Renix to H.O

Jim Malcolm · Oct 20, 2018

Jim Malcolm said:
The only difference is the head. The camshaft grind didn't change until '96 for more torque at lower rpm (225 ft-lb @ 3000 rpm vs at 4000 rpm). Getting a newer model head would give you back the 13hp you're losing out on by using the '88 motor compared to a stock '93 (177 vs 190 hp, torque is same)

If you're going to get a later model head, the 0331 is your best option. Stock torque went up 10 ft-lb (even while adding pre-cats right after the exhaust manifolds (in the manifold if you compare it against the earlier version). The only issue is lack of a hole at the back driver's side for the coolant temperature gauge sensor. It's not a big deal to drill and tap if you're so equipped (or willing to pay), but it's easier to add a hole to the thermostat housing and extend the wiring. There's a nice flat spot that make it easy.

I'd shop around to see what's available to you, For not much more than a junkyard motor, you can replace the rod and main bearings, lifters, timing set, oil pump, gaskets, etc and have a freshened up motor, or you can have another unknown. Granted, swapping a 4.0 can be done in a day if you're so inclined. Rebuilding-in-place, not so much. I'd have to drop the pan, inspect the crankshaft journals and decide if new rod and main bearings will get it good enough or whether the shaft needs polished/cut. Or you can just replace the head with a rebuilt newer model and then down the road when the bottom end goes out, the head will likely be reusable as-is. I'd take a rebuild head over a junkyard motor; same price around here ($400-$500). But if you find a low mile 99-01 XJ motor (TJ & WJ won't work) for a good price, it may be worth it to swap the motor given the known age of your '88 motor. Just make sure any 0331 head is a TUPY or aftermarket model. Turn and run as fast as you can from a stock head 99-01 XJ 4.0 unless you're planning to replace it.

I stand by my initial post except for the '99 thing.

Around here, a 7120 head would be $125 and $200 to hot tank it, inspect the guides, replace the seals, basic valve seat job (maybe not even cutting). A rebuilt aftermarket 0331 head is $500. $175. How much is that fancy air filter kit? What's the status on your exhaust manifold? Aftermarket? Already cracked? If both answers are no, run what you got until either one changes and then swap in the dual outlet style without the cats. The 99+ intake is worth it for the tensioner alone.

Pushrods are the same, but get them if they are free/cheap , select the best set. Get the valve cover (97+ is better) and the plumbing attached to it if yours has an aluminum cover. Less oil in the air filter housing. Have you pulled a few lifters and inspected them? Good time to think about a cam if you're replacing the lifters. How's your stock cam faring with this reduced zinc oil? Timing set at that point. It snowballs quickly. Never seen a complete kit. Rockauto is great, even if just for finding PNs and compatibility. The molded rubber oil pan and valve cover gaskets are worth their price. The oil pan gasket can be purchased in a kit. There's a thinner head gasket available, price may be worth the gain, but it's not much.

Me personally, I'd contact Russ P. over at jeepstrokers and have him do a mild port/polish job on an aftermarket 0331 head, and then start thinking about building a stroker long block at some point in the future. If I had known you were putting mounts on it, I would have suggested a 02-04 WJ motor. Your "best" bet is to get that 7120 for $40, freshen it up and run it till it dies.

Renix to H.O

Jim Malcolm

NAXJA Forum User

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