Make distributor cap in Lexan/Polycarbonate

[MBfreak]

Thinking about turning/milling distributor caps in Lexan.
Do our material Gurus consider this possible?

The reason is that the Bosch distributor caps for MBs from the 90´s are very poor quality and the aftermarket even worse.

Sits in a MB 500 V8 ( W140) 1994

Ola

 

[sberry]

I got to admit, this would have never occured to me.

 

[APEowner]

What's the failure mode of the Bosch caps?

 

[MBfreak]

The failurre mode is that the part is hygroscopic and thus loses the insulation properties. The spark voltage is shunted to earth thru the material in the cap.
By heating the cap to 110 Centigrade for several hours, then wiping it with acetone and spray it with clear varnish it can be reused. For som months, at least.
But this only if the cap has not been used for several minutes trying to get the car to run.
This will create carbon tracks inside the material, clearly visible if the cap is cut in several pieces.

Ola

 

[Robbie B]

Idk what brands y’all have in Sweden but here in the states the Napa brand Belden stuff is usually pretty good in the aftermarket electrical arena. I don’t see why you couldn’t make a cap out of lexan or the like but putting the electrical components in it reliably may be the hard part.

 

[metlmunchr]

Polycarbonate is hygroscopic.

 

[MoonRise]

Nope.

Polycarbonate (aka LEXAN) is NOT resistant to gasoline or acetone or benzene or methanol or ethanol or propylene glycol (antifreeze).

http://www.bayplastics.co.uk/downlo...carbonate Chemical Resistance Information.pdf

Maybe try PTFE (aka Teflon)?

Good chemical resistance and good electrical properties, but not as 'strong' or stiff as polycarbonate.

https://www.professionalplastics.com/professionalplastics/ElectricalPropertiesofPlastics.pdf

https://www.professionalplastics.com/professionalplastics/content/Teflon_PTFE.pdf

Although I see that distributor caps are 'typically' made of alkyd plastic or mineral-filled plastics.

Although I would just get a NEW Bosch 03367 distributor cap.

example (no affiliation, just a quick web search on "Bosch 03367 cap):

https://www.carparts.com/distributor-cap/bosch/bs03367

 

[bushmechanic]

High density polyethylene can be shaped well and resists a hell of a lot of stuff; it's about one step down from Teflon, in a way.

Coolant bottles and other reservoirs are often made of the stuff. It's incredibly durable when reinforced, thick, or buttressed for lighter weight, and can indeed be machined.

 

[APEowner]

The failurre mode is that the part is hygroscopic and thus loses the insulation properties. The spark voltage is shunted to earth thru the material in the cap.
By heating the cap to 110 Centigrade for several hours, then wiping it with acetone and spray it with clear varnish it can be reused. For som months, at least.
But this only if the cap has not been used for several minutes trying to get the car to run.
This will create carbon tracks inside the material, clearly visible if the cap is cut in several pieces.

Ola

That's pretty un-Bosch like. I can't help but wonder if there isn't something else going on.

In any case, Polycarbonate is probably not not the best choice for a replacement material. My first thought for a material is Ultem. It has the required dielectric strength and chemical resistance. I cant remember what the temperature range is off the top of my head.

 

[rbgearz]

Just got the plans for building a mini v8 engine and a mini inline 4 engine. Going to start on the 4 cyl. soon. Plans call for the distributor caps to be made from acetal. Lots of guys building these mini engines using acetal.

 

[laser3kw]

how would a 3D printed one compare? Not sure of what is available as far as materials.

 

[240sxguy]

Dump the distributor, add a crank sensor and run an aftermarket ignition arrangement.

 

[californiamilleghia]

interested as there are many cars where caps are not available , or you need to find in Europe

 

[MBfreak]

Thanks to all.
Looks like acetal or teflon are the relevant materials.
Will try to get hold of material and make a trial.
240 sxguy Your suggestion to install aftermarket ignition is good. The car has three ECUs for fuel, ignition and ****** that are running with bus communications for basic functions but also for pretty complicated drievability programs. Taking the ignition EZL/AKR out kills the entire driveline. Pity
Even the most advanced aftermarket system would be a PITO to set up.

