Electrolysis , used it for several petrol tanks and one oil frame. Works great. I use a washing soda solution as an electrolite. Set up a heavy duty car battery
. Set up a pos and neg electrical connection with some crocodile clips , install an in-line 10amp fuse in one side for piece of mind.
Use a bit of flat steel bar , about 1/4" x 1" as long as possible to dangle into the frame cavity. I made a wooden collar for the flat bar so I could jam it into the frame filler hole and keep it isolated.
Connect frame to neg side.
Set up flat bar and test between frame and flat bar with a multi-meter to make sure there's no continuity before making final connection to battery
Remove and clean flat bar every few hours, but can be left overnight when you need to.
Keep running for about 24 hrs and then change for fresh electrolite solution.
Run for another 24 hours , or as long as it takes.
Flush out , dry out and rinse with an oil solution.
Works great and you'll be amazed at what comes out.
I used an endoscope for interim and final inspection to see if it was clean enough.
Plenty of info on Google about the process.