A Bull Dog Gets Its Spark Back
Editor’s note: This two-part article is a supplement to Peter’s four-part series on restoring a 1-1/2 HP Bates & Edmonds Bull Dog, which began in the October/November 2008 issue of Gas Engine Magazine.
I had identified at an early stage of my Bates & Edmonds Bull Dog restoration that the engine was originally supplied with a Webster oscillating magneto, and had therefore been on the look out for one for well over a year.
I needed an M type magneto for this small engine and had failed to win a number of them I had bid for on eBay. Eventually, I was successful, though the Webster I found was an AMM – not the exact model I needed (having the more common clockwise rotation when tripped). But it appeared to be in good condition and was described as hot.
I understood that it was an “easy” matter to convert the magneto’s rotation for the Bull Dog so I then started to advertise for the appropriate igniter bracket, with the reference number 303M50. I was not in the least surprised that I failed to get any response and so, rising to the challenge, I decided to make one.
I had two options: make a pattern to get a casting made or try and fabricate one. I still had a small part of the old bracket and I scaled a number of photographs that I had of the correct bracket for the dimensions. Thinking that I might not get all my estimated measurements completely right, which would have forced me to alter components when assembling it, I decided to fabricate the mounting. In addition, I had a small part of the original bracket that had been used as a blanking plate on the engine, and I could make use of this, thus preserving another small part of the original engine.
The next step was to think about the order of work. I could see that care would have to be taken to ensure the trip lever of the Webster was aligned properly so that it would be activated by the trip arm and would then hit the movable electrode. I therefore planned to make the valve rod clamp and trip arm fittings first, so that I could then clearly identify the width measurements. I then built up the main body of the igniter and the electrodes, and then made the platform on which to mount the magneto.
While some dimensions could be accurately accessed from current parts, there was some estimation of others by scaling numerous photographs, the shape of some parts being set out in the line drawing in the Bates & Edmonds instruction leaflet I had purchased.
Valve rod clamp and trip arm
The first part to be made was the valve rod clamp of the trip mechanism, which fits on the end of the push rod and is also bolted to the exhaust pivot arm.
The valve rod clamp was machined from a 4.50-inch length of 1.00-inch square steel, turning one end, which was then tapped with a 0.375-inch-by-16-inch thread for the push rod. This steel was then cross drilled with a 0.500-inch hole, for a bolt that would secure it to the exhaust pivot arm, before using the milling machine to remove surplus metal and shape it. The rounded part of the body around the cross hole was then profiled using files, two washers being made to the required diameter to act as templates. Finally a 0.125-inch hole was drilled in the top aligned with this pivot hole and counter bored for oiling the pivot pin.
A second piece of steel was brazed to the valve rod clamp as the upright part with some 0.562-inch round steel brazed in a hole drilled at the top as the a pivot pin for the push rod journal. A 0.281-inch hole was drilled in the underside, into which was brazed a piece of the same sized steel that had been bent 90 degrees, to act as the anchor for the spring.
The trip rod journal was machined from 0.875-inch square bar first turning and drilling the hole for the 0.312-inch-by-24-inch thread of the trip rod. A cross hole was drilled for the 0.562-inch pivot pin before filing the bar to shape, again using steel washers, also known as filing buttons, as a guide. When finished, a 0.125-inch oil hole was drilled in the top and counterbored, with a 0.188-inch of strip steel, shaped, drilled and brazed in position on the underside to hold the spring.
The trip rod itself was some 0.312-inch square bar turned and threaded 0.312-inch-by-24-inch, and a piece of drill rod was used so that the end could be hardened. To harden the tip of the rod, it was heated to cherry red, before quenching in oil, agitating the trip rod to speed up cooling. After cleaning it back to bright metal it was heated again to a pale straw color then it was allowed to air cool slowly to temper it.
The wedge was the most difficult part to complete. A 0.312-inch hole was drilled lengthways through a block of steel (0.625 inch wide, 1.050 tall and 1.650 long) for the trip rod and the wedge shape at the tip was formed by sawing off the surplus material. This left a block that could still be easily held in the lathe 4-jaw chuck, enabling the drilling then tapping of a 0.250-inch-by-20-inch thread in the top for the wedge set screw, and then profiling of the raised center section.
