more than you asked for and Im glad you got it solved, here a bit more general info and you may gather a little from it. most of the experienced guys will already know this but maybe some of it could help others..
you can carefully examoine the start of the threads and look for crossthreading.
sometimes filing a little of the buggered up area will allow one to start the thread by hand.
if you have a tap and die of this thread and pitch chasing the threads is the easiest way to straighten things out.
some sets are created only for chasing threads, the difference is they don't have a slot and probably arent; as good quality as real taps and real dies. these do not cut threads but I thought it worth mention
link just for a view , get them if you like from your own source but there is such a thing as thread files. each side has a different pitch and once you know the pitch you can carefully hand file the threads and remove any offending metal.
you can spend several hundred on a "proper" tap and die set. a good one comes with three of each tap meant for taper ( starting) plug ( medium) and bottoming taps. bottomong taps are for threading to the bottom of a blind hole.
a fairly cheap tap set may include many of the most common metric and standard fasters, its probably a good choice to own one of these and if you have a good chance to buy some older tech's full set of english or german made taps , don't think twice..
meanwhile the ones sold at most general parts stores will work ok for you. just byuy singles as you need them.
a place where you need good taps is things like threading stainless where many taps just wont stand up well. for many just buying the ones you need when you need them is a good solution.
the thread files are for fixing threads, they cover most standard and metric sizes their only purpose is repairing damaged threads.
in a pinch you can cut a slot in a nut with a dremil or hacksaw through one side, or cut a slot in a bolt and use that. its not perfect but when steel is put up against brass, steel wins so thats a free way just make your own.
heat is of no help here.
of course this is a common "learning experience" that experienced mechanics will often refuse to fall into again..
always start bolts and threaded fasteners by hand, using the wrench when the threads are not enguaged properly will cause this and it has to happen to everyone at least a few times before one changes their habits,, this is quite normal and part of learning , I doubt there is a person here that has never rushed things a little and got caught by stripping a thread and spent resulting time cursing themselves while fixing it.
This is is particularily common where there is hard line and any offset helps make the task of thread alignment more difficult.
a thing that often helps is if there are several threads and fasteners that are combined, use your hands to start them all before you pick up a wrench. this enables more wiggle room to get them all started, then you can torque.
nothing at all wrong with holding the bolt wiht a socket, or with wrench it is just a description , sure use a wrench carefully ,, the ratchet makes you loose some feel so try starting threads by hand and sure still use a socket if its easier to hold them.
the correct tool for a threaded fastener at the end of a pipe like a brake line for example is a flare nut wrench. Its mainly needed for loosening when there is some fear of the wrench slipping. for this job you don't need or want a lot of torque anyway.
when loosening stuff off if its rusted locktite-ed and just won't undo then heat helps heat will cause expansion and retraction if you heat things too much you can also change the metalurgy if its hot enough that you can throw water on it and hear it hiss that's enough to create expansion and you can always repeat that, the microscopic movement helps bolt loosener find a way in. once there is a little crack to enter, capillary action takes it in.
another thing that helps with stuck fasteners is to combine torque and shock and heat. Ill often try , it wont undo, then some heat without going too crazy, then maybe another cycle , then I'll apply as much torque as I dare but while the torque is applied take a small hammer and hit the wrench. if you can land your other wrench in some spot there it is supported ( braced) and cant turn further , that helps. use a small ball peen hammer, apply torque and at the same time shock it , the shock will most often break it loose then its home free. If you instead apply more torque then your wrench strips or the fastener breaks and you end up in deeper trouble ;-)
if you heat brass and quench it you just made it softer , if you heat it and allow slow cooling the brass will be harder, iron is generally opposite.
higher carbon steels are hardenable things like drill rod, tool steel High speed steel , planer blades and such. they can be machined and then heated quenched and thus hardened. you dont need to harden anything here, just realize that heat can change hardness of metals so in generally don't go to the red stage , just hot enough that water will hiss, thats usually enough heat.
around a carb dont use heat. its probably alunimum or an alloy and not very heat proof. you can melt alunimum with a plumbers torch.
how does one learn how much torque it actually takes to strip a fastener, go strip a few, its an aquired skill to learn the limits and a feel for it.
