Here are some practical tips on how to remove a broken bolt from a motorcycle's aluminum cylinder. The same instructions can be used if you have a broken ferrous tool inside a block of aluminum.

While disassembling the cylinder of an old Honda motorcycle engine (a 1989 NTV 650 Revere), a 6×20 mm bolt fixing a water joint, broke. The bolt was quite rusted, the thread was stuck into the aluminium body of the cylinder and the bolt head came off. About 8 mm of the bolt remained protruding from the aluminium, but grasping it with grip pliers wasn't enough to unscrew it.

Using loosening oil, heating the piece with a heat gun, hammer blows, … nothing worked to loosen the bolt.

I attempted a mechanical fix: I drilled a hole in the bolt with a 2.5 mm bit, then tried to unscrew it with a left-handed broken bolt extractor. The remedy was worse than the damage: the extractor tip broke off inside the bolt: instead of just a normal-steel bolt snapping off in the aluminum, I now had the broken bolt plus about 4 mm of extremely hard HSS steel stucked in it.

Drilling out the extractor tip was impossible; perhaps only a very expensive CBN grinding pin could have filed it. Unfortunately, such pins are very expensive (CBN is a super-hard material, second only to diamond), and I don't know if it would have been possible to chip away the 4 mm of stump thickness.

It seemed the only solution was a chemical method: using a solution capable of dissolving steel without attacking the aluminum. This is what alum can do, a salt commonly used in small sticks to stop microbleeding during shaving!

The theory goes that a solution of potassium alum (double sulfate of potassium and aluminium) dissolves ferrous materials but does not attack the aluminum. However, the practice required solving several problems and questions:

• How to immerse the cylinder body into the alum solution without damaging the cylinder barrel which is made of cast iron (a ferrous material)?
• How to heat the solution so that I can use a more saturated solution to speed-up the chemical action?
• How many hours/days will be required to dissolve a bolt, which is 6 mm in diameter and 12 mm long?

To carry out the first tests I proceeded to:

• Purchase 90 g of alum, advertised as a natural product useful for shaving. The price was 2.50 €.
• Purchase a 12 v heating pad, which was rated to work at 60 °C. The price was about 10.00 €.

I made some tests on a similar bolt: using demineralized water I made a 37% solution of alum (100 ml water, 37 g alum) which is the saturation point at 50 °C. The solution was placed into a glass container put above the heating pad. The bolt, once submerged, began to produce small gas bubbles, a sign that the chemical action was working.