Making a spark plug...

[jgedde]

Not being satisfied to leave well enough alone, I decided I wanted my Farm Boy engine to have all key parts shop made. Including the spark plug. After a bit of research, I sat down and designed my own using Alibre.

There was inspiration in George Britnell's and Gail in NM's designs.

At the end of the post there's a link to the engine running on this spark plug and dimensioned drawings in case you want to try making one!

First step was to make the body starting by turning it diameter:
http://www.homemodelenginemachinist.com/attachment.php?attachmentid=64640&stc=1&d=1380315861[/IMG

Next I made a aluminum slug tapped with 1/4-32 thread as a thread test piece and a holding fixture for use later.
[IMG]http://www.homemodelenginemachinist.com/attachment.php?attachmentid=64641&stc=1&d=1380316029

Finished one side of the body

Now it gets screwed into the holding fixture for further machining

http://www.homemodelenginemachinist.com/attachment.php?attachmentid=64644&stc=1&d=1380316250

More turning

Cutting the hex

Rough drilling a blind hole

Making a flat bottomed hole with an end mill. Depth is critical since this is part of making the tang

More in next post...

John

 

[jgedde]

Now mill away material off the end forming the tang.

The completed tang

Now make a recess for the insulator. That completes the body.

Now onto the terminal. I won't annotate every picture since what's happening will be evident.

More to follow in next post....

 

[jgedde]

Start making the insulator from Teflon

Screw in the terminal. Per the drawing, there is a small groove cut into the thread on the terminal using a Dremel cut-off wheel. This allows the threads in the insulator to be formed using the terminal. The insulator will cold flow into the groove and keep it from backing out easily.

Solder in the electrode.

Ready to assemble

More to follow in next post...

 

[jgedde]

Swage the insulator into the body. This was done with a suitable and dull lathe bit. The lathe was turned by hand in reverse while pressure was put on the tool to roll the body over the insulator.

The completed spark plug (except for the top nut)

Installed in the Farm Boy

A video of the Farm Boy running with the new plug:
[ame]http://www.youtube.com/watch?v=rQgIcqR0YwU[/ame]

More to follow. In the next post, I'll provide drawings...

 

[jgedde]

And last but not least, the drawings....

Enjoy!
John

View attachment body.pdf

View attachment electrode.pdf

View attachment insulator.pdf

View attachment Screw cap.pdf

View attachment terminal.pdf

View attachment Spark Plug.pdf

 

[gmac]

John;
Very timely post and well done! Thanks for taking the time to document the build and for sharing the drawings, I'll get right at 'er.
Cheers
Garry

 

[AussieJimG]

Well done John, I have downloaded your drawings and noted the thread which has some good information about
constructing these plugs. Thank you for showing.

Jim

 

[jgedde]

Well, back to the drawing board. Spark plug v1.00 still works but the engine was getting hard to start, then it blew the top terminal an electrode right out of the insulator. The tip of the insulator looks like it melted.

I guess it's time to try Corian. The Teflon is SO soft... Any other ideas?

John

 

[stevehuckss396]

Corian is the way to go. I have corian plugs in my V4 going on 4 years now and still in great shape. I also get white corian pieces at a great price so thats one more reason for me to use it. It is also nice if you need something other than white. Corian comes in many colors.

 

[jgedde]

Beautiful spark plugs, Steve! Where did you get the Corian?

I'm a purist, so I'd want white...

I'll give it a try if I can source some...

Thanks!
John

 

[jgedde]

Found some white off cuts on ebay. Enough for many plugs!

Thanks again!
John

 

[stevehuckss396]

Found some white off cuts on ebay. Enough for many plugs!

Thanks again!
John

I was going to suggest Ebay but also my pal Jimmy makes a monthly stop at the local cabinet shop where they give him scraps. In return we send over projects made of corian. I have sent steam engines and Ron sends pens and pencils.

 

[Krutch]

I was wondering if the Teflon would hold up. I kind of thought it wouldn't stand up to compression and heat. But then, it isn't a high compression or constant hit engine.
Still very interesting thread, thanks.

