The thread of the postwar fork bottom cap is rather sensitive. To provide rust, take teflon tape, available from plumbing accessories. This helps a lot to use somewhat damaged threads, too. Spoiled threads can be renewed with aluminium solder. To increase the heat, you can use a stoke cooker combined with a blowtorch. I am thinking about an oil drain screw to facilitate oil-changing not need of frequently opening the cap. To heighten the strength of the thread into the cap, aluminium sold can be used, too.
