I've run two Thorley headers. One on my 84, and one on my 89 2wd. I didn't have one problem with the header on the 84. It was a fully smog legal header, with O2 bung and air injection rail. The air injection rail was tapped and plugged with bolts until I needed to install the rail for smog. The 89 header has been a different story. I spent the longest time getting the header flange to seal. Come to find out, if you torque the center bolts to spec, the ends of the flange will pull away from the head. This is because of the welded O-rings. The flange height at the orings is higher than that of the center bolt holes. This can be remedied by adding a washer behind the flange for those center holes. The other problem on the 89 that I ran into is constant leaking at the collector flange. Its kind of a puny flange, so it warps easily.
One common problem I've noticed on thorleys as well is the lack of a downpipe to bellhousing bracket. The stock exhaust has this bracket. Thorley deletes it. This tends to stress the header near the area where it turns to go under the truck. I've seen multiple header tubes crack there. I recommend adding some type of bellhousing bracket there.
Thorley header gaskets are darn near indestructable.
Thorley headers are a Tri-Y design, which is supposed to improve torque over a 4 into 1 header. The stock manifold and downpipe are a Tri-Y design.
As far as I know, only thorley and NWOR offer a Tri-Y header. The NWOR header has beefy thicker flanges, and a ball & socket collector flange. Other threads here though have posted about poor customer service from NWOR though. And, they are more expensive than Thorleys
Good luck.
