Author Topic: Spark plugs (8-36 thread)  (Read 8187 times)

Offline gbritnell

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Spark plugs (8-36 thread)
« on: February 12, 2014, 12:38:14 AM »
Gentlemen,
I posted a short note in the 6 cylinder thread about the spark plugs but decided to start this separate thread to make it easier for anyone doing a search for spark plugs.
These plugs are some of the smallest I have ever made, 8-36 thread. The basic design came from Steve Huck's Demon V-8 build with a few modifications to work in my engine.
I started with .375 diameter mild steel and turned the bodies. The hex is .219 so I could have gotten by with .25 stock but the little larger diameter makes the threading cuts a little more rigid.
I first did a clean up cut to .290 diameter then several other steps for the area under the hex and the thread O.D. .164 diameter.
gbritnell
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Offline gbritnell

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Re: Spark plugs (8-36 thread)
« Reply #1 on: February 12, 2014, 12:43:39 AM »
The next steps were to center drill, drill and ream for the through hole (.094 dia.) The area for the ground stap was then turned. The thread was chase on the lathe so that it would be concentric with the other diameters. I had the lathe in back gear turning at a moderate speed but still puckered up trying to get as close to the shoulder as possible. I made about 5 passes to finish the thread.
gbritnell
Talent unshared is talent wasted.

Offline gbritnell

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Re: Spark plugs (8-36 thread)
« Reply #2 on: February 12, 2014, 12:47:36 AM »
I made up 10 blanks to provide for any mishaps along the way. With all the initial turning complete the blanks were mounted in the dividing head, indicated and the hex cut. (.219)
Completing this operation the plugs were put back into the lathe to be cut off leaving about .03 for cleanup later.
Talent unshared is talent wasted.

Offline gbritnell

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Re: Spark plugs (8-36 thread)
« Reply #3 on: February 12, 2014, 12:58:38 AM »
The plug blanks were moved over to the small lathe to qualify the length, drill and counterbore for the boss on the insulator and then the crimp diameter was turned. I drilled for the counterbore staying about .04 away from the finished depth then with a homemade flat bottomed counterbore went in and flattened the bottom surface (.169 dia.)
Talent unshared is talent wasted.

Offline gbritnell

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Re: Spark plugs (8-36 thread)
« Reply #4 on: February 12, 2014, 01:03:08 AM »
I had to cut the extra material away for the ground strap so to hold the pieces I made a brass bushing with the 8-36 thread in the end. I then counterbored it from the other side to a depth that would allow the ground strap diameter to stick out far enough to be trimmed. The bushing was mounted in the dividing head and the calculation made to leave .03 width for the strap.
Talent unshared is talent wasted.

Offline gbritnell

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Re: Spark plugs (8-36 thread)
« Reply #5 on: February 12, 2014, 01:15:09 AM »
The extra strap was nipped off and filed flush. You remember I said that I made extras well the next step was the reason. I have never had good luck bending ground straps over, especially with 12L14 steel and today was no exception. Out of the first 4 tries 2 broke off so another method would need to be used. I turned up a bar that would fit inside the plug body but protrude just far enough to give me about a  .020 gap. I then heated the strap up with the propane torch and bent it over. From there I had no more breakage.
Talent unshared is talent wasted.

Offline gbritnell

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Re: Spark plugs (8-36 thread)
« Reply #6 on: February 12, 2014, 01:20:15 AM »
The insulators for these plugs are made from Corian. I have another material that I normally use for my larger plugs but in talking with Steve Huck he said he's had real good luck with the plugs in his Demon V-8 so I thought I give it a try. (Thanks Steve for the Corian shipment). Steve says he drills his insulators with a .020 drill. My plugs are longer than his and no matter how hard I tried I couldn't drill all the way through the Corian, even from both sides. Time for a re-engineering. I went up to .03 diameter as I had some straight piano wire that size. This went much better but still took a long time pecking and cleaning the material from the drill flutes.
Talent unshared is talent wasted.

Offline gbritnell

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Re: Spark plugs (8-36 thread)
« Reply #7 on: February 12, 2014, 01:33:32 AM »
The next step was to crimp the insulator into the plug bodies. Normally I roll this flange on the lathe but with these plugs being much more fragile I used Steve's method of making a small press to fold over the flange. It worked well.
Talent unshared is talent wasted.

Offline gbritnell

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Re: Spark plugs (8-36 thread)
« Reply #8 on: February 12, 2014, 01:39:17 AM »
I took a picture of the center electrodes but when I started going through all the pictures those were out of focus so I can't show them to you.
I cut all the center wires a little long, soldered on the brass caps and then fitted them to each plug. They only varied by .005-.008 but enough that they had to be fitted individually.
Ok, 8 plugs made, 6 needed and several that made it into the trash.
gbritnell
Talent unshared is talent wasted.

Offline stevehuckss396

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Re: Spark plugs (8-36 thread)
« Reply #9 on: February 12, 2014, 02:12:59 AM »
I have a brass bar that I insert into the base to bend the strap. Mine has a small radius instead of the square corner. After putting the radius on the rod I had no trouble with breakage.
Do not be like the cat who wanted a fish but was afraid to get his paws wet.

Offline ths

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Re: Spark plugs (8-36 thread)
« Reply #10 on: February 12, 2014, 08:43:07 AM »
Thanks George, the process was half formed in my mind, now it's complete.

Hugh.

Offline tangler

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Re: Spark plugs (8-36 thread)
« Reply #11 on: February 12, 2014, 11:10:49 AM »
George,

Thank you very much for taking the trouble to document your method, I'll be having a go.  I've already got some Corian pen blanks from ebay.  Not pure white unfortunately, they've got some sparkle in them - should be quite pretty  ::)

cheers,

Rod

Offline wheeltapper

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Re: Spark plugs (8-36 thread)
« Reply #12 on: February 12, 2014, 11:48:51 AM »
Make sure the sparkle isn't metalflake, I imagine it's conductive.

Roy.
I used to be confused, now I just don't know.

Offline Graham Meek

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Re: Spark plugs (8-36 thread)
« Reply #13 on: February 12, 2014, 12:30:10 PM »
I am currently working on a smaller version of my 1/4-32 design, using exactly the same build technique which uses a Ceramic bead (Fish Spine) for the insulator nose and a PTFE insulator for gas sealing. The new size is a 4.5 mm thread, ( approximately 0.010" bigger than 8-32), for my Fiat Tractor and I will be using a cone seating to seal the plug into the valve bonnet in the cylinder head. The three photographs show how I tackle forming the single side electrode, it is quick and gives a consistent profile.

My best regards
Gray,

Offline gbritnell

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Re: Spark plugs (8-36 thread)
« Reply #14 on: February 12, 2014, 12:57:02 PM »
Hi Gray,
Thanks for the pictures. When I make my larger plugs I use a method similar to yours. For these small ones I thought that with the thinner ground strap it would bend easier. If I have to remake them I'll be using the blind counterbore method. This is one of those times when a small CNC lathe would sure come in handy.
gbritnell
Talent unshared is talent wasted.