When steel and aluminum stick to one another...

Question:

I am restoring a 1967 Corvette with an L79 327 cid engine. It has an aluminum intake manifold and my problem is that it has a steel nipple that will not budge. How do you advise that I remove this nipple without damaging the manifold? Should I leave the manifold on the engine or remove it? Should I heat the manifold or the nipple, or freeze the manifold or the nipple? I tried freezing it but that didn’t help. Should I try a chemical rust remover?

Answer:

You don’t say why you want to remove the steel nipple and if it’s a matter of cosmetics I would advise you to leave it alone and do your best to improve its appearance without removing it.

The problem is that an electrolytic reaction has occurred between the dissimilar metals, effectively welding the steel fitting to the aluminum intake.

The nipple is not stuck to the manifold in the way that steel parts normally get stuck from rust.

Instead, the metals have actually joined one another and the force required to separate them is almost always more than what is required to “tear” the aluminum, which is the softer of the two metals.

So, regardless of what you do to encourage the metals to separate, the end result will be damage to the manifold.

The only practical way to get the nipple out is to carefully drill, machine or cut it out while doing as little damage as possible to the threads in the manifold.

Assuming the manifold hasn’t been damaged, use a pipe thread tap to clean up the threads so the new nipple goes in properly. If the threads do get messed up you have three choices; buy a replacement manifold with a good nipple, drill and tap the hole larger to accept a bushing that’s sized correctly to accept a new nipple, weld the hole closed and then drill and tap it to accept the new nipple.

One other thing—you’re probably better off working on the intake with it off of the engine.