After living with my third exhaust manifold crack since new (once on factory manifold, once each on two "improved" aftermarket manifolds with the expansion joints) long enough for my muffler and converter to finally wear out, I decided to try something different and swap the whole setup for the new two piece design.
Chrysler changed the manifold to a two piece design during model year 2000. Apparently through 1/24/2000 they used the one piece design, then switched on 1/25/2000 to a two piece design with a "Y" shaped pipe from the manifolds to the single catalytic converter and stayed with that through model year 2001 when they added pre-cats to the "Y" shaped portion of the setup.
Since my catalytic converter and muffler were going at the same time, I did my homework on the model year changes and decided (based on what parts changed when) that the "late model year 2000" setup would bolt into my 1998 as long as I switched out everything from the manifolds to the muffler at the same time. I realized I couldn't bolt in anything later then model year 2000 because all the later models require hookups for three oxygen sensors total rather then the two I was wired for.
Long story short, I was able to buy...
A set of Model Year 2000 iron manifolds used on eBay
Walker number 53461 for the "Y" pipe with the hole for the single sensor
Walker number 31625 for the gasket between "Y" pipe and cat
Walker number 16085 for the catalytic converter(*)
Walker number 21456 for the muffler
(*) not 100% sure on this part number
...and with an excellent writeup I found at www.dailydriventj.com
on the basic exhaust manifold change, a Sawzall to cut the old parts out, and a whole lot of sweat (and some cursing) was able to bolt in all the parts as expected, no welding.
Most of my research was done in Walker's excellent online catalog at www.walkerexhaust.com
, entering the different model years and looking at the part numbers. The local dealer was no help at all, refusing to participate in or discuss any "modification" to the vehicle.
A few notes:
I did have to extend the wires on my upstream oxygen sensor by about 14" because the hole for that sensor is on the other side of the transmission from the original, so longer reach is needed.
The manifolds I bought used were ported for an O2 sensor in each one, which I wasn't expecting on model year 2000. I had to buy special plugs threaded for those holes (I think it is 22mm taper, exactly like a spark plug, and I could have plugged a spark plug in there if I didn't care about looks). Those plugs were an astonishing $22 each. Maybe I was really sold a later manifold set?
I reused my original tail pipe, and found I needed to use a little 3" long extender (technically an adapter from 2.25" O.D. to 2.25" I.D.) I got for a few dollars at Kragen to make the reach from the muffler to the leading end of the pipe, but I think the reason I came up short at that junction was I was a bit sloppy with the Sawzall work.
The clearance between the leading edge of my new muffler and the skid plate was really tight, millimeters when it should have been inches, so it rattled when the motor shook at startup or shutoff. I ignored it for a while, then paid the local Johnny Franklin Muffler shop $20 to give me a bit more clearance. They just heated the front pipe with a torch to soften it a bit, then pried the muffler back a bit to add clearance. Took five minutes. No problem. No rattle any more.
There really is no gasket either between the "Y" pipe and the manifold downpipes or between the muffler and catalytic converter. All are the "ball and socket" metal to metal style. The only gasket is the three hole Walker 31625 between the "Y" pipe and the catalytic converter.
I would have appreciated if Walker had included the two 10mm x 1.5 pitch nuts needed for the muffler with the muffler itself. I had to buy those, along with three suitably long (30mm) 8mm bolts and nuts to join the "Y" pipe to the catalytic converter. That was it for hardware except for a few 2 1/4" clamps.
I am very hopeful that unlike my past repairs with better and better imitations of the flawed factory design (that all cracked anyhow), this two piece design now represents an improved factory setup will finally free me from this endless cycle of manifold cracks.
One other nice thing in returning to the "factory" manifolds versus the heavy tube aftermarket replacements is that the access to the lower bolts is much more "open" (all are visible and reachable from below) on the smaller cast factory manifolds, so getting to those bolts on reassembly was much easier then on disassembly.