We see an awful lot of busted exhaust bolts in these heads. And we see a lot of failed repairs. We also see a lot of corroded flanges that create significant exhaust leaks, as well as grooved flanges from the factory steel gaskets digging into the flanges as a result of exhaust manifold/header shuffling from heat, vibration and expansion & contraction.
On 3.4 Cayman and 3.6 911 heads we are seeing some with low spots around the large secondary air passage. In some cases this dip is as deep as .8mm.
As the photos indicate we’re Bridgeport Mill fixtured to remove broken exhaust studs and machine for Heli-Coils.
We prefer Heli-Coils over Time-Serts for this particular repair. Heli-Coils will not back out with the fastener as Time-Serts sometimes do. This is an especially common problem when converting from bolts to double ended studs and copper-clad lock nuts for manifold/header mounting. During installation, some copper-clad exhaust lock nuts grip the stud with such force that the torsional load required to install the nut jams the stud into the insert. It’s very common for copper-clad exhaust nuts to lock onto the stud so tightly that the stud will come out during removal of the manifold/header. If this happens, the swedged bottom of the insert galls on the way out making a mess of the aluminum threads. I have seen this countless times.
This same stud & copper-clad lock nut scenario when coupled with a Heli-Coil will not do this. The wedging of the stud shoulder against the top most end of the wire will cause the wire to move away from the stud and dig into the aluminum, thereby locking it into the head. It will stay put when the manifold/header is removed.
We can also repair damage done by other shops in their attempts to repair busted studs as these photos show.
Click on photo for larger image.