How to make a distributor cap from raw materials?

Has anyone ever seen anyone make a distributor cap from scratch?

I have an antique tractor that has a magneto with very uniquely shaped distributor cap. I’ve searched for a replacement cap on antique tractor sites with no luck. I’m wondering if there is any way to fabricate a new distributor cap for this magneto? The old cap is still in tact and the tractor is still running, but I thought I’d get a jump on trying to fabricate a new one before this one finally breaks. It’s very old and fragile.

If anyone has any information on how to mold or manufacture a custom distributor cap I would be very grateful. I have a machine shop so I could make fixtures to mold one, but I’m not sure what materials to use or how this might be done. I know the cap has to be electrically insulated for high voltage. But I’ve never cast anything like this before. So any suggestions or knowledge would be very welcome.

Thank you.

Not knowledge, just ideas to help with further research…

I don’t know the base material they would have used in the original; I think today alkyd casting is common for the base material. I have a vague recollection that maybe older models were phenolic but I have no idea where I got that idea, and no idea whether either is practical for home shop work. Both are thermoset plastics, and for high temperature use applications a thermoset plastic can be important; thermoplastics are intended to melt at high temperatures.

But for a tractor, where it might not be tucked up right against a hot engine block and might be exposed to lower temperatures, I would think that some of the higher-termperature thermoplastics might be OK. Acetal and PEEK come to mind for high-temperature thermoplastics that could be molded or machined and have good chemical resistance generally.

If I were going to make one myself, I would hope to machine it out of acetal rather than mold it. Acetal (e.g. Delrin) machines easily and is widely available for reasonable cost.

I don’t know what kind of brass to use for the contacts. I would think phosphor bronze but I have zero experience here and I could be completely wrong.

@John_Bump, it occurs to me that you might know more here?

1 Like

Thanks for the ideas Micheal. I actually had an idea of making it from flat sheets in a box shape and either gluing the box together or using some type of screws. This cap, while having nice rounded corners is actually fairly rectangular in shape. Not round like on most engines.

I haven’t had the cap off in a while. In fact, I need to take it off to put a new set of points inside which I was able to find new. I’m hoping it will come off and go back on without breaking. It’s so old I’m thinking that it could probably break fairly easy. Especially being on a farm tractor that sat outside in the sun a lot. This magneto is exposed to the weather and sunlight. And I know that sun like can play havoc with various plastic-like materials.

I was even thinking of trying to incorporate an existing distributor cap in the mix and maybe trying to just build the rectangular base around a round distributor cap. But there’s another problem I almost forgot about.

I haven’t had this cap off lately, but if I recall, it’s not like a normal distributor cap that just has brass electrodes sticking down where a rotor flies by close and jumps the spark to the capt electrons. Instead I think it has some kind of spring-loaded rods (probably similar to motor brushes), that actually ride on the rotor and are activated only when the metal part of the rotor passes under them .

If that’s the case it may be extremely difficult to duplicate those aping-loaded electrodes. I’m going to be putting the new points I have in the tractor fairly soon. While I have the distributor cap off I’ll try to take some pictures of it.

Also, I don’t have a 3D printer, nor am I aware of whether a cap like this could be printed from suitable material. But if that was an option maybe I could draw up the cap in CAD and have someone print it out for me? As I say, I don’t even know if 3D printers could print a suitable material?

In fact, if they are that good I might considering buying a 3D printer myself.

As I say, currently the current cap is still functional. But I use this tractor almost daily, not all day long. But I’m always uses it for something. I’d hate to see this tractor die just because I can['t get a new distributor cap for it.

I can buy a whole new magneto, which is actually different from the one that is on the tractor, but will supposedly fit on this tractor. It has a different type of distributor cap. But even so they want $550 for the new magneto. I hate to spend that if I could just make a new distributor cap for the one I already have/

I pride myself on fixing and/or making a lot of things from scratch. But this distributor cap is a brand new challenge for me.

I don’t want to rattle on too much. But there is another idea I have which would probably work, but I would only want to go there as a last resort.

To begin with, this magneto died a long time ago. So I’m just using the points and distributor cap as a regular distributor with a coil and battery providing the spark. So this has already been a make-shift repair from many years ago.

As a last resort, I’m thinking that I could just take this magneto off altogether and just find an easy-to-obtain standard 4 cly distributor. And then hack that distributor onto the shaft that used to drive the magneto.

In other words, I have ways to fix the problem if I go to extreme lengths like this. A lot of work to be sure, but something I know I could ultimately do with the tools I have available.

But I’m thinking that making new cap for the existing magneto might be easier?

Obviously if I had a new cap to just slap on it that would be the easiest fix of all.

High-temperature thermoplastics (you’d probably be looking at Acetal, PEEK, or maybe ASA) can be 3D printed with a high-quality, high-temperature printer. For one-offs, you definitely would want to order from some place like Shapeways. For DIY, you can expect to invest on the order of $1K minimum in a printer that could handle those plastics, and common expectation is that you start by learning with less expensive plastics before moving to more “exotic” filaments.

The hardest part is probably measuring the existing cap and doing the CAD for modeling. You really don’t want to try to reproduce the same thing. You have different design needs for injection molded, machined, and additive manufacturing (3D printing). Learning how to design well for 3D printing is different from learning how to design well for molding, which is also different from machining.

