The stem of the fuel mixer was broken off, leaving part of its threaded end stuck in the cylinder head. I stripped down the mixer and cleaned out the inside before squaring up what was left of the thread and recutting it with a 0.750-inch NPT die. There was not enough thread length to refit the mixer using this short section of new thread, as the large head on the fuel adjustment needle would rub against the cylinder head. I had to make an extension collar to screw onto the remains of the mixer and then into the cylinder head.
I placed a length of 1.250-inch diameter steel in the lathe chuck and drilled through with a 0.750-inch drill to a depth of 1.40 inches. I turned 0.750-inch of this length to a diameter of 1.050 inches before cutting a 0.750-inch male NPT thread on it. I started this thread using the threading tool on the lathe before finishing off with a die. I cut this bar to a length of 1.30 inches and reversed it in the chuck.
Next, I screw cut a 0.750-inch NPT female thread to a depth of 0.340-inch to match the length of the remaining thread on the mixer. I bored out the remainder of the new collar to 0.80-inch to match the bore of the mixer tube. I trial fit both parts to ensure that the mixer was correctly aligned in the vertical when screwed in tight. I also made a check to ensure that it did not foul the pushrod. This showed I needed to lengthen the lathe cut thread by 0.010-inch to ensure the mixer lined up correctly.
Once the fit of the new extension piece was correct, I rounded any rough edges with a file and then fit it to the mixer, using Loctite to hold it in place. I covered the part of the new thread that fit in the cylinder head with tape for protection and later, when all work on the mixer was finished, I rusted the collar and any exposed thread on the mixer to match the original using reverse electrolysis (learn more at Reverse Electrolysis Primer).
The tip of the mixer needle was badly worn and pitted so I had to replace it. It was difficult to measure the old taper, and I found I also needed to recut its seat. I used a 40-degree included angle for the new needle. After measuring the length of the old needle, I sawed off the corroded one and drilled a 0.1875-inch hole in the threaded section of the head.
I turned a new needle using some 0.1875-inch brass rod. With the lathe compound slide still on the same setting, I also made a lapping point so the seat could be trued and matched to the needle. I brazed the new needle into the hole in the shank of the old head, keeping the overall length the original size.
The choke plate fitted to the mixer was made from some copper sheet that was not a good fit because it was bent in several places. The mixer body was not square where this fit so I smoothed it up with a file before making a new choke plate from some 0.0625-inch thick metal sheet.
Igniter and trip
When the engine was damaged during shipping, the stem of the moving electrode of the igniter bent, the arm on the igniter trip broke off and the catch for the trip rod to push against chipped. The peg for the trip arm spring on the trip assembly was broken and the push rod bent as well.
My first task with the igniter was stripping it to remove the damaged parts. The only way to extract the damaged movable electrode was to saw through the stem below the bent section after first removing the fixed electrode.
The thin shroud from the electrode body, which covers the first half inch of the moveable electrode shaft, was partially broken and the remainder soon fell off. One way to repair this is to fully sleeve the passage for the electrode shaft, but the hole was not badly worn and did not need any remedial action. I instead decided to fix it by making a small collar and fitting this in a short recess bored out around the hole for the electrode shaft.
I clamped the igniter to the milling table and then centered the hole for the moving electrode under the boring tool. I cut off the remains of the old shroud and bored the hole for the electrode out to 0.580-inch diameter, the size of the old shroud, and 0.250-inch deep. I used homemade plug gauges to verify measurements when boring the hole.
I turned a cast iron collar on the lathe 0.582-inch diameter with an undersize central hole of 0.368-inch drilled through. This would be reamed to size, 0.375-inch once fitted.
I chilled the collar overnight in the freezer and placed the body of the igniter in the range cooker overnight. The next morning the new sleeve was easily pushed in place. Once cool, I used the reamers up to 0.375-inch to get the finished size.
I placed the head of the moving electrode in the 4-jaw chuck on the lathe and used the dial gauge to center it on its shaft, not an easy task as the shaft was a little uneven. I sawed the shaft off and drilled a 0.375-inch hole through the head. I brazed a length of 0.375-inch drill rod, slightly longer than needed, to this old head. I would trim this new shaft to length and drill the hole for the taper pin when all other parts were finished and ready for a trial fitting.
The contacts on both electrodes appeared to be made from copper or some similar soft alloy, and they were badly misshapen. I drilled both out and made new contacts from an iron nail and then press-fit them in place, hammering the stems over to lock them securely.
The repair to the igniter trip was more of a problem. The trip was made of hardened iron and any repair would have to stand the shock of repeated tripping as well as resisting wear from the continual action of the trip rod.
One option is to make a completely new trip from high carbon steel and then harden and temper it. Before this, however, I attempted to repair the damage to the trip catch by cleaning it then welding with ordinary rod, which would absorb some of the carbon from the cast iron and harden it a little. As usual, I would have to carefully control heating and, more importantly, cooling to avoid making the cast iron brittle.
I secured the trip on a short length of threaded rod to make handling it a little easier. I heated the cast iron in the oven for half an hour before welding it and then used the welding rod. As soon as the weld was finished the part was put in a hot oven for an hour, and then into a cooler oven for a further hour. I tested the weld by hitting it several times with a hammer to shock it to see if it dislodged or if the cast iron shattered. Nothing moved so I cleaned up the weld to match the original shape. I did this using a small grinding wheel and then grinding points before finally using diamond files to clean up and finish-shape the hard weld.
