Shop built sparkplugs for model engines

[doc1955]

Something to keep in mind with a points-type or simple transistor ignition is that energy is being built in the coil during dwell - that's the time the points are closed or the transistor is ON. In a multi-cylinder engine the dwell is necessarily limited to tens of crankshaft degrees since the coil has to service other plugs. The resistance in series with the coil is often minimized in order to get enough current through the coil to build enough energy in the time allotted for each plug.

In a single or dual cylinder engine, the dwell is huge. The points spend most of their time closed, and if an automotive coil with insufficient series resistance is being used, the energy built is much more than needed in a model engine. When the points open, and all this energy is dumped into a single cylinder model engine plug, you're going to greatly shorten the life of the plug. If you also run a really wide plug gap, that excess energy will show up as some terrifically high voltages during compression that can result in a rash of arc-over problems that a typical plug isn't designed to handle. (i.e. it doesn't have a 3 inch long insulator).

I have found that after I went to a coil running on volts on my single cylinder engines that have shop made plugs the burning has pretty much gone away. I started winding my own coils that will work on volts. Anyway you are correct in saying that the coil itself is the cause of small plugs burning up (the coil and the dwell time) You would be better off putting in a wasted spark to lessen the dwell time.

 

[Bentwings]

If you check horizon hobbies under miniature spark lugs you should be able to find 1/4/32 spark plugs used in Rc model engines .

 

[stevehuckss396]

Hey Brian.

Take a look at peek. It's Temps are double what corian is. It machines like delrin. I bought a 3/8 rod about 3 years ago and it was about a dollar an inch. Would work good for your insulator. Much cheaper than ceramics. Think I got it from u.s. plastics.

 

[stevehuckss396]

If you check horizon hobbies under miniature spark lugs you should be able to find 1/4/32 spark plugs used in Rc model engines .

He's looking for a plug that's roughly 1-3/4 long. Just a guess as the insulator is 1.4 inches.

 

[johwen]

 

[johwen]

I use wiper sniper coils not car coils as they will cause flash over and you need extra long insulators which to me are out of proportion in the models we make. The insulator ceramic tubes i use are approximately 1 inch in length. If the plug oils up or fuel floods is easy to clean. I have one hit and miss engine with the same plug now for over ten years and is run regularly.
Give one ago. John

 

[Rocket Man]

I am in the process of making a small sparkplug for a model internal combustion engine. It seems that Corian is the best material for the insulator, but I'm having a hard time finding Corian locally. Does anyone know of a different material than Corian that will work suitably? I need 1/4" diameter material, but if I can find it in scrap rectangular pieces I can turn it in the lathe. I can't find a source on the internet that sells Corian in 1/4" round sections or I would buy some on-line. any help would be appreciated.---Brian

Fire clay works great. You only use a tiny amount of water 2% or 3% then compact it very tight under pressure. Build a die squeeze it hard in a vise. Let it dry natural for 1 week then bake it in an oven. There are 1000s of different size spark plugs I bought some with 1/4-28 threads many years ago why build a spark plug. I am getting old very near end of the road my time is too short to be making things that I can buy for $1 instead of a weeks work trying to build it.

 

[dsage]

If you're looking for a suggestion for testing your plug(s):
I made an adapter to put them in my lawnmower. Gives them a good workout. Probably more than most models would give them. The ones I made were a really odd size. .401dia 40tpi
Engine is here
http://www.davesage.ca/olympus.htmllook for the "click here to watch it run" below the picture.

 

[Brian Rupnow]

Today I made up two sparkplug insulators from Corian. One turned out fine, one broke. That seems to be about on par for me---A 50% screw up rate when make anything from Corian.---Brian

 

[peterl95124]

If you're looking for a suggestion for testing your plug(s):
I made an adapter to put them in my lawnmower. Gives them a good workout. Probably more than most models would give them.

Brilliant !!!

do you have any test results for Corian ?

 

[dsage]

Brilliant !!!

do you have any test results for Corian ?

Unfortunately no. I made these particular ones with Macor because Albert Hutton appears to have made his from some sort of ceramic-like material. I wanted to make them true to his design. The Macor was crazy expensive so I wouldn't use it again. Nice stuff though.
I have Corian but as others have mentioned unless you need an odd plug it's easier and cheaper to buy them and avoid the uncertainty of self-made plugs troubleshooting a brand new engine build.
I think Corian should be fine. Models really don't get that hot.
Many of my engines use CM-6 plugs which are dirt cheap or the long reach rimfire (?) plugs also (used to be) a reasonable price in low numbers.

