how should i do this

[gaz_moose]

so ive got given this little job to do.
its a section of my brothers lotus exhaust it has cracked all the way around the weld where a section of pipe comes out the cat.looking at all of the welds none of them have any penetration.

anyway it seems to have been welded with a mig before but i only have a tig to fix it with.in the past when i have tried to tig over mig it has just made the old mig fizz up and then not produce a decent weld.

the tollerences are pretty tight so i cant just wack it back togeter any which way.my query is when i grind off all the old mig how can i make it so that i keep the broken off section in the correct location?
you can see in the pics that i attempted to knock up a half fashoned jig but im going to have to take it all apart to grind off the old welds and that is where i have become stuck.

the cat section is mild steel and the rest is stainless

 

[pedrobedro]

I draw lines on with marker pen that extend past the join then line them up again after. You could mark them on your jig and on your parts for a double check. There's usually enough play in exhaust mounts to stand a bit of misalignment, some aftermarket ones need quite a bit of persuasion to fit. What rods are you using for the weld out of interest ?

 

[gaz_moose]

i usually draw witness lines when doing these kind of jobs but i will have to grind them off when i remove the old mig.thinking about it now i guess i could draw on one side then grind t'other.
as for what rods i only have one box of rods cant remember what the number is off the top of my head will look when i get home.hahaha

 

[zxr400]

Could you grind a small area, which you can get to, whilst the rod is on?

If so, you could tack weld at that point, then remove the rod and grind the rest of the area, then re-fit the rod and weld. If you can tack in a few places before removing the rod, it should be strong enough to keep it there whilst grinding the rest of the weld.

EDIT: I realise now that you want to remove the pipe from the cat to grind it!

Could you use a centre punch and dot mark two points either side of the area to be ground?

Cheers,

Mark

 

[Matt the sign]

How about 4 drill holes through both sections when assembled .....

 

[penfold]

How about 4 drill holes through both sections when assembled .....

What he said, except you could probably get away with just 2 or 3; I would tap them as well to hold it together precisely. The weld appears to have failed due to inadequate penetration; make sure you get plenty of heat into the joint when you do the repair.

 

[gaz_moose]

looking over the entire exhaust assembly most of the welds are undercut and or have little to no penetration.not bad for a 20 grand car.the cat section seems to be well made i think they out sourced that part!.the hangers for the rear silencer etc.. are made out of mild.

i just figured it would make my life more easy if i could pull the stainless section out of the cat to grind it.think im going to grind off the welds from the hard to reach bits off the job then blast it up then grind the old welds off the back.how does that sound?

 

[gaz_moose]

ok the rods that i have are sifsteel A 15
a quick bit of research shows these are good for stainless ive been using them for everything

 

[wyn]

I've got a tube of sifsteel A15 in front of me and they are for mild steel.

 

[gaz_moose]

yeah a bit more research like looking at the sif PDF shows they are for mild.i guess thats why they weld mild so good.since im joining dissimular metals what rods should i use? i really dont want to have to buy a box of rods im not going to use all that much.
do someone a favor and end up out of pocket,how many times have i had that happen.

 

[Paul.]

.since I'm joining dissimilar metals what rods should i use?.

A 309 stainless filler rod would be the ideal choice for mild to stainless.

There's some in the shop

 

[gaz_moose]

yep i spotted those was just waitin on someone more knoledgeable to give them the nod.

 

[hotrodder]

The witness line method... after marking with a pen/paint etc i often then score the end of the pipe (for when i absentmindedly degrease the parts). Hacksaw nicks would still be visible after cleaning up the pipe sections unless you go nuts with the grinder

Another way would be to use some angle to create a more repeatable jig, tack to the pipe section in the vice for example and use one of the mouting tabs on the other section

 

[pedrobedro]

I've mig welded a few jobs like that and they stick alright with mild steel wire but 309L would be the correct choice for tig welding it. If you buy a small pack for £20 they will always come in for the next job and can be used on mild steel as well although they cost more.

 

[gaz_moose]

id rarther do the job right the first time as my brother knows where i live

i will get some of those rods from weldequip as my local supplyer are no help at all. think im going to grind off the old weld from the hard to reach places then tack it together then grind off the rest of the old weld then weld it up.

cheers gents.