Seeing all of the work I had already completed this week gave me the inspiration to do even more. Funny how that happens. While I had the free time available, I though it best to put it to good use and get the engine closer to that magical moment of being able to run it for the first time.

Having done all the plumbing for the combustor, it was time to locate and install the spark plug. Spark plugs can be finicky things in these engines and trying to find just the right location to mount one can be frustrating to say the least. In one spot, it may not light the engine. In another spot, it may light but burn the electrode off of the plug. There is a balancing act that occurs to find the spot where ignition occurs easily, and the plug is out of harms way.

Spark plug mounting plate

My first step in installing the plug will be to fabricate a suitable mounting plate in which to thread the plug. I had a large piece of steel plate which had lived a previous life as a combustor end cap I was using for testing another engine. This material was suitable, so I started out by drilling and threading the hole in the plate to mount the spark plug. After threading the hole, I tightened the plug down on the plate to make sure that the threads would be strong enough to keep the plug in place.

Chucking up the part for turning

I cut the center out of the plate so that I could turn it to the proper size on the lathe. To mount the piece in the stock I first tried to use the socket for the spark plug as a holder, but soon realized that there would be way too much wobble. After all, I am still pretty new to the whole machining of parts thing. I quickly realized the error of my ways and saw that I could just attach the whole assembly to the lathe chuck using nothing more than the sparkplug itself.

Turning the part

It becomes clearly visible in the photograph above that this was a much better solution to hold the part in the lathe chuck. Sometimes, the simplest solutions are the best. With the part firmly in the chuck I turned down the plate to match the diameter of a small pipe I would be attaching to the plate. The pipe will allow me to recess the spark plug mount inside of the combustor which you will see as the part progresses.

Turning a step

Once the part was turned down to the proper size, I created a stepped lip on the face of the part so that it could mate with the tube it would be welded to. This would assure a nice tight seal with good weld penetration.

Fitting the tube

By making gradual passes, I soon had the step turned down far enough to accommodate the tube. It is a tight fit and will hold the tube snugly on its own. The lathe, which is also a very new addition to my shop, is an indispensable tool just like the mill. It truly amazes me how I ever survived without it. The quality of parts that you can produce is just so much better, even with these mini machine tools.

Welding the assembly

Once the parts were all turned to the proper diameter, I used the mig to weld the mounting plate firmly inside of the tube. Although the weld bead looks large in this photograph, it is actually that the part is just smaller than what you see here.

Fully welded

Once I had a good weld bead all the way around the mounting plate, I allowed the part to cool before attaching it to the lathe again. By turning down the outside of the tube for the full length, I could clean up the diameter of the entire assembly as well as remove any excess weld bead.

The finished mount

After lathe turning the assembly, the whole diameter was nice and even. With the part mounted in a vise, you can see how much of the plug will be inside of the combustor. By taking measurements of the threads on the spark plug, I was able to remove most of the threaded part in the lathe and leave enough to still thread into the mount. Removing the threads gave me a spark plug with a long center electrode which could arc to the side of the flame tube and give a much bigger spark.

Drilling the combustor

Even though spark plug placement is very important, most of it boils down to trial and error. By taking a few measurements and combining them with best guess estimates, I came up with a mounting location that would place the plug between the primary and secondary holes of the flame tube. I laid out the hole on the outside of the combustor housing and used a hole saw to make the opening for the spark plug mount.

Flame tube

I allowed the center drill of the hole saw to penetrate right into the side of the flame tube and make a pilot hole. This hole would be the exact location that the spark plug would pass through to the inside of the flame tube and allow ignition. I then enlarged the flame tube hole just big enough for the electrode end of the plug to pass through. You can see the bottom of the plug just peeking into the flame tube in the picture above.

Welding the recessed mount

With the recessed plug mount located and the flame tube drilled out, I tack welded the mount into place. After my alignment checks showed that the plug had not shifted due to the metal warping from the welding heat, I finished the weld. I left the plug in the mount while welding to keep the threads from becoming damaged.

Finishing off the mount

With welding completed, I used a grinding wheel to bring the edges of the mount flush with the outside of the combustor housing. I then turned to the trusty flap disc on the grinder and polished things to look nice and shiny. The entire operation was a success, and the mount looks really good. The top of the plug barely protrudes from the housing and should be nice and safe in its new home.

If this mounting arrangement looks familiar to some viewers, it is because it is very similar to the one used by Don G. at www.rcdon.com for his GR-7 engine. I liked the look of it so much that I decided to use the same approach and hope that he doesn't mind.

Ignition system

Sadly, when preparing the combustor for atmospheric testing I managed to burn out my ignition unit. This ignition was built using the parts from a "stun gun" and seemed to work rather well. Russ from www.badbros.net found out about the ignition and had this to say..

"Electronics only work because of the Magic Genie in the little chips and wires. When you mess up and let out the smoke, that is the Genie getting out. Things just never work the same once the Genie gets out!"

I will have to build a new ignition unit later, and find another means of igniting the combustor for atmospheric testing.

Fuel system for tests

It is a shame that my ignition burned out, because I already had the fuel system ready to go for testing. I am simply going to draw fuel from the gas can and run it through a bypass valve setup to regulate pressure to the nozzles. I am using one of my trusty ShurFlo pumps for the fuel during the tests.

Combustor on test stand

The combustor is already on a test stand and ready to go, but it goes back to the whole issue of an ignition source. As soon as I find another way to get a hot spark to the plug I will be firing this thing up and getting some good pictures and video.

Keep checking in, as I should be able to make an atmospheric test run sometime very soon. Thanks for stopping by, and if you appreciate what we are doing, please contact me about making a donation to the site! Potential sponsors are always welcome!

Gary Richards

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