Author Topic: Spark plugs (8-36 thread)  (Read 8136 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
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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.
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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.)
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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.
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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.
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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.
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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.
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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
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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.
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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
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Offline Doc

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Re: Spark plugs (8-36 thread)
« Reply #15 on: February 12, 2014, 06:11:13 PM »
That is the way I do my plugs now. I had a terrible time trying  to bend them with lots of fails. I found this works very well.
« Last Edit: February 13, 2014, 03:20:24 AM by Doc »

Offline jerry kieffer

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Re: Spark plugs (8-36 thread)
« Reply #16 on: February 13, 2014, 09:17:56 PM »
George
          If you were to go to a much finer thread such as 56 TPI, you would be able to increase the through hole by about .025"-.030" making the plug much more efficient and dependable.

An example can be seen in the attaches photo.

Jerry Kieffer


Offline ken rector

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Re: Spark plugs (8-36 thread)
« Reply #17 on: August 13, 2014, 07:04:00 AM »
George,

I've made some plugs this size but haven't had a chance to try them yet.  I too had problems bending the side electrode, even using the torch.  Eventually I silver brazed a piece of pre-bent .030 piano wire to the edge of the plug.

Now I can hardly wait to see if the combustion chamber gets hot enough to effect the silver braze (1150 degrees).  What do you think?
Member, So. Cal. Home Shop Machinists
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Offline gbritnell

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Re: Spark plugs (8-36 thread)
« Reply #18 on: August 13, 2014, 12:47:07 PM »
Hi Ken,
I don't think it would but I'm a little skeptical about having it fail and fall off in the cylinder. The problem is you wouldn't know it because the plug would spark from the center electrode to the body of the plug until it was too late. (bent valve, scratched cylinder wall) When I started making plugs many years ago I never used a ground strap, I just let it jump from the center to the side wall of the plug. They worked ok but had a greater tendency to foul.
gbritnell
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Offline Ian S C

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Re: Spark plugs (8-36 thread)
« Reply #19 on: August 13, 2014, 03:13:54 PM »
Rather than a bent electrode, maybe an annular one, just a narrowing of the opening around the centre electrode. :thinking:    Ian S C

Offline IanR

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Re: Spark plugs (8-36 thread)
« Reply #20 on: August 13, 2014, 04:21:31 PM »
Len Mason made plugs as described by Ian S C, down to 2 or 4BA, with epoxy resin as the insulator, and found they worked well and were less prone to fouling than bought plugs.

Offline mnay

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Re: Spark plugs (8-36 thread)
« Reply #21 on: August 22, 2014, 08:28:40 PM »
Guys,
I just read this thread today.  Thanks for all of the great information on making spark plugs.  I have yet to try, but I will know how to tackle it in the future.
Mike Nay

Offline Laurentic

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Re: Spark plugs (8-36 thread)
« Reply #22 on: June 14, 2019, 09:03:26 PM »
Hi gbritnell.  Read this thread with interest.  I know thge last post was something like 5 years ago, but do you have a pic of the clamp/crimp you used to secure the insulator inside the steel body (your post of February 12, 2014, 01:33:32). 

Trying to get my head around this and each time something says no!

Many thanks,


Chris

Offline Art K

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Re: Spark plugs (8-36 thread)
« Reply #23 on: June 15, 2019, 02:40:44 AM »
I have to admit to being one of those who likes to pick my battles, spark plugs not being one of them. For the time being at least. I have been thinking about it though, Steve has been feeding me the idea of how easy it is to make spark plugs....Out of curiosity why not make plugs with the double sided ground strap like in Gray's first photo? does it just add complexity?
Art
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Offline Graham Meek

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Re: Spark plugs (8-36 thread)
« Reply #24 on: June 15, 2019, 10:35:33 AM »
Hi Art,

Making the plugs with a double earth electrode is much easier than a single one. Plus the life of the plug is extended as the spark has twice the number of electrodes to jump to.

Since I first wrote on this post the 4.5 mm plugs have been made and I have attached a few photographs for the forums perusal.

Regards
Gray,

Offline gbritnell

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Re: Spark plugs (8-36 thread)
« Reply #25 on: June 15, 2019, 02:02:36 PM »
Hi Chris,
Here's the tool and procedure for crimping the insulator into the spark plug body.
The first picture shows the components.
On the left is the crimping die. This is made from drill rod and hardened. I have tried to use them without hardening but the edge of the flange on the spark plug digs in and erodes the chamfer.  In the center is the threaded die that matches the spark plug thread. On the right is the guide bushing. The guide bushing size can be anything you like with a standard reamed hole through it. I use a .375 reamed hole that way I can use standard drill rod stock for the crimping die.
In the front is a spark plug body with the crimping flange shown. I make this flange about .006 thick.
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Offline gbritnell

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Re: Spark plugs (8-36 thread)
« Reply #26 on: June 15, 2019, 02:18:14 PM »
The first picture shows the spark plug screwed into the holding bushing. This is then slid into the guide bushing. (Picture 2) Picture 3 is the crimping die. In picture 4 it is slid into the guide bushing. You will notice that the threaded bushing and crimping die bushing are sticking out of the guide sleeve, this is so they can be squeezed together to crimp the flange on the plug. The crimping die needs to have a hole slightly larger than the diameter of the insulator on the spark plug other wise if there is any misalignment with the parts it could break the insulator when crimped. The chamfer angle only needs to be enough to fold over the flange, generally about 30 degrees. The outside diameter of the chamfer is the same as the O.D. of the plug flange.
gbritnell
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Offline b.lindsey

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Re: Spark plugs (8-36 thread)
« Reply #27 on: June 15, 2019, 03:02:26 PM »
Nice to see this older thread resurrected and thanks for the additional pictures of the crimping set-up and tools George.

Bill

Offline Laurentic

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Re: Spark plugs (8-36 thread)
« Reply #28 on: June 16, 2019, 02:13:41 PM »
Many thanks for the pictures and description of the crimping device - all is very clear now.  Nice solution, simple and effective.

Cheers,
Chris