I repaired the broken trip arm with ordinary mild steel. I drilled a 0.109-inch hole 0.250 inches deep into the remains of the broken trip. I turned a 0.109-inch diameter spigot on the end of some bar that I then roughly filed oversize to copy the profile of the original arm. I brazed the two parts together and then finish-filed to shape once cool.
I trial-assembled the components of the igniter so I could mark the position and alignment of the hole for the taper pin and measure the moving electrode shaft so it could be trimmed to length. To align the parts, I assembled the igniter without springs and set the points at the rest position, a gap of 0.050-inch. To hold the contacts in place, I put a shim between them and used a clamp to hold them together. I marked the position of the pin hole with a center punch through the outer collar before drilling through and reaming with a homemade taper reamer.
New springs were necessary as one was missing and the other broken. So the new springs did not look out of place, I cleaned all oil off both of them and then allowed them to lightly rust to dull their shiny finish. When the color was right, I oiled them to arrest the development of any further rust.
Once I had completed these tasks, I assembled the components of the igniter using new mica washers and tube to replace the original oiled ones. Then I checked the operation of the igniter by testing it with a battery and coil.
Before fitting the igniter to the engine, I made a new copper gasket to replace the old one. One thing to watch out for when refitting this igniter with its small mating surface is to make sure it seats flush by tightening the two nuts evenly. If it is not even it is possible that compression leaks will occur.
The original dome muffler appeared to be 6 inches in diameter. It is possible to form each of the two replacement halves by beating out steel, although it takes some time and is a little noisy!
To form the first side, I cut out some 0.125-inch thick plate, slightly oversize at 6.250 inches to make it a little easier to hold around the edges. I marked the center point on both sides and then slightly scored a 5-inch diameter circle on one side. I marked this into four sections to give reference points when forming the metal.
It is possible to beat out the dome shape by resting the steel on a sand bag, but an alternative method is to use a section of pipe with a 5-inch internal diameter. The oddments of pipe in my workshop were not the right size, so I rolled a length of 0.125-inch thick steel then welded a section of 5.50-inch pipe inside. I trued this using the lathe before chamfering the inside edge and giving it a smooth, rounded profile that would not gouge the metal when forming.
I used a ball peen hammer to hammer out the steel, first cleaning the ball of nicks and burrs with emery cloth to prevent marking the metal. To form the dome in the metal, I held the sheet centrally over the prepared forming tube before hitting it with a doming hammer. I started at the edge and worked around it in a circle. I always finish doming at the starting point, by drawing one of the lines out from the center so I know where to start again to get an even indentation for the dome. In addition, I take care to always hit inside the circle, then keep working around the edge until the dome has started to form, then it will rest inside the circle of the former.
Once I completed this, it was a case of patience and methodical work, round and round until the shape started to appear. When I felt that the metal had started to stiffen, I annealed it then cleaned it again before continuing.
If thick sheet is being used, say thicker than 0.125 inch, it might be necessary, once the dome has started to form, to get the metal red hot to hammer the rest of the shape. With thinner material the full dome can be formed without heating.
I used a cardboard template to monitor progress and to check the profile of the dome from time to time.
The outer edges of the metal started to curl from the hammering, so I leveled these out by hammering flat on the anvil once the dome was nearly shaped.
Once the dome was the right shape and even, I planished the metal to level out the surface and remove the hammer marks. This required a domed stake, in this case I used the rounded end of a chisel and a small, light hammer. Working out from the center of the dome in a concentric circle, I use VERY LIGHT blows on thinner sheet metal. With the thicker 0.125-inch plate used for this muffler, I used a small hammer with a significant tap. I moved the metal continuously around the stake at the same time I was hitting it, repeating the process until there was a reasonably smooth finish that would look perfect under a coat of paint.
Once satisfied with the finish of the dome, I rough-trimmed the edges to leave a diameter slightly bigger than the required 6 inches. I first cut the surplus using the hacksaw before finishing with a grinding wheel.
I formed the second half of the muffler exactly the same way. I placed each half of the muffler in the 3-jaw chuck so I could accurately mark the position of the three bolt holes using the center point of each jaw, and then drilled these.
I used three bolts to join the two halves and pressed the raised dome back to give clearance around the bolt holes. I trimmed a length of steel on the lathe to make a forming punch, making the tip the diameter of the hole, 0.250-inch, fitting inside a similar sized hole in an iron block. I turned the next part to 0.625-inch diameter, slightly rounding the bottom edge so it did not cut into the dome when used.
Next, I turned some 1.50-inch diameter steel to form the socket for the short pipe nipple used to fix the muffler to the cylinder head. I reduced the inside of this steel to 1.250 inches.
After drilling a pilot hole through the middle of one of the domes, I bored the hole out to 1.250-inch so that it would slide onto the steel ring and then brazed this in place.
The original mufflers had two baffles in each half and spacer rings cast at the edges. I made the spacer rings by simply machining six 0.080-inch thick washers that I brazed in place.
There are two ways to make baffles: either by cutting lengths of steel tube the correct length or by rolling steel strip to the required diameter. I tack-welded these in place to check alignment and fit before final welding, and then cleaned them up with the grinder.
Read Part Two of Peter Rooke’s tale of gas engine restoration in Fuller & Johnson Rejuvenation.