Fabricated Ignition Distributor for 4 cy. model i.c. engine

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Brian Rupnow

Design Engineer
Project of the Month Winner
May 23, 2008
Reaction score
Barrie, Ontario, Canada
This morning I'm looking for a good "how to" article on building a distributor for a model i.c. engine. I have printed out a drawing provided by George Britnel on Aug 16-2023, and it has good information, but I am looking for more. I have tried searching on Google and on the forum, but my google-fu is not working very well for me. Can anyone please direct me to a link with more information about making my own distributor for a model i.c. engine from scratch.---Please----Brian.
I've taken the time to go over George's drawing, and it's very good. There is enough info there to build a distributor from. Thank you again George. It seems that this design will do for any number of cylinders. I haven't made up my mind to build a distributor yet, but the information on George's drawing will be a great help.---Brian
Seems like 3D printing would be idea for a distributor cap, especially since you can use any number of different materials, such as nylon, etc.

And you have the 3D modeling thing down, and have for a long time, so no problem there.

3D printing might just be the cat's meow, but I don't have a 3D printer. I'm still kicking the distributor idea around in my head, but haven't decided on a course of action yet.---Brian
Hello Brian,

I have resisted the urge to buy a 3D printer also.
I needed a distributor cap on a small V-8 engine model. Other builders used CNC and Perspex and they came out great too.

bakelite cap.jpgteflon cap.jpg

Please consider making a cutter tool from 0-1 tool steel to form the spark plug lead towers. The tool is shown in the image above. The two prongs are milled and stoned after hardening to cut the OD kind of like a woodworkers "plug cutter".

I tried several different type of plastic before I settled on a scrap of white plastic that I believe to be Teflon. The Bakelite machined wonderfully but I kept breaking the towers when drilling through for the wire.

Congratulations on the magazine articles! Big fan
Another option:
I have made a distributor cap by moulding two pack resin. I made a two part mould from aluminium. The mould supports all the conductors in place and the resin is poured over them and then the mould is closed.
I made the first one in clear resin, so I could check for any errant sparking between the conductors on a bench test.


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I am sure you have all developed working distributors. But a word of caution to the unwary, that can cause hours of grief trying to find a failure that you don't expect. Material is most important for high voltages.
E.g. If the electrical clearances are quite small where electrodes are exposed - as in distributors - you can get flash-over,
  1. e.g. from a terminal where the spark should be going to an easier earth, because the compressed charge in the cylinder to be sparked has a higher breakdown voltage than others just in uncompressed air....
  2. Between electrodes and earths through the material as an incredibly small pin-hole, or larger explosive puncture. Usually because of sharp metal edges and thin plastic -filled gaps between electrodes.
  3. Just normal gap-jumping from common electrode to appropriate cylinder generates Ozone, a highly corrosive version of oxygen. It degrades plastics far more rapidly than normal life. Distributors trap the Ozone inside the cap, unless ventillated. So car makers always used an Ozone resistant material, which was the most significant development from the 60s, to 90s, after which electronic Low voltage distribution was adopted on high volume cars. (Individual sparking plug-caps, etc.).
  4. All arcs create conductive molecular sized dust. Surface clearances have to be large enough that the surface tracking voltage is much higher than the spark-plug firing voltage.
  5. Arcs across the surface of plastics create tracks of degraded plastic, usually of conductive material where a hydrogen and/or oxygen molecule has been blown off the hydrocarbon molecule leaving an exposed and conducting carbon.... These track encourage more tracking until the surface fails with black lines.
  6. Nylon is hygroscopic. It absorbs water. (Measure a piece, soak in water for a few days and measure again, it may have grown!). In high voltage stress the water can cause the arc to track, externally or internally.
  7. PTFE ablates under arc temperatures. It simply turns directly into gas. So stays clean.
  8. So select a material that is suitable for high voltages, with adequate designed clearances against arcing across the surface and by penetration through the dielectric. My train may have told me the best materials, but that is forgotten from 1980s.
  9. Cheers,
  10. K2
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I make my caps from black Delrin. I kame the aforementioned plug cutter to cut the bosses. For a 4 cylinder cap you can generally make the teeth on the cutter wide enough to remove all the material. With an 8 cylinder cap there is a little cleanup to be done. For the electrodes (contacts) I make a button rivet withe a washer. I insert the contact into the cap from the outside, place the washer over the shank and rivet in place. When all the contacts are assembled I chuck the cap in the lathe and skim the excess shank off. For the wires I solder a brass button on the end then insert them into the cap. I cut a short piece of shrink tubing and slide it over the wire and up against the terminal boss and shrink it. This enlarges the wire diameter. I then cut another piece of shrink tubing,, one that will fit over the boss and long enough to cover the first piece of tubing. I then shrink it on the boss. This generally will have a nice snug fit to hold the wire in place. For the rotor I machine it from Delrin also. I make the boss diameter large enough to accommodate at least a 2-56 set screw. I mill a slot in the top to hold a piece of phos bronze strip ,010 thick. Itcan beheld in place at the end with a tiny rivet. Before securing the strip putative small offset bend in it so it will contact the center electrode. The phos bronze is springy so it will give when the cap is installed.
Between the fixed central contact and rotor arm, do you put the tiniest dab of copper grease or carbon to lubricate the contact and carry current "arc free"? I once had to remake a spring contact for a switch and carefully used a centre punch (resting the job on my leather welding gauntlets as a cushion) to create a dimple on the obverse with a small contact projection on the reverse - which slid from contact to no-contact, etc. I have used both pencil lead and copper grease on switches to minimise wear and maintain/assure continuity. But only where it won't cause shorting or flash-over!
Traditionally, cars had a spring-loaded carbon brush from cap to rotor arm. You could make a tiny one from a piece of pencil?
I use a drop of way oil. I have never had a conductivity issue or had the strip wear out. My 302 V8 engine has probably close to 40 hours of running time on it. I do lubricate it twice per season.
Timely post. I was just about to draw up GB distributor & some others I've seen in Strictly IC. Terry Mayhugh & other builders have also done nice work. But admittedly I don't have a very good grasp of the electronics.

