If this hole is at the top of the down (front) pipe, near the manifold, then it is almost certainly the lamda (oxygen) sensor.
A replacement just screws in as it is threaded, though there appears to be a nut involved.
My 20v 1998 blew after approx 76K, and then some, the breach was just where the welded the flange join is at the end of the front pipe. At first it hissed and then it growled and then finally it sounded like a dragster, shaking the whole car through the rev range but only upto about 3.5k rpm as beyond that power just dropped like a stone.
Luckily a local welder fixed it for £20.
He discovered that a) the down pipe is a strange double pipe system, i.e. one pipe inside the other???? and that there is no securing mechanism along most of the the middle section.
This would explain why, when i replaced the front pipe two weeks ago the new front pipe has a flexible section where it meets the middle section.
These flange joins must have taken a pounding under acceleration/deceleration.
price shope around and play one fast-fit against the other and from £200 plus at the local fiat to 160 and then eventually 105 for a 2 year gaurantee s-steel.
lamda price.....pass. but shop around!
let us know...
james