nothing at all wrong with putting a bolt and a nut in a vice and intentionally stripping them just to see if they can approxinmate where to stop before it becomes destructive.
everything has a torque number I think feel is as important. and most mechanics dont start out with that as some childhood sense, it is learned.
often if I have a bolt that wont start Ill just look at the end at the threads, Ill take it to a vice grab a fine file and put a taper on the starting threads, sometimes Ill use a grindstone being careful it doesn't take flight.
often that's enough so I can start it by hand. I think your brass nut is actually captive by the line so you have to use a small file and work a bit more delicately but same idea. brass isn't hard to file, the first bit of threads won't matter much. you can file it a little. try with your file on about a 45 degree angle and go around the start of the thread, it probably just has a little burr..
in place of a thread file a small triangular file can get intothe root of the tread and then you can follow the threads a bit..
the thread files are a special tool you won't need them often but they will fit most thread pitches youll ever find.
you can buy a thread pitch guage to help indentify the thread pitch thats another tool . it may come wiht a set of taps and dies. .
often Ill look for another bolt of a size and pitch I can determine , maybe a new one from a box, or one I recognisze.. then just lay one bolt agaist the other or try a the fit of a nut and bolt, thats how you can ID it easily without spending.
often brass fittings will have tapered threads, some will have a flange on the pipe or a tapered seat, if it has a flange on the pipe like a brake line or if it has a tapered seat or a copper washer, its not a tapered thread. Tapered threads are often used in plumbing fittings as they enguage the thread gets tighter, that makes the seal instead of a flange or a tapered seat. its usually one or the other, not both. if it has tapered threads it won't also have a flange or a seat.
in your case I htink you are dealing with a line that has a fitting similar to a brake line so that won't have a tapered thread. If its a pipe or a sensor entering the block with no other seal then it would most likely be a tapered thread. if its a sensor with a copper crush washer then it won't be a tapered thread.
to strip fasteners the torque required to cause trouble varies with size it also varies with the grade of bolt.. if you like you can try some various fasteners of different types ..maybe try a 1/4" grade five and then a 1/4" grade 8 and maybe some ungraded chinese made bolt.. or a brass bolt.. if you put each in a vice and intentionally strip it youll see how much it takes, having a sense of that isn't really numetric, its a learned feel..
you can use a torque wrench if youd like to quantify the torque but I think the most important is getting a feel for how much is too much. its not really stuff you learn from a book but nothing wrong with experimenting, just to gain a better feel. whats a few stray bolts worth? not much!
so a mechanic who is tightening a high grade bolt will take that into allowance and if it's brass he'll recognise that and go a bit easier because he's gathered an intrinsic sense of what it takes to go too far, from experience. if its a bolt holding a driveshaft together you can bet its higher grade than some offshore or hardware store quality fastener. so its a hard bolt with a purpose so it can be a higher torque then.
I see machines all the time and many of the new ( offshore quality) ones are full of 6 mm metric fasteners.
Metric fasteners use a mark with a number to show thair grade, if you see no marking then its probably a low grade bolt.. so go easy then !
if its standard then there will be lines on the head, the grade is generally the number of lines you see plus two, three likes is a grade 5 bolt. stainless bolts have two lines indicating stainless not the bolt grade. more lines means harder bolt. if its metric it will isually have a number stamped in to the head of the bolt, That's an easy way to identify a bolt as metric or standard. if it has lines, its standard, numbers mean its metric, usually.
allen head bolts or the proper term is "socket head cap screws" are harder to ID because there is no place for the markings, but in general are usually decent quality , not always.
intentionally stripping bolts int he vice isnt; a normal thing people do but its completely harmeless and I thik a good way to hone your senses, try to guess where youll feel trouble , or try to guess what torque it will require to strip it, thats good knowledge to aquire. next after you have broken a few bolts off notheng wrong with practicing techniques for removing them this can be fun, ,why would you bother to do this? just to learn, just so when you do break a bolt off in some critical place you have the skills and have tried and made a few booboos on stuff that doesn't matter or cost anything. Id rather break three "ez outs" than break one off in some critical place. try drilling out a broken off easy out. Ha! yes thats yet another way to get good and stuck .. ;-)
