Sparkplug HV Feedthrough

Edited from original material by Richard Hull posted on the Fusor Discussion Board (16 Nov 1998) (

One of the parts required for any fusor is the critcal high voltage vacuum feedthrough. This connection must be absolutely vacuum tight and made of material which will not outgas, creating virtual leaks. The part, if puchased at the lowest price from Duniway or Lesker will be $135.00. (20kv insulation with conflat 2 3/4 flange.) For real heavy neutron production, even this is a little weak as one might hope to hit 30-40KV and those insulator/feedthroughs are all over $500.00 each.

I have just finished a high voltage feedthrough based on a spark plug!

It was a lot of work and fitting, but I was fussy and wanted a professional looking item. I have the machine tools in my lab, I have the time and skills, but not a lot of money to throw away on something that I might make which will serve as well as a store bought item.

Spark plugs are sealed for hundreds of psi engine compression. The deepest vacuum never exceeds 15psi. The best plug to use is an AC 44XLS - cost $1.80. Why the 44XLS? It has a very deep thread on it, that's why. You must cut this entire threaded portion of the base off carefully with a hack saw. This leaves a normal plug with a nice long ceramic insulator sticking out of the bottom. (Possible equivalents for the AC 44XLS are the Champion N12Y or the NGK BP5ES. I'm not sure if these are NON-resistor plugs, though -jl)

*******NOTE******* no matter what, do not use a resistor plug! Most all cars use them now. To test, should you not have the 44XLS, use an ohmeter and measure from the center firing electrode tip of the plug to the insulator terminal contact on top. Resistor plugs measure several thousand ohms. You should measure less than 1 ohm....Really, close to zero ohms! If you don't, it is a resistor plug. You can't use them as they will quickly burn up and all your effort will be wasted.


From here you are on your own with attaching it to your system.

I did the following:

  1. After cutting off the threaded portion, I put the plug in my lathe and machined the base flush with the insulator and trued up the bottom.
  2. Next, I silver soldered a 4-40 threaded through round bushing of 1/2" length to the firing tip terminal. This left me with a plug with a long insulator ending in a threaded metal sleeve. This will allow the central grid stalk conductor to be threaded in or out for changes, modifications and upgrades.
  3. I now took a threaded conflat 2 3/4" flange blank off and machined a precise hole in the center of it for the plug's round steel body. I then inserted the plug (tight slide fit) and TIG welded it to the flange. The non-TIG welder folks could probably get away with brazing here. Epoxy is an option, but make sure to ground the plug body to the flange in some fashion or risk electricution.
  4. I next epoxied, (use low vapor pressure "epoxy patch" - Duniway), a TIG welding torch alumina gas collar to the base of the plug to create an insulator cup shield around the main insulator post. (just like the high dollar jobs.) The gas collar I used was a Weldcraft #WCT10N46 - $2.45. This can be found at most any welder supply store.

The final step is to make the stalk conductor. I used 1/8" 304 stainless welding rod with a 4-40 threaded end. This will have to be insulated over its entire length. I use a tight slide fitting alumina tube from Mc Master Carr with a 1/8" ID. It will requre a diamond dremel cut off wheel to cut the foot long tube to length.

The grid and its attachment are up to you.

This whole process was worked through on the fly, first time, yesterday and took 6 hours of labor from start to finish and the thing should be good for 20KV, no problem.

I also epoxy filled the small outer lip where the insulator exits the metal plug body for good measure. Do all expoxy work after, and only after, all welding and brazing operations are complete!!!

On Feb 01, 10:35 pm, Scott Stephens wrote on the discussion board:

FYI - A sparkplug with an extremely long reach (over 1 1/2 inches!) is a Champion F121501 used for kerosine heaters.



hv/sparkplug.htm - 2 Feb 2000, Jim Lux

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