Seems they are warped after welding and now causing exhaust leak no matter how much I tighten the clamp

Any fix for it ? Some thin multilayer gasket or something ?

 

This is sadly quite common. best thing I have found is to take a hand grinder, with a brand new (flat stone). Has to be one where the nut that holds the stone on doesn't hang down lower than the stone (recessed). There is a bit of technique to it, but you have to run it around on the flange quickly and evenly. This works on all types of flanges that get warped after welding.

 

Two things when using v-band flanges:

1) need to use a known good quality of SS material for the flange, such as 304
2) need to clamp the flanges to together in the as used configuration during welding, which will help prevent the flanges from warping. If you just weld the flange onto the pipe without having it clamped to the other flange it will warp.

 

In your case you can search for v-band flange gasket.

http://store.034motorsport.com/v-ban...-gasket-3.html

 

if they are just a little off... a fine coat of copper silicone on the mating joints will do it.

 

they state:
Note, we do not recommend these gaskets on the turbo outlet, only when used sealing connections in the exhaust system.

then I've found this

http://www.ebay.com/itm/Stainless-Steel-High-Temp-V-band-Sealing-Gasket-3-/221382762543?pt=Motors_Car_Truck_Parts_Accessories&hash=item338b70e02f&vxp=mtr
Any good, anyone use it ?

 

a little OT, but does anyone make Titanium V-band flanges?

 

You need a material that will compress. For $8 I would try the aluminum one, but if you can find them in copper then I would go with that. Stainless is a hard material and will not compress easily to create a seal.

 

I've seen people wrap the flanges in aluminum foil and then clamp it. Idk if it works but its worth a try.