Making a distributor cap

[Peter Twissell]

Not sure it looks good, but it will serve the purpose for bench testing the ignition system.
The polyester is the cheap variety, sold on Ebay for making paperweights with things embedded. I think it was about £8 for 500ml and I used 50ml to make this - so 80p per moulding!

The final part will be in a more suitable resin, after I have learnt how to do the process reliably.

 

[awake]

I've not done any resin casting up to this point, so I am asking out of ignorance - what would be a more suitable resin, and why? Better looks, or ??

And also asked out of ignorance: must a distributor be fully enclosed? Certainly for "real" use one would want to keep out water &c., but for model use, could one make an "open" distributor??

 

[Peter Twissell]

Ideally I would use a phenolic resin, because it has a high dielectric constant (resists arcing through) and does not absorb water.
I will probably end up using polyurethane, which is almost as good and is available.

There is no reason why a model engine distributor needs to be fully enclosed. The rotary engines used in early aircraft had a 'distributor' which consisted of a fixed exposed terminal from which an arc jumped to terminals mounted on the spinning engine, each connecting to and passing current to the spark plug electrode.

 

[jeff10049]

I came across this thread as I need to make a magneto slip ring with good dielectric strength and was searching for a castable resin that would work. Nice job on the cap have you cast any in urethane yet? if so how are they holding up?
I did come across this potting resin and thought it might be promising for casting such things it's affordable enough but I haven't tried it yet thought Id post it here for others as a possible option. Or if you're looking for another option.
Jeff
https://www.amazon.com/Epoxyseal-Electronic-Grade-Potting-Epoxy/product-reviews/B00VXV9YZ2

 

[Peter Twissell]

Hi Jeff,
I have not cast a cap in urethane yet. I have taken a hiatus from the radial engine for a while.
I am still hoping to source some phenolic resin to do the job 'properly'.
I can only find safety data sheets for the Epoxyseal 9000 - no dielectric properties.

One significant discovery I have made is that phenolic resin is more hygroscopic than I thought and that commercial distributor caps rely on a coating of laquer to keep them dry.
I suspect most plastics could be used with an appropriate surface seal.

 

[Steamchick]

A fascinating thread...
I designed a distributor with a magnetic link to the HT coils per cylinder, from a single primary coil, which eliminated distributed HV sparks. Patent belonged to the company. But the "better" option was individual coils on spark plugs, as per most cars since then.
I think you are brilliant to make your own distributor caps.
K2

 

[Peter Twissell]

A fascinating thread...
I designed a distributor with a magnetic link to the HT coils per cylinder, from a single primary coil, which eliminated distributed HV sparks. Patent belonged to the company. But the "better" option was individual coils on spark plugs, as per most cars since then.
I think you are brilliant to make your own distributor caps.
K2

Ooh, I like the magnetic pole switching idea!

 

[jeff10049]

Thanks for the reply, I also have not found a way to cast phenolic other than thermally set types that come in the form of beads molded under heat and pressure in the dies. The link to the potting resin was just an example I was hoping to find something with more data and traceable certifications. I assume it may exist I think I will search avionics potting resins and see what I find. That's interesting about the hygroscopic nature of the phenolic maybe I am overthinking this.
I remember seeing clear distributor caps at car shows they were a novelty thing but people ran them just fine as far as I know. I wonder what they were made from perhaps acrylic?

You know the more I think about this some of the automotive caps from standard ignition are a blue color and don't seem to be phenolic they will bend but not break phenolic usually breaks in my experience but they may still be some form of phenolic. I think I will send an email right to the standard ignition and see if I get a response ill keep you informed if I find anything good.
BTW the mold you made is fantastic with some modification it could potentially be used for the thermoset phenolic.

 

[Gordon]

I made a distributor cap on my 3D printer. Worked OK and easy to make.

 

[Steamchick]

Peter, possibly a modern neodymium magnet passing a pole would work if with a small clearance - like a magneto - but would it be accurate enough for timing?
K2

 

[Peter Twissell]

Gordon, I have no doubt a model engine will work fine with a 3d printed cap in almost any plastic.
However, my 2.3 litre radial is intended to be mounted on the front of a road vehicle, so it will need to tolerate weather.

 

[Peter Twissell]

Peter, possibly a modern neodymium magnet passing a pole would work if with a small clearance - like a magneto - but would it be accurate enough for timing?
K2

Works for lawnmower engines!

 

[Ken I]

However, my 2.3 litre radial is intended to be mounted on the front of a road vehicle, so it will need to tolerate weather.

Oooohhhhhh - pictures please - now you've got my interest piqued.

Mentally I'm picturing something like a Morgan V-Twin but noisier and way cooler.

You can't just drop something like that into a conversation.

Can't wait.

Regards, Ken I

 

[Scott_M]

You can't just drop something like that into a conversation.

My thoughts exactly !!

Do tell

Scott

 

[william_b_noble]

instead of phenolic, I think you should consider FRP, you can build it up and cut it with carbide tools and it is commonly used in high voltage applications. There are also machinable ceramics that you can machine to what you want, and then you heat them to make them hard and rather strong. and Finally(!) most UMHW plastics are suitable, aren't they? just avoid the ones with carbon as a filler

 

[Peter Twissell]

FRP is fibre reinforced plastic.
Strength is not a problem.

 

[Peter Twissell]

The intended application for the radial engine (which this distributor is a part of) is a Morgan layout three wheeler. I have it designed in CAD, but no building until the engine is up and running.
The trike has a bit of a trick up its sleeve.
But that's another story entirely.

 

[william_b_noble]

The intended application for the radial engine (which this distributor is a part of) is a Morgan layout three wheeler. I have it designed in CAD, but no building until the engine is up and running.
The trike has a bit of a trick up its sleeve.
But that's another story entirely.

if this is a full scale model, it might be worth checking with the Morgan clubs, there are active 3-wheeler clubs (one is near me) - I have a 4-wheeled one. there are also some smaller production engines with distributor caps - if you were to find one that met your needs aesthetically, it might save you a lot of maintenance hassle if you adjusted the design to use an existing part for both the distributor cap and the rotor. Alternatively, you could hide coil packs as used on more modern cars and motorcycles somewhere and route the wires through a "fake" distributor to get the look you want.

as an aside, I'm never going to do this, but for decades I have thought that a BMW motorcycle engine (of the type with a driveshaft to the rear wheels) would be excellent for a 3-wheeler type of car, with front leaf springs shackled at the front and back of the cylinders. done right it would look amazing, and probably handle about like a Morgan (e.g. an excellent car for drifting in a turn but not nearly as fast as a Porsche or similar car on the same turn - I have done the experiment ...) now back out of that dream .... "hello real world"

 

[Peter Twissell]

William, if you are aware of any engine which has a 7 outlet distributor, I would be very grateful for the pointer.

 

[l.dullum]

I have machined several distributor caps out of acetal. Machines easily, the only issue is a high coefficient of expansion, about 4 times that of aluminum, which has to be taken into account.