As a side note, when I look at these commercial Chinese engines popping up in a multitude of cylinder configurations, seems like they pretty much have the miniaturized ignition system figured out, at least to the extent I would be happy to adapt to my future projects. The spark plugs are small (off the shelf, available in 1/4-32 'glow plug size' through RCEXL & the likes). Same for the CDI modules, they are evidently up to the task; small footprint, driven by compact LiPo battery or whatever. If I understand correctly, they can be had with or without integrated advance/retard curves for the usual reasons, speed/idle/starting.

But what really catches my eye is the small (scale) physical size of the distributer bodies. They look right relative to the engine scale. Same goes for the harness wire gauge, looks to be standard high strand regular copper wire. No bulky external metal sheath (grounding? shielding?) like on every RC gasoline engine. And no biggish steel boots which have some kind of resistor in there? (I might have that wrong). From what I've seen of YouTube videos when people get these engine 'kits', they just hook up the CDI high tension wire to distributer center terminal & a skinny ground wire from distributer to engine body. Flick the switch & away they go. Has anyone had the opportunity for a close-up view or care to speculate how this is being implemented? The pickup sensor is right adjacent to distributer, very close to a rotor assembly. This V10 has 5 cylinder terminals in pretty close proximity, but the magic smoke is staying inside. Are they doing anything fundamentally different?

I don't have a firm grasp of some of the earlier shop made model engineering distributers, specifically why many seemed 'big', say 1.5-2" vs <1" dia just to spitball a number. Were they trying to integrate the equivalent of mechanical points which occupied more real estate than hall effect pickup? Or maybe size was related to where arcing grief was tamed & related to using what was available - car or motorbike coils? Maybe these coils output higher voltages vs. todays model CDI's or duration or voltage profile or... & that was the challenge to be overcome?


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Raveney, it is a simple thing really. Simply decide if "model making" is about the process, or finished object, as top priority.
For me it is the process. I have less motivation once finished than when just starting. So "low tech" - actually some was hi-tech decades ago - and hands-on manufacture suits me. But if the final solution is what matters most, A computer-based hi-tech route (often where the machine makes the part to the CAD that you input to the computer) is an obvious choice. Brian has the interesting half-way house where he uses CAD to make the drawings, then "basic manufacturing tools and skills" to make the models. Which seems to be your choice as well.
Enjoy MAKING MODELS. However you choose. Keep posting , I really enjoy it all.
@Steamchick hopefully my post was not misleading. I have no desire to buy a finished kit like this. I am just an amateur model engineering builder. But my blind spot is spark ignition systems, which is where I want to go next. I'm just specifically mentioning the distributers & underlying ignition systems being implemented on these commercial engines. Developing my own circuits, making my own boards, scoping & diagnosing issues... is outside my knowledge or interest for that matter. I think I could chimpanzee a working distributer with drawings & all the parts identified, but that is different than designing an ignition system.
All of my distributors are right about 1.00 diameter. The first multi cylinder (4) that I built I made and still use points. When I advanced tu Hall trigger ignitions I never went back and updated the 4 cylinder. When I built my 302 V8 I wanted to keep the distributor somewhat to scale and I made several styles of points for it.
The engine would start and run but it wouldn't go over about 2000 rpm. The reason being that the cam was so small thar I couldn't get enough dwell for the coil at higher rpm's. This was the late 80's before anyone fooled with Hall triggers. The engine sat for a long time without running. When I learned about Hall ignitions I changed the distributor over keeping the same size cap and body. The rest as they say is history. All of my engines now use a Hall trigger ignition, even th small hit and miss ones. I only have points on my Holt engine because that's the way it was designed. When I redesigned the Holt I made it for a Hall setup.
Thanks George, that is useful history. The 4-cyl OHV plans I have from you (I don't even remember when I bought them, they have been in the 'fantasy one day' pile for a while now haha). But I pulled them out the other day for a looksee & was coincidental to this post subject. They show a 1.124" OD distributer with Delrin point base, brass point arm, spring etc. I assume this is the original design? Are you saying you just left your personal 4-cyl as-is, but if you did it over again today you would use hall sensor configuration like the V8?

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