Anyone out there used nitric acid to remove a broken tap from aluminium. I have one jammed in an engine. Or has anyone got another idea for removing it.                                                                                                                                                                             

Use a sledge-hammer ???                                                                                                                                                                              

How did you snap a tap on aluminium ? As for removing it depending on size can you get long nose pliers down the sides? then apply heat from a blowlamp to warm the surounding area and remove that way                                                                                                                                                                             

I didn't snap the tap off, i just noticed the bolt head was missing thinking someone had tryed to remove the bolt and snapped it, under closer examination i can see the flutes of the tap in the hole. You can't get pliers near it. Alice, i think the previous owner used the sledgehamer approach!!!                                                                                                                                                                             

only other thing I can think of is open up the hole down to the tap so you can get hold of it then after removing it helicoil back to standard size or get it plugged then drill and tap                                                                                                                                                                              

can you post a photo? what size thread? how far down t'hole is it? is it easy to get to?
 it is quite easy to snap a small tap in aluminium. it tends to grab the tap and not let go. when tapping ally always use parrafin or maybe wd40 to lubricate the tap and prevent this happening.

damn forgot this bit.
 have never heard of nitric acid to remove a tap, sounds decidedly dodgy to me, and may end up removing fingers or skin too.
                                                                                                                                                                             

New one on me. Of the top of my head, the acid would eat into the ally more than it would eat into the tool steel of the tap, so at the very least when the remains of the tap came free you'd still have to drill it out oversize and then tap and helicoil.
Concentrated nitric, as xj says, can leave nasty burns on skin. Bright yellow, too.                                                                                                                                                                              

Where in the engine?  If there's sufficient room to work you could try cutting a slot in the snapped off shaft to take a screwdriver using a slim Swiss file, heating the area with a blow torch and backing out the tap.

If that doesn't work you could try punching a slot with a small sharp cold chisel, heating the area and using an impact driver - you're almost certainly going to have to have a helicoil insert put in anyhow so you've nothing to lose.

If you go the Nitric acid route you want a 50% solution and a syringe (with no needle) to gradually trickle it in past the snapped tap.  A couple of guys on one of my Porsche forums have done this with snapped off steel bolts in aluminum that has suffered galvanic corrosion- with varying measures of success.  Nitric acid doesn't affect aluminium but it does oxidize steel - and skin - it really loves skin. So you need goggles, protective gloves and sodium bicarbonate solution to neutralize spillages.  Oh, and if you're using pure or fairly pure stuff you're going to have to water it down outside because it's a fuming acid and VERY VERY bad for your lungs.  All in all more trouble / danger than it's worth.                                                                                                                                                                                

if it's not causing you any trouble (a leak or any thing just leave it alone)                                                                                                                                                                               

if you can get someone to weld a nut to it you be able to get it out or there is a process used in engineering were you break the tap up but for the life of me cant remember want it called                                                                                                                                                                              

there is a process used in engineering were you break the tap up but for the life of me cant remember want it called   Hammer and chisel                                                                                                                                                                              

Spark erosion.                                                                                                                                                                              

Just drill it smaller drill bit than the tap steady hand and go slow half the time when you reverse the dril the tap comes out to                                                                                                                                                                             

taps are made from cobalt steel, which is harder than the high speed steel general purpose drills are made of. If it is a through hole the easiest way is to knock the tap through and helicoil the thread. Spark erosion is expensive and finding someone to do it difficult. You could get/hire a set of tap removers but chdck whether you need three or four flute removers. Maybe a pair of radio pliers ground to fit in the flutes or if not causing a problem leave it. But not if its on the main bearings. How far from london are you ?

Now i know its there, its got to come out, i could ignore it but its like an itch thats got to scratched, im moving towards the nitric acid, sounds hazardous but have some marrigold rubber gloves. Thanks for all the advice guys & gels. Ill let you know !!!!!                                                                                                                                                                             

good luck. make sure to count your fingers before you start and after you have finished just in case .