I drove around to the local auto parts stores and purchased all the Dorman #47408 1/8" x 7/32" right angle vacuum fittings I could find because, to me, they looked like T-18 spark plug boots. Then, I experimented with various modifications to adapt them to both the CM-6 and the 1/8" diameter 20kV plug wire that I'm using. This wire is distributed by S/S Machine & Engineering.
Straight out of its package, the small diameter end of this fitting is a perfect size for gripping and strain relieving the 1/8" diameter plug wire. However, a simple shop-made insertion tool is needed to actually get the wire into the boot. The i.d. of the body, though, is too small to fit over the CM-6, and so it must be opened up some. This is much easier said than done in rubber, and I was only able to come up with a passable solution using tools I had on hand.
There are several ways, involving various degrees of complexity, to make an electrical connection to the top of the spark plug. The simplest way is to pull about four inches of wire through the boot, strip off 2-1/2 inches of insulation, and then coil the bare wire up into a small pillow. The wire is then pulled back through the boot until the pillow is at the bottom of the boot where it can contact the electrode when the plug is pushed into the boot. The boot will grip both the wire and the plug tight enough to maintain a reliable connection on practically any model engine. I torture tested a few boots made up this way because I was really interested in using this simple connection for my application. What I found, though, was that after twenty or so installs the ultra-fine strands making up the 1/8" diameter plug wire began to break away. I was more worried about this debris finding its way into a combustion chamber than I was about the integrity of the electrical connection. For most users where the plugs won't be frequently removed from the engine, this simple contact using this particular wire is probably very adequate. In my case, though, several plugs will frequently come in and out of the engine; and so I wasn't entirely comfortable with it.
After trying unsuccessfully to make compression springs work, I finally arrived at a tubular contact design similar to that found in plug wire kits for early era custom cars. The sequence of photos shows the steps I followed to make up four plug wires using the last four fittings left over from my experiments. Additional fittings for the rest of the wires are on order from an online supplier.
I started by sliding the boot onto a 3/16" diameter mandrel in my lathe and cutting off a portion of the body to give the boot a final height of 3/4". As mentioned earlier, the i.d. of the body must be opened up to fit over the plug body. The next photo shows the three tools I used to do this: a 1/4" rougher, a 5/16" rougher, and a 9/32" annular cutter. I found that 3-flute cutters produced a nicer hole than the 2 and 4-flute cutters that tended to create polygons. I manually held them in end-mill holders to reduce hand fatigue and marked the depth of cut directly on the cutter to avoid going too deep. A bi-directional twisting motion was used to grind away the rubber and open up the hole. I found it useful to first chill the fittings in a freezer for an hour or so. I started with the annular cutter using the rear of a drill bit as a pilot to get things started straight. I then went deeper using the 1/4" rougher, and followed that with the 5/16" rougher. I tried to grind a special purpose cutter, but I wasn't able to improve upon the grinding action of these cutters. When the boot slid onto the plug with the 'right' snug feel, I stopped and went onto the next part.
The contact I designed starts as a length of 9/32" o.d. brass tubing purchased from a local hobby shop. After cutting a .360" long piece, both ends were slightly beveled by manually rotating a 45 degree countersink in them. The bevel on the top end will later help guide the contact onto the plug. On the mill, a 1/16" wide slot was cut along the entire length of the tube. As shown in the photos the corners were chamfered using a pair of sharp side cutters. The bottom-end chamfers are wide enough to provide a clearance slot for the plug wire. An .040" diameter hole was drilled for the plug wire by passing the drill through the slot and drilling close to the bottom end of the contact. A half-round file then cleaned up all the sharp edges. Finally, the contact was slid onto a #3 drill and pliers were used to carefully reduce the diameter and close up the 1/16" slot while maintaining the circularity of the contact. The chamfered wire clearance slot was re-checked.
To prepare for soldering, the wire insertion tool was pushed into the boot. This tool is just a short length of .160" o.d. x .130" (reamed) i.d. brass tubing pressed into a convenient holder. This tool is a very snug fit going into the boot, but it will allow the plug wire to easily slide into the boot where it can be grabbed with pliers, or preferably, a hemostat and pulled through the boot. The insulation was was then stripped back about 1/8" and the wire was soldered into the hole that was drilled earlier. Care has to be taken to not allow excess solder to flow down inside the contact where it can block the insertion of the spark plug. Any wire left protruding outside the contact after soldering was clipped off and the sharp stub was rounded with a file.
The plug wire passing through the wire insertion tool was then rotated to orient the attached contact so the slot runs parallel with the insertion tool. The contact insertion tool was then used to push the contact down into the boot. This critical dimensions of this tool include a .218" dia. x .250" long nose followed by a .250" dia. x .250 long shoulder. The tool was inserted into the contact and then, with a coordinated effort between both hands, the contact was pushed into the boot while the excess plug wire was continually pulled out through the wire insertion tool. When the contact was fully inserted, the wire insertion tool was removed. The boot was then slid over a test plug several times to check the mechanical fit, and an ohmmeter was used to verify continuous electrical continuity while the boot was twisted on the plug. If the hole in the boot was enlarged correctly there will be enough spring force created by the surrounding rubber to hold the contact snugly closed around the plug electrode.
I found it interesting that all my NGK plugs showed zero resistance as expected while the Rcxel plugs I have typically showed tens to hundreds of ohms of resistance. I got the exact same non-zero results measuring directly across them. I have no idea where the resistance is coming from. The CM-6 is not a resistor plug and the RcXels I have are not labeled as being iridium plugs.
If there is any problem with the contact, it can be removed without damaging the boot by snipping the plug wire outside the boot and grabbing the contact with a pair of long nose pliers. Using a twisting motion the contact can be collapsed and pulled out of the boot so a new one to be installed. - Terry
The patience you exhibit is as impressive as the build.
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