While the block was held in the bench vise, small square files were used to rough shape the lengthways-drilled hole into a square hole for the trip rod, taking care to keep all cuts parallel and not to over cut. The end of a piece of 0.312-inch square High Speed Steel was ground to a 45-degree angle ensuring it had sharp edges. This tool was then pressed into the hole, using a vise, thus finishing off squaring the corners. When it had built up a raised metal burr, resistance was felt, so the tool was removed and the raised metal burrs were filled off. The tool was then turned through 180 degrees before repeating the process. After a few minutes effort and removal and replacement of the cutting tool several times, there remained a neat square hole.
A file was then used to complete the final shaping of the wedge, the taper to the base of the wedge and rounding the bottom of the angled section.
All that remained of the original igniter bracket was the flange clamped on the cylinder head, the remainder being sawn off long ago. All the holes were welded up so that it could be used as a blanking plate when the engine was converted to high tension ignition.
Having decided to fabricate a repair, the first step was to drill out all the weld used to seal it and face off the inside on the lathe to clean it up. Where the flange had been welded, the rust and weld were filed off to bright metal enabling the join between the weld and the cast iron to be identified. The weld could then be accurately center punched, and a carbide drill was used to remove it as it was quite hard. The original hole for the movable electrode was badly pitted so it was drilled oversize to 0.500 inch so that a sleeve could be fitted. This sleeve was turned from a length of cast iron, half an inch longer than needed and drilled undersize, 0.312 inch. Final drilling and reaming could take place when finally in position.
The bracket was then placed with the inside surface flat on the milling machine so that the face of the sawn off section could be milled flat as a reference point and to make it easier to join it to a new block. The thread for the priming cup was cleaned out using a 0.125-inch NPT tap.
The next step was to draw a plan of the bracket with detailed dimensions. As reference points, I had several photographs of an original bracket which were scaled using the measurements from the remains of the old bracket and the Webster magneto.
The next decision was the material to use. While the original bracket was cast, it would be easier and stronger for me to fabricate one from steel. A block of steel was found to provide the basis of the first section of the bracket, which would be joined to the original, using two pieces of steel plate being for the horizontal platform. The cast iron sleeve made for the movable electrode would extend through the steel block, providing a continuous surface for this electrode and stop steel from rubbing on steel.
I intended to braze this block to the original part, but to give it added strength there would be a hidden bolt which would also provide a temporary means to fix it for trial assembly. First a 0.500-inch hole was drilled in the new block for the movable electrode sleeve, which was then inserted to temporarily hold the new block in place. A 0.215-inch hole was then drilled in the new block, through and 0.500 inch deep into the old part, taking care to position it as far as possible from the electrode holes. This hole was then opened up to a 0.250-inch clearance hole in the new block and counter sunk for the socket head cap screw, a 0.250-inch-by-20-inch thread being tapped in the old block.
The old and new blocks were then assembled so that the hole for the fixed electrode could be drilled from the inside, using the original hole as the pilot.
The new block was then coated in engineers blue before being mounted on the old part so that its profile could be scribed, the sleeve for the movable electrode being used with the bolt to ensure correct positioning.
The milling machine was used to start the shaping of the new block, then grinding wheels and files leaving it slightly oversize. It was reduced to a perfect match after it was permanently joined.
A scale drawing was made of the side profile to work out the length of 0.375-inch steel plate required for the first part of the platform. This was cut slightly wider than needed so that when welded it could be filed to a seamless fit. Deep recesses were filed in both ends and the new block, which was then bolted to the old part. The igniter body was then bolted to an angle plate to help correctly align the other parts which were clamped to the work bench. The first part of the platform was tack welded to it, to check the angle, before filling each “V” in both sides of the plate with weld.
The surplus weld from these joints was ground off, smoothing the area of the joint to match the original rounded profile in the photographs of original brackets.
