 time in total.
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Post by tschultz »

Very nice-- what is the expected life of the rotor? I have had the same one in my E28 for the past 5 years and 60k miles (100k km). I'm thinking about replacing it with the new one I have sitting in my storage unit...
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Post by GripGreg »

That's MR. MacGyver!!! =D> :wink:
And I totally agree with Ralph! He just beat me to it! 8)
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Post by GripGreg »

While we're here; does anyone know what size rotor I need for my Buster?
I hear there two sizes? Is it front to rear or side to side? Build date is 9/81.
Thanx in advance,,,,Greg
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Post by Brucey »

my theory (and it is only a theory) is that the insulating material (which is described in some places as 'polyester'; is this right...?) expands and contracts somewhat with changing conditions.

Temperature, certainly, perhaps moisture content too. It is also possible that there is some kind of long-term distortion or shrinkage in the moulding which may cause it to crack.

My failed rotor had cracked one mounting lug right off, and there were many cracks in the arm itself. The common feature with these places is that these positions were also those where there were metal inserts in the moulding. Weirdly it seemed that the cracked parts at the broken lug ought to fit together again, but don't; they are held apart by the metal bushing, as if the plastic had shrunk, causing it to split.

So it might be that any such arm may fail eventually; it may be that it is something to do with the moulding materials or even the design; there are several different designs and many different makers for this part. I have noted several reports of similarly mysteriously shattered rotors, but relatively few where someone has found a rotor cracking before it failed.

It is a source of some comfort to me that the new rotors I have ( I bought two; one to fit and another to have in the car as a spare) are German made and look to be of good quality.

I'd suggest a careful inspection of an old rotor and replacing it if there are any signs of cracking, or any disband between the metal parts and the insulating material.

If anyone has any pictures of similarly failed rotors maybe we can figure out what causes them to break, or if some brands are more likely to fail than others.

cheers
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Post by Brucey »

GripGreg wrote:While we're here; does anyone know what size rotor I need for my Buster?
I hear there two sizes? Is it front to rear or side to side? Build date is 9/81.
Thanx in advance,,,,Greg
I think you need the earliest Motronic version (9/80 to 1/84) which is

12111 286 072

http://2009.bmwfans.info/parts/catalog/ ... strib_c-3/

This (and the cap I think) are different to the later versions. I don't know if they are more or less reliable on average but I have anecdotally seen and heard of fewer of these earlier rotors that have smashed themselves to pieces, but seen more caps that have cracked. Swings and roundabouts, then.... :roll:

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Post by jacko »

I'd be surprised if they were not Bakelite.
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Post by Brucey »

jacko wrote:I'd be surprised if they were not Bakelite.
well that is what I always thought before, and (to me) that is what the cap still looks like; you can tell it is filled with something because if you scratch it, it goes powdery in a particular way. The smell is distinctive, too.

I shall check my busted arm again but I don't think it had either characteristic.

The new one was listed as being in polyester resin. That might be true; such materials do exist, e.g.

http://www.matweb.com/search/datasheett ... 5d9043e1a0

edit; a few examples of rotors;

the old rotor arm was this pattern;

Image

but there are lots of others; perhaps they are not all in the some kind of material...?

Image

this Bosch one looks a bit manky but isn't actually falling apart despite being very old;

Image

But this bosch one (from an audi UR-Quattro) has gone as stage or two further. I think the construction is similar to the E24 part.

Image

cheers
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Post by Alpinacsi »

GripGreg wrote:While we're here; does anyone know what size rotor I need for my Buster?
I hear there two sizes? Is it front to rear or side to side? Build date is 9/81.
Thanx in advance,,,,Greg
Early motronic came with a slip on rotor and the respective cap and the later ones used a bolt on rotor and respective cap. But many of the early ones were converted to the newer style (I believe this was a BMW campaign that dictated the change). Plug wires were also dictated by the cap used.
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Post by GripGreg »

New info:
My Buster has the Motronic distributor that is tri-angular.
My Rosallina has the round distributor that I've seen forever.
Is there another style, like early Motronic & later Motronic?
Buster is a Series 1.
Will someone post pictures of early & later styles? Or, was this info enough? :-?

Also, I can't find any parts for a 635; only 633! Is there a difference in the electrical area?

I see the distributor pictured.
Thanx in advance,,,,Greg
Ps,,Napa has the Bosch cap made in Spain selling for about$165. PartsGeek has the Bosch cap for $65.
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Post by jacko »

Brucie, you can blame the greens..... I believe filled PBT (and PET ) alloys are being used as substitutes for Bakelite now but some. Note... they ARE more brittle .... but then I guess you already KNEW that! ;-)
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Post by Brucey »

GripGreg wrote: Is there another style, like early Motronic & later Motronic?.....
if you look at the BMWFANS ETK link I posted earlier you will see that you can go to adjacent pages (using the 'next illustration' and 'previous illustration' controls) and there you can see the different parts used. I have pulled up an 'all dates' listing there for the Euro 635 model.

There are at least two different motronic distributor caps (that look near-identical externally) but have different rotor arms inside them. Contemporaneous 633 ignition parts are usually the same as 635 parts (and you can check this by clicking on the part number in the ETK listing, which will list other BMW vehicles using the same part)

If early parts were exchanged on a recall campaign for later ones as ALPINACSI suggests, then yours could already be changed for later ones.

Bottom line; you will have to look and see your own parts to be sure of what you have.

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Post by GripGreg »

Thanx very much, Brucey and Alpina Dude!!
The next & previous illustrations made a major difference!
They showed me that I need to look at my rotor to know for sure.
I like the way you can post a quote into your message. It's very trick.
I guess because I don't know how.:-?

It looks like Buster has been updated. But, there appears to be a nut on
threads holding the cap on, and I don't recall seeing an outer cover.
Thanx for your input,,,,Greg
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Re: Rotor Arm; Lot-o-Harm?

Post by plip1953 »

Resurrecting an old thread, is there any particular reason why there is no mention of OE rotor arms? Or is just that they are very expensive relative to adequate quality aftermarket versions?

Also, is it correct that any kind of cleaning/scraping, smoothing of the rotor edge is not a good idea?
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Re: Rotor Arm; Lot-o-Harm?

Post by Pod »

plip1953 wrote: Fri Aug 17, 2018 6:08 pm

Also, is it correct that any kind of cleaning/scraping, smoothing of the rotor edge is not a good idea?
I’ve always cleaned rotor arms with fine emery cloth..............
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Re: Rotor Arm; Lot-o-Harm?

Post by Jono B good »

Or you can get a jewelers file and magnifying glass and also get the contacts inside the cap as well. The brass contacts on the cap and rotor should be shiney.
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Re: Rotor Arm; Lot-o-Harm?

Post by Brucey »

the contacts on the end of the rotor arm and inside the cap have sparks jumping across the gap all the time, and this causes them to wear. It isn't vitally important that the metal parts are clean and shiny, but they probably wear a bit more slowly if they are smooth rather than rough. Note that the debris from the wear is potentially more of a worry; this, if enough of it is present on the insulator, can help to promote tracking.

Eventually the metal parts wear enough that the gap is bigger than one would like, and it is time for new parts.

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