We are now considering a somewhat later ( 1997?) W140 system which is COP for ignition .

Ola

 

[matt_i]

Depending on your capabilities, you could always graft the "top end" of another distributor onto your existing base. I'm guessing in the high voltage top there's not really any "sensitive electronics"...its just "get out of the way!!"

I would however try the parts cannon first, can you get a spendy OEM version, can you get on RockAuto (know its not right around the corner) and see what other secondary market suppliers are selling.

 

[rsanter]

Depending on your capabilities, you could always graft the "top end" of another distributor onto your existing base. I'm guessing in the high voltage top there's not really any "sensitive electronics"...its just "get out of the way!!"

I would however try the parts cannon first, can you get a spendy OEM version, can you get on RockAuto (know its not right around the corner) and see what other secondary market suppliers are selling.

This was my though.
Perhaps an adapter ring can be made to allow a different cap,to be used on your engine.
You may have the modify the rotor

 

[Ign]

You can "smear" polycarbonate with chlorinated Brakleen

Acetal is pretty squirmy (you'll drive yourself nucking futs if you try to machine a perfectly square block) and can be subject to some fairly significant expansion and contraction due to temp changes (although it's way better than HDPE)

Interesting project, please follow up with what you do!

 

[4xdog]

Transparent distributor caps were available as aftermarket accessories *many* years ago, ostensibly for fault-finding around spark.

Here's an ad for the Remax Klearcap from the Earl's Court Motor Show edition of The Autocar in October 1954.

I have yet to find one of these for my old Triumph TR3, but I'd like to try it.

My best guess, knowing polymer science back in the day, is that the typical blackish cap would have been made by a phenolic thermosetting resin (like Bakelite phenol-formaldehye) and the clear cap most likely from acrylic (like Lucite or Perspex polymethyl methacrylate). PMMA wouldn't be a bad material choice, BTW, for a hand-made distributor cap.

 

[MBfreak]

APE owner.
I fully agree that the cap problem is rather Un-Bosch like.
There is certainly nothing else going on, it is a common fault on this version of the MB 140 luxury sedan. A couple of years later the driveline electronics were updated , no more distributors. Each OEM Cap was around 2200 SEK, ie about $400 for the two needed.
I consider BOSCH a very competent company and they have made a lot of outstanding subsystems for cars and trucks, often in close cooperation with MB.
Like the mechanical direct and manifold fuel injection systems. Also they had the entire market for diesel injection systems in Europe and many othe parts of the world.
ABS was another breakthrough.

However, they have some clusterfxxxs
The hybrid fuel injction system KE Jetronic
The early COP units.

Ola

 

[matt_i]

The failurre mode is that the part is hygroscopic and thus loses the insulation properties. The spark voltage is shunted to earth thru the material in the cap.
By heating the cap to 110 Centigrade for several hours, then wiping it with acetone and spray it with clear varnish it can be reused. For som months, at least.
But this only if the cap has not been used for several minutes trying to get the car to run.
This will create carbon tracks inside the material, clearly visible if the cap is cut in several pieces.

Ola

Just thinking some more, if you had a new part that you could thoroughly cleanup, if you masked + painted the OD a couple sprayed coats that might keep moisture away from the base material.

 

[californiamilleghia]

Did they put special "Mercedes" material in the plastic injection machine ?

if Bosch had this problem on one part , why would the next batch of say Audi distributor caps not have the same problem ,

Am I missing something ?

 

[MBfreak]

california.
I have not the slightest idea.
It may be as simple as the EZL/EKR ignition module with the igniton coil MB selected has a rise time and a peak voltage that is overstressing the material.
At pulses where risetimes are several kV/µs and tops out at the spark ignition voltage, probably 15 kV, my knowledge does not even come close to predicting the behaviour of material.
So I will not speculate and do what I understand a little bit, ie make a cap in teflon.
And sink another bunch of cash in two new OEM caps.
The car is a joy to drive and ride.