 

[Jasonb]

I think temp depend sa lot on teh type of engine, a slow running Hit and miss that should be pumping air in and out of teh cylinder more that it is firing won't get that hot and Corian should be OK but put it into a 4-stroke aero engine that is firing at a far higher rate and temperatures will be a lot higher.

"Fish spine" ceramic insulators can be used and have the hole already up the middle as does glass Tube which is easily cut to length with a diamond Dremel wheel, couple I made earlier assembled with Loctite "Black Magic" To Nick Roland's design (RMC) that's a tungsten tig electrode up the middle.

 

[Bentwings]

View attachment 148474

I often wondered if you used glow plugs by just carefully cutting the element and gapping it as needed if you are really good TIG welding you might be able to fashion a ground electrode . I suspect the coil electrode is high resistance so maybe not a good idea worth a try however

 

[Brian Rupnow]

I had a HUGE piece of 1/2" Corian from a cut-out for a kitchen sink. Today I went searching for it. I found about a million things, some stuff I'd even forgotten that I had. I searched my office---I searched my book cases--I searched my hot tub pumping room---I searched my garage and every cupboard in it---I searched my office bathroom---I searched my sauna--(I actually discovered a new grindstone for my Drill Doctor that I have been looking for for five years.) Finally I went back and searched my hot tub pumping room again-(The hot tub is long gone to make room for an upstairs laundry room, but the pumping room adjacent to my office has a dozen long shelves and is the general $hit collecting room for the entire house.)--And what do you know--I found it!! Maybe this week I will make my second sparkplug.

 

[Drillrod]

I had a HUGE piece of 1/2" Corian from a cut-out for a kitchen sink. Today I went searching for it. I found about a million things, some stuff I'd even forgotten that I had. I searched my office---I searched my book cases--I searched my hot tub pumping room---I searched my garage and every cupboard in it---I searched my office bathroom---I searched my sauna--(I actually discovered a new grindstone for my Drill Doctor that I have been looking for for five years.) Finally I went back and searched my hot tub pumping room again-(The hot tub is long gone to make room for an upstairs laundry room, but the pumping room adjacent to my office has a dozen long shelves and is the general $hit collecting room for the entire house.)--And what do you know--I found it!! Maybe this week I will make my second sparkplug.

Happens to me all the time. It's always the last place that I look in that I find what I'm looking for.

 

[DKGrimm]

Brilliant !!!

do you have any test results for Corian ?

I love working with Corian. It's a little brittle, but it machines very well with sharp steel tools. Back sometime about 2008 I made some long-reach 10mm sparkplugs for my Red Wing 1/4 scale. They looked great; I was very proud of them. The first time I started the engine, though, I could smell burning plastic. The plug shorted out within a minute or so. the problem was I designed the plug to be very "hot" with a long skinny insulator. Corian does not conduct heat very well, so the material was seeing combustion temperatures with nowhere to dump the heat. The one on the left in the picture ran just a short time and you can see the erosion on the insulator tip.

In 2009 I made an ignitor for the same Red Wing engine. Here I also used a Corian insulator, but it was recessed into the ignitor body and hidden from the worst of the burning combustion gasses. It has run ever since, accumulating over 500 running hours, and is as good today as when it was made. (The electrodes were made from nickel welding rod and are also still running, albeit with some wear.)

 

[Brian Rupnow]

I found my Corian and machined the bodies for two sparkplugs. Had to go downtown and buy some silver solder, as I was out. Yeowwww--that stuff is expensive. I bought 5 ounces of 1/16" diameter silver solder "Easy-flo 45" and it cost me $174. Canadian. That cleaned out my Paypal account. Sometimes it's hard to live in the land of the loon and the beaver!!!

 

[Brian Rupnow]

And---we have two beautiful sparkplugs ready to go when the j.b. weld has dried 24 hours. Note---if I were to sell these sparkplugs, I would have to charge $150 apiece for them, based on the time it took me to make them. Fortunately, when I'm making parts for myself, my time is free. Tomorrow I will test these plugs and let you know how they worked.---Brian

 

[Brian Rupnow]

Today I tested the sparkplugs and they both work very well. I have been screwing around with this all week, and just found out this morning that the battery I have been using is stone dead. I stole the battery out of my garden tractor, which I know has a full charge, and Presto-Changeo--big fat spark!!!

 

[Brian Rupnow]

The sparkplugs work well, and to see one of them running an engine, flip over to my "v-twin maybe v-4" thread and have a look.---Brian