My experience with this began when the head on my 4.0 ’01 Limited XJ cracked. This was quite a surprise considering how well I had taken care of this truck. I had tormented and abused my previous ’90 XJ during many wheelin trips and that one made it to 300,000 miles with only the typical maintenance to be expected from a broke college student (which is less than desirable). When I bought the ’01 Limited after college I actually took great care of it and kept it off the trails – this is also when I built up my SJ specifically for wheelin. Regardless, the head cracked after 93,000 miles of very typical daily driver duties.
As you’ve probably learned by now, the reason wasn’t how I treated the engine, it was just that sometime in 2000 the head and exhaust were “updated” to the 0331 design and well – it’s just a bad head casting design that has a high probability of developing a crack that allows coolant to flow out of the water jacket and into the crank case. The typical location is between the third and forth cylinder (right under the valve cover cap) and that is exactly where mine was. Unfortunately for me, mine was actually bad enough that the coolant leaked down into the block onto one of my exhaust valve lifters and caused it to fail.
I really didn’t want to put a new 0331 head back in there and I also wanted to keep the stock exhaust system (especially the O2 sensors) so that I wouldn’t have any issues with emissions. I ended up installing an 0630 head and found myself with giant exhaust leak because the exhaust ports on the 0630 were not fully covered by my exhaust manifold. I made a thin steel plate that sandwiches between the head and the exhaust (and intake) manifolds. I started selling these adapter plates in the forums in order to help others out and have been doing so ever since.
Other applications: People have fit these plates in Wranglers and Grand Cherokees that had the 0331 head. There have also been successful installations when replacing 0331 heads with 7120 heads as well.
So far it has worked in all situations that involve replacing an 0331 head with an older (0630 or 7120) head when the vehicle still has the stock exhaust/intake manifolds. If you want to try it out in a new application, my policy is to ship it out, let you see if it fits and then allow a return with full refund if it doesn’t work out. You pay the return shipping.