John Smyth 4-1/2 hp Restoration – Part 2 of 5

Peter Rooke makes a replica muffler and cart for his 1914 John Smyth engine.

| April/May 2016


There appear to be many different muffler styles for this engine; a plain ball muffler, a ball muffler with a ridge around the gap and a “tin hat” style. Most photographs examined showed a ball muffler for this size of engine and a comment noted during the research said that the plain muffler was used on the early engines to around 1915.

The diameter for a 4-1/2 hp engine was 7 inches, but mufflers this size are nonexistent in the U.K. Indeed, there was already a 5-inch ball muffler sitting in our son’s garage in Houston, Texas, waiting for someone with spare weight allowance to bring over to the U.K.!

Thoughts then turned to making a replica muffler, the difficulty being to make the walls thick enough so it would sound like an original. With no press equipment there was no way to form a steel muffler with a 0.25-inch wall thickness, so rather than make patterns for casting I decided to fabricate one.

The first step was to make a wooden pattern for one-half of the muffler, so several bits of hardwood were glued together, ready to be turned and shaped on the lathe. This would be 6.5 inches maximum diameter to fit inside the 0.25-inch thick walls.

The radius tool for the lathe could not be set for a 6.5-inch diameter. A series of calculations were made to generate the diameter of the dome at 0.125-inch intervals so the adjustment dials could be used on the lathe to create a rough dome. This was then sanded to give a fairly smooth finish. A pattern was cut from cards to get the shape for 10 pieces to create the dome before cutting these out of strips of 0.125-inch thick steel.

These segments were then bent to shape to fit the dome, first bent on the long axis around a small pulley, and then the short axis first using the vise and then finishing off with an engineer’s ball peen hammer. Once all 10 segments were shaped, the wooden pattern was soaked in water before resting the strips on it and tack welding them all in position.