During lockdown, I decided to complete my long overdue Wyvern petrol engine. The 2 items remaining were the carb and silencer (muffler) I looked at the aluminium casting and drawings of the carb and decided that was going to be really difficult. I had one casting and machining the very rough casting was going to be a challenge. An alternative option was to use a vapour carb but I wanted the carb to be visually in tune with the Wyvern. Looking around I found a design by Jerry Howell. It seemed to be the correct size and visually compatible with the Wyvern. Partway through the build, I found that I would need to drill 2 holes 1mm diameter at an angle and have these intersect within the carb block. I thought long and hard about a method to achieve these critical drilling angles and concluded I couldn’t do it with any certainty of success. It was going to end in tears! More research and looking at many Wyvern/Nemett pictures on Google led me to the Malcolm Stride carb. It seemed very simple but drawings were impossible to obtain. I purchased his book, “Miniature Internal Combustion Engines” which had further photos but no details. On the HMEM site, I found an outline design by Brian Rupnow which was useful.
With all this info I set about drawing the parts using AutoCad. The design is very basic. I used a gramophone needle for the fuel flow. A 15 SWG stainless steel tube (from a horse syringe – ouch!) for the fuel head. I also incorporated an O ring into the barrel to provide a smooth throttle motion. The basic brass block was a 15mm cube.
The attachment to the Wyvern was a challenge due to the size of the flange, the need for strength and the size of the bolt heads. I decided to use 7BA thread with 8BA head. To achieve this the carb-to- Wyvern coupler needed to be an oval tube which can be seen in the photos. I also used an internal silver solder ring** to ensure that the joint between the coupler and the carb block looked good with no sign of silver solder overflow. Solder resist was used but proved not to be necessary since the wicking stopped at the joint edges.
A similar approach was taken with the silencer but this required more tricky machining to match the curvature of the silencer body to the coupler.
As a finish, I sandblasted the brass. I am undecided on the final finish. Clear lacquer could be an option. Polishing looks good but quickly tarnishes.
Question to the Forum:-
What is the best way to finish brass?. Sandblasting gives a very even colour/texture but will tarnish with time. Polishing is a pain and tarnishes very quickly.
What is the best way to finish/protect the aluminium silencer?
What is the best way to preserve the bolt heads from tarnish?
** Wasteful of silver solder but gives very clean joints. The trick is to minimize the silver solder used in forming the ring.
With all this info I set about drawing the parts using AutoCad. The design is very basic. I used a gramophone needle for the fuel flow. A 15 SWG stainless steel tube (from a horse syringe – ouch!) for the fuel head. I also incorporated an O ring into the barrel to provide a smooth throttle motion. The basic brass block was a 15mm cube.
The attachment to the Wyvern was a challenge due to the size of the flange, the need for strength and the size of the bolt heads. I decided to use 7BA thread with 8BA head. To achieve this the carb-to- Wyvern coupler needed to be an oval tube which can be seen in the photos. I also used an internal silver solder ring** to ensure that the joint between the coupler and the carb block looked good with no sign of silver solder overflow. Solder resist was used but proved not to be necessary since the wicking stopped at the joint edges.
A similar approach was taken with the silencer but this required more tricky machining to match the curvature of the silencer body to the coupler.
As a finish, I sandblasted the brass. I am undecided on the final finish. Clear lacquer could be an option. Polishing looks good but quickly tarnishes.
Question to the Forum:-
What is the best way to finish brass?. Sandblasting gives a very even colour/texture but will tarnish with time. Polishing is a pain and tarnishes very quickly.
What is the best way to finish/protect the aluminium silencer?
What is the best way to preserve the bolt heads from tarnish?
** Wasteful of silver solder but gives very clean joints. The trick is to minimize the silver solder used in forming the ring.
