Is there a plumbing tool to cut or break the head off of a rusty bolt?

Answer 1

Then you'll have a sheared stub of bolt... it may be easier to undo it.

Patience is the key... resist the urge to use the longest spanner you own, at least initially, you'll probably snap it.

The first thing to do is clean away as much rust, dust, paint and other contamination as possible to allow lubricants into the hidden part of the thread and avoid contaminating the thread when things start to move. If it is a nut and bolt clean everything. If it's a bolt into a threaded hole clean whatever is showing.

The next thing to do is lubricate the area thoroughly. Look on your favourite site for "penetrating lubricant" aka liquid wrench, freeing oil. This stuff is specifically formulated for your intended purpose. It uses "capillary action" to work its way into the thread and is made to be as thin (low viscosity) as possible but still really slippery. Also its formulated to interact with metal and rust in a way which helps.

Have a drink while it soaks in and lubricate the offending item a few more times.

Try your spanner again "lefty loosey"...

If the bolt (or nut) moved a little then rock it "righty tighty"... add more lube and keep rocking it back and forth so it moves a bit more each time. If you see power emerging from the thread clear it away with the brushes you used in the first step.

If the standard penetrating lubricant didn't work search for "freeze release spray". It works in a similar way to the stuff you use to temporarily freeze pipes but has lubricant qualities.

There are two keys to using this: 1) Be generous, it's expensive but needs a sustained spray to work (read the instructions). 2) Wait for 30 seconds to a minute then apply force to the spanner while the item is still cold.