Good points about the special design considerations for 3D printing. The new cap wouldn’t need to have the exact same shape as the original. It could be more boxy without the fine curves used for esthetics. It could also have additional external ribs for additional strength and support. So that might be a way to go. I’d need to find someone (probably a 3D printing business, that has a 3D printer suitable for the correct materials. Then do a bit of studying up on how to best design it for 3D printing.

I might add here that a new cap for similar magnetos cost about $70. That’s a lot of money for just a distributor cap. I guess that due to the antique nature of it and supply and demand issues.

If it was a popular distributor cap it would cost about $5 to $10.

In any case I can’t even find this exact cap even at the $70 price. But I thought I’d toss that out there just for general information.

This is why I decided to toss the idea out here to the makers. Some of these makers on YouTube make stuff I never thought would be possible. Including old fashioned 7-segment Nixie tubes from scratch! In comparison with that making a distributor cap should be a piece of cake I would think.

Of course a homemade Nixie tube would probably take over $1000 in time and effort to make. But I think they idea was similar to mountain climbing. It’s more about meeting the challenge than needing the Nixie tube.

In my case with the distributor cap it’s a little of both. I could definitely use the cap, but succeeding in having made it myself would be quite rewarding as well.


Have you ever machined Acetal? I’ve found it very nice to work with, other than the plastic chips sticking to everything around my mill in my dehumidifed workshop… :slight_smile:

Do you have the metrology and modeling skills to model the features that you need to match, regardless of the mechanism you use?

No I haven’t. But I do have a pretty nice Lathe/Mill combo with a lot accessories. It’s a powerful machine with a 17.5" swing on the lathe. Only about 24" between centers. And I’ve made several different fixures for making good use of the vertical milling feature. I’m very happy with the machine and I’ve machined quite a few parts on it. But I have never worked with Acetal, I’ll have to look into that material.

Yes, I’m retired from R&D and I’ve been working with CAD programs since the earliest versions of Autocad on DOS. I currently use Blender for most of my CAD work. I know that Blender isn’t really geared toward CAD, but using various plug-ins, etc., I’m able to use it for CAD pretty well.

I was using Fusion360 for a while, but for reasons I won’t go into here I moved over to Blender instead. I’ve found that I can pretty much do anything in Blender that I could do in Fusion360.

Just for information, I also design and make a lot of robotics parts as a hobby. So I’m designing stuff here on a fairly regular basis. This high-voltage distributor cap is something I haven’t really had to create before. Most things I design are fairly forgiving and there’s a lot of flexibility in how they can be designed. So while they can be a lot of work, they typically aren’t as challenging in critical details. This distributor cap need to be both, strong, and impervious to high-voltage.

Also, by asking here it was my hope that I might accidentally run across someone who already has experience making something very similar. So maybe I could just follow their recipe.

In fact, before I came here I did a search on YouTube on “How to make a distributor cap” half-expecting to find some people actually doing it. YouTube amazes me. In robotics I often feel like I’ve just come up with a new idea only to discover that people have been posting how-to videos on the same thing long before I ever thought it.

But no luck on finding anyone who makes distributor caps.

1 Like

Great! I didn’t want to assume “It’s easy, all you need to do is start by learning CAD” :wink:

I just looked up UV resistance. Acetal doesn’t have great UV resistance, so if the tractor lives outside 24/7 it might not be the best idea. PEEK and PEI are (I think) the two filaments in common use that are high temperature, UV resistant, and stiff. PEI filament is almost $300/kg, prints at 350-380⁰C on a 140-180⁰ heated bed. This is not within the capabilities of a normal 3D printer. I don’t know how much blocks of either to machine cost.

I just looked up Acetal plastic and it would be fairly expensive to buy a block larger enough to carve out this cap on a mill. Over 90% of the block would end up being scrap cuttings. Seems like a pretty big waste of an expensive material. Plus, it would the be difficult to insert the electrodes as they would need to have holes machined out for them and then some way to anchor them in place.

Just off the top of my head I was originally thinking of molding the cap using fiberglass cloth and epoxy. I think I could mold a strong enough cap that way, but I’m not sure if fiberglass would be a good enough electrical insulator? It may or may not.

The nice thing about molding a cap is that any electrodes can be molded right into it. So I’m still looking at molding as being an attractive option. Just not sure if using fiberglass cloth would work. I don’t think the cap has any serious stresses on it. As long as it can hold its shape in a fairly rigid manner that should be sufficient. One thing for sure, making a fiberglass prototype couldn’t hurt anything. It would either work or not. And if not it wouldn’t be too great a loss as the materials would be fairly cheap.

Also, if I made a fiberglass cloth cap I would only need to make a mold for the inside of the distributor cap and then just lay the fiberglass and epoxy over that mold to fashion it by hand into the finished product.

That was the first method that came to mind. Not sure if that idea would work or not. I just used the tractor today. So I still haven’t removed the cap to take pictures of it. Fortunately the tractor is still running for the moment. But it’s not running smoothly. I just rebuilt the carburetor, and put new plugs in it. But it’s still missing. I’m thinking either points, or possibly a timing issue. Also it could be plug wires leaking too. I haven’t changed out the plug wires yet.

I’m also hoping to do that points soon. I don’t like it when the tractor isn’t purring like a kitten. It’s not good to run it when the motor’s not running purrfect.