Landing gear

How do you properly solder a washer close to a plasic wheel or nylon bellcrank without melting the plastic?

Very carefully!

We have a product called "Liquid Fence" which you can get at a plumbing supply. It is a thick liquid that comes in a tube and is applied to pipes in areas where you do not want the heat from the torch, while soldering one end, to penetrate the other end. You just apply a small ring at the point where you want the heat to stop. I had a cracked frame on a bicycle where the wheel fit into the fork. I applied a bit of this 2 inches up from where we brazed the frame with a brazing torch and it didn't even scorch the paint from that point up. I just added a trim color in the area where it was brazed and it looked like new. But you don't NEED this. It is just a handy product.

Since we are soldering, the key thing is to do it very quickly. Let's say we are soldering the pushrod on a belcrank. We use a higher temperature solder (silver solder, or even a "more lead, less tin" type) to solder the back washer on before sliding the belcrank into place. Clean this first joint and wash it in water and baking soda as well. Then slide the belcrank on to the wire, punch a hole in a small piece of 400-600 grit sandpaper and slide this over the belcrank with the grit facing away from the belcrank. Now slide the final washer into place. It is important that this washer, by the way, be sized very closely to the size of the wire. If it is too big, you will get too much solder flowing on the othe side and mess things up. Now, apply some flux, and using a high grade lower temperature solder, heat the washer and wire together with the iron while holding the solder on the opposite side of the wire as the iron. When the solder just begins to flow, remove the iron and let the solder flow all around the joint. Let it cool and then spin the sandpaper a few times behind the washer to clean it on the back side, and then tear it off. The sandpaper will serve as a heat shield, as a spacer to get just the right clearance, and as a means of cleaning the back side of the washer all at once. Now clean it again with water and baking soda, and then "grease" the joint with vaseline. And you are done.

Just remember to push the washer on tight up against the sandpaper while soldering. Let the sandpaper provide the proper spacing and serve as a heat shield. And if you can get some "Liquid Fence", put a little of this on the belcrank before soldering. The latter isn't needed, but it is a plus that can be added to our modeling arsenal and used in these critical applications.