Ola

 

[larry4406]

...So I will not speculate and do what I understand a little bit, ie make a cap in teflon. And sink another bunch of cash in two new OEM caps.
The car is a joy to drive and ride.
Ola

Sometimes you have to pay to play...

 

[cleaner]

PEEK is resistant to automotive chemical exposure, dimensionally stable, and can withstand elevated temp exposure. It is a good dielectric in the polymer materials.

Ultem is used for under hood auto applications but may not have the dielectric properties needed. It is available for 3D printing as well.

 

[Tostal]

Transparent distributor caps were available as aftermarket accessories *many* years ago, ostensibly for fault-finding around spark.

Here's an ad for the Remax Klearcap from the Earl's Court Motor Show edition of The Autocar in October 1954.

I have yet to find one of these for my old Triumph TR3, but I'd like to try it.

---------------------
-------------

Do you mean like this one?:-

Tostal.

 

[Tostal]

The electrical processes which take place in the distributor (arcs, glow discharges, corona effects) produce nitric oxide and ozone gases, which are poisonous and corrosive. In order to prevent the formation of acids resulting from accumulation and concentration of these gases, the distributor cap is provided with ventilation holes.

If the ventilation is inadequate, the surfaces of metal parts can corrode and the surfaces of the insulating parts can be attacked and the insulating capabilities of these parts are reduced, resulting in leakage currents.

I.E. Whether you're experiencing failure of replacement distributor caps or even making your own, don't forget those important ventilation holes.

Tostal.

 

[Tostal]

I checked the price for aftermarket distributor caps for this M-B model, over here they cost Euro 183 each, so no saving. They're described as 'premium quality', made by Bosch and the material used is 'polyester', if this is any help to the OP.

Tostal.

 

[4xdog]

Do you mean like this one?:-

Tostal.

Yep -- it would be fun to play with one of those.

 

[MBfreak]

My last input to this thread..
After having read thru several forums regarding the failure of Bosch distributor caps in W140, but also older cars with M103 engines, and tried to get the facts from these ( not so easy, most posters have difficulties with removing a M3 torx) the conclusion is:
The cap material sweats a sticky substance that captures spark eroded metal oxides and thus creates conducting tracks that kill the cap.
Interesting enough, some of the tracking takes place INSIDE the material. Increasing´the venting may help.
So, it is a maintenance item. Change oil, rotors and caps every 15000 km.
Or palm off the car and get a newer with COP.

Ola

 

[MP&C]

Many years ago, when they were more popular, we installed a clear cap from Mr. Gasket onto a newly rebuilt engine.

My experience was that the cap is less impervious to engine heat than the standard caps, such that the holes at the top for the wires would actually deform into more of an oval and "lock" the wires in place. If you did get the wire pulled out the crimped end typically was stripped off.

 

[MoonRise]

So, it is a maintenance item. Change oil, rotors and caps every 15000 km.
Ola

Old school yearly maintenance used to be change the plugs, cap, and rotor yearly (about every 12000 miles).

I'm not even going to get into setting points.

And change the oil every 3000-5000 miles.

Had an 89 Ford 302 that I used to do that to. OEM cap and rotor were terrible, went to Accel (still remember that) and things were MUCH better in the ignition system. I think the Accel ones would go about 25000 miles before I had to replace them.

IIRC, the Accel cap and rotor had decent brass as the metal and the OEM was crappy aluminum. Made a BIG difference in durability of the cap and rotor and thus the operation of the engine.

Just an FYI.

 

[MBfreak]

Moon Rise.

The best ignition system ever made is the FORD EDIS. Wasted spark, , perfect signal per cylinder, dead simple and reliable advance pulse. Will run in DBTC 10 limp home if advance word compromised.

Available for 4,6 and 8 cylinders. Can be tweaked, using two EDIS 6 for a V12.
Have thought about fitting an EDIS 8 on the MB, but drive train has three ECUs talking to each other on CAN-bus.
Would require VERY major surgery, unfortunately.
Have fitted a 2x6 EDIS and Megasquirt 1 on a Jaguar XKE V12. Fuel by MS1. Required a pulse tweaker, Also possible if 2 36-1 pulse wheels installed.

Ola