1/8 Scale Model T spark plug

[gbritnell]

The next step is to center drill and drill the though hole for the insulator. I'm using a 1/16th drill which will be .005 smaller than the upper body of the insulator so that when it's inserted it will stop against the shoulder. I do the drilling after the threading because from my experience if you drill first and then thread there's to much load from the die and the thin wall will sometime collapse.

[gbritnell]

With these preliminary steps completed it's on to the mill to machine the hexes on the plug body. I have a collection of threaded hex bushings to hold parts in my dividing head so I found the appropriate one and screwed in the plug body and then mounted it in my chuck. This type of chuck is called a set-true chuck which means after clamping your part you can adjust the chuck so that you workpiece is exactly centered. The first picture show this procedure. The following pictures show the machining of the two different sized hexes. These early plugs had an internal thread so that the insulator could be secured by a threaded bushing.

[gbritnell]

The bushing with the plug body is now removed from the dividing head and put back in the lathe to cut the length and to cut the recess stop between the body and the threaded bushing.

[gbritnell]

A little bit of filing and a few rubs with a fine emery board and the body is finished. Now on to the insulator. I used white styrene for the insulator because if this plug were to be mounted it could be screwed into a threaded hole. If it was to be soldered I would have used Teflon for the insulator to take the heat of soldering. The center hole was drilled .04. I wanted to turn the center electrode long enough to pass through the entire insulator to hold everything securely and .04 was about as small as I wanted to go for this length. (.380) The outside of the insulator was then turned down to the needed dimensions, .062 at the bottom to fit in the plug body and .067 at the top to provide the stop shoulder. Along with this I put two spark ribs on the outside. I apologize for the detail picture as the camera didn't focus on the right spot.

[gbritnell]

The insulator was cut off and, cleaned up and set aside. The remaining part is the center electrode. The first turning was the .04 shaft to go inside the plug. After this I detailed the top. The full sized plug had a screw on nut so I put a couple of little steps on the top to replicate this.

[gbritnell]

As I was getting ready for my next segment I noticed an icon had popped up in my previous post. I don't know where it came from. Anyway, now that all the bits are finished I took a couple of pictures with them on a penny so you could get an idea of the size. I then pushed everything together and took several more shots of the finished plug. If anyone needs a model T spark plug in 1/8 scale I have one laying around.

[xken]

Great tutorial! I do hope some people appreciate the effort you put in to doing this. I know what kind of time it takes to stop and document.

One question and I may be showing my lack of thread sizes, you mentioned you cut the threads to 3-56 size; I am familiar with 2-56 and have tap and die for 2-56 . Is 3-56 just the next size up that you machinist's keep away from the rest of us?

Being a modeller first and a self taught on machines I still have a great deal to learn.

Bring it to Glenmoor and we could have some fun with people with it.

Again great job and keep up the great work you are doing!

Ken

[gbritnell]

Hi Ken, 3-48 is the next size up from 2-56 in the course thread sizes. 3-56 is the fine thread for this diameter. I have course and fine taps and dies for each size from 0-80 to 3/8-24 with numerous special threads in between.
George

[xken]

Thanks for the insights, I will have to find a thread chart somewhere. Where do you get the taps and dies for these sizes?

Ken

[Deuces-wild]

Hey Ken, You might want to try J&L Industrial supply. They carry a machinist's hand book that has all the tap/drill size charts for watch makers and also the bigger stuff. The hand book is about??? 4 inches thick and packed with lots of valuable info. The book costs about 30 bucks last time I looked. The one I have at work is the 24th edition which I bought in the mid '90's.

[modelmaker]

Hi George , wonderful tutorial on making spark plugs. Like ken I'm also a self taught model engineer and still have alot to learn , I have my own way of making spark plugs but this is invaluable information and I will take it on board , great clean result Thankyou for sharing.

Sean

[strevo]

Hey Ken, You might want to try J&L Industrial supply. They carry a machinist's hand book that has all the tap/drill size charts for watch makers and also the bigger stuff. The hand book is about??? 4 inches thick and packed with lots of valuable info. The book costs about 30 bucks last time I looked. The one I have at work is the 24th edition which I bought in the mid '90's.

Guido,
A guy I work with is a tool maker by trade and has a copy of the machinists handbook from the 60's. The funny thing is, even though they keep releasing new editions, for the most part, it's all the same stuff. It really is like 4" thick!
-Steve

[Deuces-wild]

You got it Steve!!! I"d still like to find out where those micro taps are sold at. Maybe www.smallparts.com has them??? It might be worth a look....... Yep! They got'em. Check it out!

[chuckylane]

[QUOTE=gbritnell;36887]As I was getting ready for my next segment I noticed an icon had popped up in my previous post. I don't know where it came from. Anyway, now that all the bits are finished I took a couple of pictures with them on a penny so you could get an idea of the size. I then pushed everything together and took several more shots of the finished plug. If anyone needs a model T spark plug in 1/8 scale I have one laying around. [/That is THE COOLEST,I can not wait to tell a friend of mine about it,Fantastic]