One of the important tasks in restoring a Stearman is the re-oiling of the inside surfaces of the 4130 tubes that make up the fuselage frame.
When Boeing fabricated these frames, all junctions of tubing had holes inside the welded joint so that there always an internal passage from one tube to the next.
Conditioning and protecting the inside walls of the tubing consisted of coating all the surfaces with hot raw linseed oil. One method of doing this is as follows.
Remove two diagonally opposite engine mounting studs from the frame and insert temporary studs with a 1/8th axial hole all the way through.
Go over the frame carefully, and put a small PK screw with a 1/16th ID O-ring in each hole that has been drilled in the frame tubes. There are about 20 of them. When you think that you have plugged them all, cap off one on the temporary studs, and apply air pressure to the hole in the other one. Put in about 10 psi. Now go around the fuselage feeling the tubes and listening for escaping air. Then put screws and o-rings in the holes that you missed.
Now with appropriate sized cans and hot plates, arrange to heat the raw linseed oil to about 150 F. Now if you haven’t done this before, what would your guess be at how much it takes to completely fill the fuselage? The number of 7 1/2 to 8 gallons never ceases to amaze me.
Position the frame with the tail low and the front end high, with one of the temporary studs at the top and the other at the bottom.
Run an intake tube from the heated oil can to the pump you are going to use, and an output tube from the pump to the bottom stud. Run a tube from the top stud to an empty can.
Now start the pump. If you have any small fountains, turn off the pump and put in the last (hopefully) of the PK screws. The hot oil flows into the bottom stud and downhill in the bottom tubes to the tail. Then the level starts to rise and finally reaches the top tubes. You know it is full when oil starts coming out of the top stud. You can determine where the oil has reached because the tubes get warm.
Then it is a good idea to cap off both studs and rotate the fuselage, letting it set for 15 minutes at each 90 degree point.
When that is finished, raise the tail and lower the front end. Connect the intake tube from the pump to the bottom stud, and uncap the top stud. Run a tube from the pump outlet to an empty container. Now when the pump is turned on it starts drawing the oil out of the fuselage.
When it finally draws air, you have to let it stand for a while and then more oil will be drawn out. Let it set over night in this position, then run the pump a while longer. There will be somewhere in the vicinity of a couple of quarts that you will not get back.
Clean off any oil that gets on the fuselage frame with turpentine, right away. And then flush out your pump with some turpentine so that it has some chance or working the next time. Save the oil for the next restorer to use.
It’s a messy job, but someone has to do it. Dispose of any rags used to wipe up the linseed oil outside in a safe area. They are somewhat prone to supporting spontaneous combustion.
Raw Linseed oil is sometimes difficult to find, and boiled linseed oil is not an acceptable substitute. It doesn’t have the same protective and hole plugging characteristics.