 

[jgedde]

I made the first plug from Corian last night. I like the Corian much better. It machines easier and is much more rigid. It also allows the insulator to be glued into the spark plug body. It is a bit brittle, but machines easily with sharp tools.

I didn't take any pictures... Sorry.

I do plan on making another tonight with some minor design changes. I'm going to have the insulator protrude less (electrode stick out further) to get a hotter plug. The engine misfires and floods a bit too easily if too much fuel is run.

I used JB weld to hold the insulator into the body and keep the terminal from unscrewing. I was a bit too generous with the JB and when I screwed the terminal in, hydraulic pressure cracked the insulator along its length. The plug is still usable, but I do get some phantom 3/8" long arcing along the crack line... It seems as if the JB Weld isn't the best dielectric material in the world.

That's another reason I'm going to make a new one. Third time's the charm - I hope... I'll take pictures this time.

If the design changes to make the plug run a bit hotter are successful, I'll update the drawings and repost.

So, here are the changes in a nutshell:
1) Shorter insulator at the spark end.
2) Different epoxy.
3) Thread locker to hold the terminal from unscrewing instead of epoxy.

But, Corian does seem like a winner.

John

 

[jgedde]

I was wondering if the Teflon would hold up. I kind of thought it wouldn't stand up to compression and heat. But then, it isn't a high compression or constant hit engine.
Still very interesting thread, thanks.

Admittedly I contributed to the failure. I was running the engine under load with 10% glow fuel. I was trying to have the engine hit on almost every stroke to see how the plug would hold up. Parenthetically, I was really surprised how much power the Farm Boy makes! All in all, the Farm Boy is a first class design but I think it's a bit over-carbureted.

In any event, the Rimfire VR2L held up to the stress test as did the Corian plug (although I used ethanol for the latter test).

The Teflon is just too soft in my opinion and expands and contracts too much. It tended to push the electrode out because the Teflon just didn't have the holding power. So, I'll be using Corian for sure. If that ends up being a poor choice, Macor will be the next attempt ($$$ )

John

John

 

[Cogsy]

I have successfully used corian in the plugs I made for my Upshur farm engine. I haven't yet got it running in hit and miss mode (it's shelved until I finish the Rupnow engine) and I've had some quite long runs hitting every compression with no plug failures yet. In fact, with a few hours running on the engine, I'm still using the same plug, even though I'm running 2 stroke dirt bike fuel in it with lots of oil.

I used selleys 5 minute epoxy to glue the electrode in, no swage at all. So far it hasn't moved.

 

[jgedde]

I have successfully used corian in the plugs I made for my Upshur farm engine. I haven't yet got it running in hit and miss mode (it's shelved until I finish the Rupnow engine) and I've had some quite long runs hitting every compression with no plug failures yet. In fact, with a few hours running on the engine, I'm still using the same plug, even though I'm running 2 stroke dirt bike fuel in it with lots of oil.

I used selleys 5 minute epoxy to glue the electrode in, no swage at all. So far it hasn't moved.

Thanks fore the vote of confidence on the Corian.

I like the looks of the swage but it does have one major drawback... It basically means I need to make a new spark plug body every time I change something. It's hard to undo the swage without damaging the body.

I figure using just epoxy like you did would be fine. For this run, maybe I'll just use the glue and only swage it when I'm happy with the results.

John

 

[Draw-Tech]

Hi jdedde

Here is what I designed for a sparkplug. Insulator is corian, and removable to replace burned components.

Jack
Draw-Tech

View attachment Spark Plug Assembly.pdf

 

[jgedde]

Hi jdedde

Here is what I designed for a sparkplug. Insulator is corian, and removable to replace burned components.

Jack
Draw-Tech

Hi Jack,

Now that's what I call a pretty spark plug! Nice work! I really like the rebuildable design. It's remeniscent of very early spark plugs from the turn of the century.

Is the body brass or does it just look that way in the photo?

Cheers!
John

 

[Draw-Tech]

Hi jgedde

Yes this model I made was a Brass body, .032 Dia. tungsten (tig) welding electrode. I have also made the base from Stainless, with Brass flange nut.
This can be modified to proportion of your engine design.

Jack