Silver Soldering a Crankshaft Assembly

Well, my original plans were to press fit the crankshaft parts together, and use setscrews to secure everything. That didn't work out so well. After the parts were pressed together, I discovered there was way too much runout (or wobble if you prefer)in the cranks. So, what to do? Nobody likes a wobbly crank.

After much thought I decided to try silver soldering the crank parts into one crank. The bigger problem was figuring how to hold everything in alignment while soldering. What I ended up with was two Vee blocks clamped down on the bed of my mill. I aligned the blocks by first clamping down one. Then, using a straight piece of drill rod clamped in the vees of both blocks as an alignment device, I clamped down the other.

Soldering depends on proper clearances between parts to work properly. As I had originally machined the crank parts with a press fit, I had to increase clearances by filing the round crank parts a little. I did this by clamping them in the lathe and holding a file to the crank ends and crank pin. The parts to be soldered are 1/4 inch in diameter where they fit in the crank discs. For a part that size, .001" clearance is sufficient. If you have too much clearance between parts, you'll get a weak joint. If you have too little, capillary action will not be enough to draw the solder into the joint, and again, you get a dry and weak joint.

The parts have to be clean shiny metal, with no rust, oxides or grease or oil. I used a silver solder paste, which is tiny beads of silver solder in a thick resin type flux, as best I could tell. It was purchased at a local building supply store (OK. Lowes.) According to the manufacturer's specification, this particular paste contained five percent silver, with a melting temperature of 450 degrees. There are a lot of different formulations of silver solder, some as high as 20 percent silver, with higher melting temperatures. I chose the lower temperature because I didn't want to change the temper or coloration of my crank parts, and I didn't feel I needed the strength of the solders with higher soldering temperatures for a model engine crank. It was also locally available. I have another project in the works that will depend on this sort of soldering, so my cranks served as a chance to practice.

The purpose of silver in solder is to improve the wetting and flow characteristics of the solder, by the way.

The first photo shows the crank disks and the crank pin installed in my jig with a solid piece of drill rod through the bottom of the discs. This was to insure that the two discs were aligned at the crank ends. I used the holes that originally were intended for the setscrews as a place to apply the solder paste. If you look closely at the photo, you can see the grey paste filling the holes. I heated the parts with a propane torch, the resin melted, and the solder flowed. After the parts had cooled, I rolled them over, removed the solid drill rod, installed the crank ends, clamped everything down, and soldered those using the same procedure. All went well except for the last joint on the last crank, which simply refused to flow. I had a very old tin of resin flux in the top of my tool box, and decided to try some of that. I disassembled the joint, cleaned it one more time, and painted it liberally with the new/old flux. This time the solder flowed really well. Hmm. Maybe I should have used additional flux on all the joints?

The second photo shows how I measured the runout at the crank end. I got about .002" on either crank. I can live with that. If you look closely at the area where the crank end meets the crank disc, you should see a bright white line. That's silver solder drawn through the joint by capillary action. That's what you want to see.

The last photo shows the cranks installed in their bearings and the engine bases. The silver solder is in the syringe in the middle, above the tin of flux I bought years ago for a purpose I don't recall. If I have to do this again, I'm going to try to find a similar low temperature silver solder in a wire form, and spend more time investigating fluxes.