There are a few jobs that need doing to 'Hans' our 30 year old BMW R65 Scrambler project to get it through its MOT.
The first thing we need to do is get it running, but a quick look into the petrol tank shows that a year left empty has lead to a fair amount of rusting. It's not about to corrode through, but it will taint the petrol and block up the filters.
After a little bit of googling around and reading classic bike and tinkerer type forums I found a few ways to clean the inside of a rusted tank:
Option one: Fill it with a medium, the recipes vary on what this should be, gravel, ball bearings, loose nuts and bolts, nuts and bolts tied in string etc. The tank should then be agitated. Ways to do this include shake by hand, roll in material and put in a cement mixer, tumble dryer... you get the idea
Option two: Use water to which is added a range of highly corrosive and noxious chemicals including caustic soda, spirits of salt, industrial strength drain cleaners, phosphoric acid etc. Agitate, or don't agitate, again your get the idea
Option three: Electrolysis. Water, a screwdriver some wire and a battery charger
Option four: Vinegar. Simply fill the tank with Malt vinegar. It contains acetic acid which will eat away the corrosion. Then rinse out with water, then fill with water and baking soda to neutralise the acid. Rinse again, then add WD-40 or petrol to coat the tank and drive out any final moisture.
Option four seemed like a winner. It's cheap, is unlikely to end in a trip to the hospital due to acid burns or electrocution and is unlikely to melt through the entire tank. This method has appeared on a number of websites and the results look good if not spectacular - well what do you expect with just household vinegar.
44 half litre bottles of vinegar from Ocado later (I'm surprised they didn't think our order was a mistake) and we set to work.
After we had filled the tank with vinegar, we agitated and used a bottle brush to reach as far inside as possible.
The smell of the vinegar made us hungry, so as soon as we were done we made use of some of the unused vinegar by ordering fish and chips from around the corner!
We'll give it a couple of days and see what the results look like.