now i done this on a peugoet but the principal is the same, i done this on an e34 for the mot and it went through fine so it works.
so heres the problem pipe, i first went over it quickly with a wirebrush
then i cleaned it up with a wirewheel on the drill and the angle gringer, at this point i also go around the whole exaust tapping it with a chisel to see if there are anymore holes, luckily i only found pin holes wich i can fix with just a bit of weld but for now ill concentrate on that big hole.
here you can see just how big the hole actually is, now me putting my finger in the hole(ahem) isnt just to illustrate how big it is, it also gives me a marking point so i can cut a bit of metal to go over the hole, i now know its roughly my index finger long.
to cover the hole im going to use an old computer case, its flexible but thick enough to be able to weld with relative ease but it is a bit tricky is your not that handy with a welder so just take your time and itll be fine, i also want to point out at this stage, although the hole is only my index finger long i will need atleast an inch either side so that i can weld to good metal on the backbox, cant weld rust!
a quick trial fit shows that im amazing and got it right first go, if its not quite right its easy enough to trim it down or to cut a new piece.
now i had to bend it to match the shape of the backbox, on this pic you can see its not round, the backbox is an oval shape, even worse its offset a little bit to the curve, because its flimsy metal its easy enough to shape, just dont put it on the backbox and beat it with the hammer!
another trial fit shows im amazing still, time for the fun bit!
now im lucky enough to have a decent welder but any welder will do the job just practice on some scrap first, another thing here is ive taken it outside to avoid filling my house with fumes, something i advise you to do, plus its a nice day why not blind the neighbours with a welder.
ive welded the one edge first, this way i can still shape the plate as i go down, because the metal is thin it distorts due to the heat so start at the one end and work your way down, in this you will see i got lazy and didnt bother using a test piece and had the settings wrong, quick fix and i was away.
because the metal rises and falls a lot due to not being a perfect curve i advise to tack and then hammer the edge down as you go this way you will always get a good contact between the 2 pieces of metal, i ran down this side and then the other side before moving onto the last edge.
last edge done, if you squint it looks fine, its under the car so it dosnt have to be pretty!
thats it all done, get yourself some heat resistant paint and jobs a good un!