The main part of the platform was then cut from another piece of 0.375-inch steel plate and again both sides at the joint were cut so there would be a deep “V” to increase the surface area for the weld. The holes for the magneto mounting bolts were drilled before shaping the “U” cut out at the end of the platform. This was accomplished by first using a hacksaw to cut the straight sides, then chain drilling round the circular part, before finishing off with a half round file.
Again, the igniter was mounted on the angle plate and the last piece of the platform was clamped to the work bench before it was tack welded on both sides. After checking everything was square, each “V” was filled with weld before being profiled using a grinder and file.
Before finishing the bracket, it was necessary to make the eccentric roller assembly that is used to retard the ignition when starting the engine. Fortunately I had just acquired another engine that had a Webster magneto mounted on an igniter bracket. I was therefore able to remove the roller assembly in order to make an identical copy. In any event, work was necessary on the new acquisition, and it needed stripping as the spring on the control lever was broken and the moving electrode bent.
The roller was a simple turning exercise, but the pivot pin and its eccentric layout required a little more thought. The pin for the roller was 0.500 inch diameter, whereas the pivot fitting the hole in the bracket was 0.375 inch and offset from the center. A piece of 0.500-inch steel was set to run true in the 4-jaw chuck, before moving it 0.062 inch on one axis. This resulted in the required 0.125-inch offset when turning it down to 0.375 inch diameter, creating the eccentric part.
The control lever was milled from some 0.437-inch-by-0.500-inch bar, the round piece for the pivot pin being cut from a section of round bar and brazed on to the end of the arm which had been filed to match its profile. This was later assembled on the pivot with a 0.125-inch hole drilled for the taper pin. Finally, two 0.125-inch holes were drilled in the lever arm with 0.375-inch centers for the new spring, which was on order from Hit & Miss Enterprises. The broken spring from my other igniter bracket was used as the template.
The next step was to mount the igniter on the engine along with the magneto to check the position of the roller for the trip rod, so that in the starting position, the roller was lower thus retarding the ignition.
After marking out the position of the hole for the pivot pin I realized that I had not made enough allowance for the spring to latch in the wall of the side piece. Fortunately, the edges on the new block were still square, so an “L” shaped piece of steel was milled out and brazed into position. This part of the bracket was then shaped and milled out to provide the recesses for the control lever spring to locate in either the “start” or “run” positions.
The next step was the important task of fixing the new block and platform to the remains of the old igniter. To supplement the hidden bolt, I decided to braze the two parts together as I do not like to weld old cast iron. Brazing cast iron is not easy as it needs to be scrupulously clean. First, the mating surface of the cast iron was given a quick pass with the file to expose clean metal which was then cleaned with a brass brush, including the inside of the oversize hole for the moving electrode. The steel piece was cleaned with new wet and dry paper, as were the threads of the bolt. The mating surfaces were then given a liberal coating of flux paste. The outside of the cast iron tube for the moving electrode was also cleaned with the brass brush after scoring the surface with a file to ensure flux would be retained when inserted through the old and new blocks. Flux was also applied to the threads of the cleaned bolt, and the bracket was assembled and the bolt tightened.
After checking that alignment looked correct, the bracket was then put on the brazing hearth and surrounded by fire bricks. It was then heated to brazing temperature and care was taken to ensure that all braze had flowed to all sides of the block. At this point, it was left covered in the fire bricks overnight to cool down gradually and reduce the risk of making the cast iron brittle.
The following day the bracket was inspected to check that braze had flowed to all sides, and after cleaning a thin strip of braze was evident. The cap screw could not be turned with a wrench, so that was securely held in place. To finish, a spot of weld was put on top and filed smooth.
A Dremel and a small grinding pint were then used to smooth off the last of the surplus metal on the new block, and a 0.094-inch hole was drilled through the wall of the sleeve of the movable electrode for the oiling point. With this part of the bracket completed the next step was to reverse the rotation of the magneto, and make the electrodes and other fittings.
In Part 2, Peter works on the Webster magneto.