How to dissolve steel without affecting aluminium alloy Disclaimer: I have very limited knowledge of chemistry

My problem is the following: I have motorcycle part made from some sort of Aluminium alloy - most probably 6005, 6061-T6 or 7005. And inside of this aluminium part there is steel part which is simply stuck there and it's really hard to get it out physically without damaging the aluminium part.

After a bit of googling, I've found out several ways of doing that:


By putting aluminium part with stuck steel part into water and start adding concentrated nitric acid until steel part starts to dissolve. But I've also read that dillute nitric acid (which will occur after mixing with water) will also dissolve aluminium or I'm wrong here?
By using some sort of Alum: either by using Potassium alum or Ammonium alluminium sulfate. Some guy on the forum suggested to dissolve above alum in hot tap water and then put aluminium part with steel part into resulting solution and let it stay there for a day. It should dissolve or at least damage steel part without affecting aluminium part which may ease physical removal of steel part.


Could you please advise which of above solutions is better? And in case of using alum - which of them should I buy - potassium or ammonium one?

Thanks in advance for any help!
The part of the steel insert that is stuck to the aluminum is not accessible to water.  Let's look at linear coefficients of thermal expansion,

http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

Aluminum is 22.2 ppm/K. Steels are smaller, the largest being 304 austentic at 17.3 ppm.  Ambient temp is around 24 C, a 400 F kitchen oven is around 204 C.  That should not be hot enough to lose temper in the alloys.  If the aluminum hole is an inch in diameter, worst case increasing the temp 180 K times 4.9 ppm/K net gives you 882 ppm or 22 microns of slack.  The steel insert pushes out.