dump that thing in an electrolysis tank. will easily and safely remove rust from any steel item. Avoid its use with aluminum and other alloys.
How to build an electrolysis tank:
Use a plastic or non-metal container large enough to fit your part(s). I have a plastic coffee can, a 5 gallon bucket, a 20 gallon rubbermaid storage bin from wallyworld, and a broken 250 gallon cattle plastic water trough. Step 1: fill the tank with water. Step 2: add lye, which can most easily and inexpensively found as sewer main line cleaner at Home Depot in the plumbing department for about 10 bucks. Use about a cup per 5 gallons. Step 3: find some scrap metal to use as electrodes. I use an old valve in my coffee can, and some pieces of 2" square tube for the larger ones. Step 4: roll your 12v battery charger and pull out a set of old jumper cables to submerge and attach to the parts. Step 5: affix the negative clamp to the part that you want de-rusted. Step 6: attach the positive clamp to the electrodes. Step 7: hook up the appropriate clamps to the jumper cables, neg to neg, pos to pos and turn on the battery charger. Turn it on and let it be. Very soon you will start to see bubbles forming and a foam at the top, that means its working.
Basically, the lye acts as an electrolyte and adds to electrify the water. when you negatively charge the part and you charge the electrodes as a positive, it will draw the rust away from the part and will affix to the electrode. It is an amazing process. In the case above, where it is gears in a case, point the opening toward the electrodes. do not let the two touch.
My first positive experience was with a John Deere B tractor. Each tractor came from the factory with a set of "shutters" which mounted in front of the radiator which could be opened and closed to heat and cool the engine. This was because these tractors were designed to be started on gasoline and then when the engine was to operating temerature, switched to distillate fuel such as kerosene (when kerosene was available and cheap). They are a series of about 20 shutters rivited to a frame and made to flap like venitian blinds.
Mine were seized up. 3 cans of PB Blaster and large plyers did nothing to free them up. After an hour in the tank, pulled them out and operated them easily by hand!
It is a great, inexpensive process to derust parts. It leaves them with a black oxide that can be cleaned and painted or oiled.
You can use baking soda as well, but they lye seems to react a lot better and will also dissolve and remove paint.
