OK, an update.
I asked Garrett about the cracked plane, he first said don't worry about it, if the plane works fine, but I explained I was worried that the crack would get worse, he had a look at it and flexed it against the bench and agreed that if I could silver solder it, it would be good. He said he has several planes that have been fixed, not by him, that work just fine.
I got out my 1/4" thick steel plate I use for welding on, and my torch, this did not work well, the steel plate was acting as a heat sink.
next I tried some wooden blocks, this was not bad actually, but not great, and not the blocks only singed a little.
I then put the plane on my small hot plate stove and cooked it from below and torched it from above, still not enough heat, so I got out my LPG tank and big burner, that added a LOT of heat, and things began to glow, but I was not really able to get the temp up high enough to melt my silver solder.
I did really try, and was partially successful.
I was worried about the plane cooling to quickly, so I bought a 30Kg sack of silicone sand, the dry stuff, and dumped it in a 5 gallon bucket, but the bucket was not deep enough, so I also put in a piece of pipe......
I put some sand in the bucket and pipe, then dropped the hot plane bed into the pipe and filled it with sand, and the bucket too.
Several hours later the sand was still warm, but not very, so I took out the plane bed.
I put it back together and started lapping the sides and bottom on my float glass and sandpaper set up.
I hope you can see the crack, on the right side of the picture you can see the silver solder has partially filled the crack, that is where it got hot enough I guess, the top of the cheek, the area with the least amount of metal.
The silver solder that I was using has a melting point of 780C/1436F, I guess I could not get enough of the plane up to temp. The larger LPG burner I have runs at 2000C but I guess it is more about BTUs to heat the whole plane up. The only other option I have is my forge, but the plane will not even sort of fit in it.
Plan B, I have ordered some different silver solder, two types, one has a melting temp of 650/1202F, I hope I can get the plane body up to that temp, I think I can, if not the second type I ordered has a melting point of only 220C/428F. I'm sure I can melt this one, but it is not as strong, but then again, how strong does it have to be?
So I have three types of so called Silver Solder:
High Temp 780C/1436F Tensile strength 78,180 PSI
Middle Temp 650C/1202F Tensile strength 63,960 PSI
Low Temp 220C/428F Tensile strength 31,910 PS!
These planes are made from gray cast iron, as far as I know, and from looking round on the net, I find that gray cast iron as an ASTM number from 20 to 60, and the range of tensile strength is then from about 22,000 PSI to 62,000 PSI. Now I doubt that these old Stanley planes have the hardest, strongest gray cast iron in them, so if I can get the middle silver solder to flow at 63,960 PSI I think I would just about guarantee the silver solder joint would be stronger than the gray cast iron it is joining. Heck even the low temp silver solder at 31,910 is going to be about the middle of the range of the tensile strength of gray cast iron. Who knows that quality of gray cast iron was used in these old planes....?
After I try one more time to solder it, I'll then spend some time lapping the sole and then refinish it. I'll put it in the electrolysis tank tonight to make sure it is super clean for the second attempt at silver soldering the crack. FYI I did use the special flux and lots of it, for this type of silver solder.
Cheers!