Time to rebuild my 258 /4.2 engine and I'm wondering those of you here who have done it what would you do again what not to bother with?

[Scarry D]

Time to rebuild my 258 /4.2 and I'm wondering those of you here who here who have done it what would you do again what would you do different what would you not bother with?
It's a 1988 YJ I've owned since new I'm probably going to do a 4.0 head swap possibly an RV cam but I don't want to kill my fuel economy although right now I'm only getting 16 MPG . I already have the Mopar MPI kit. It's a daily driver a work vehicle I work on large Farms it's what's gets me and my tools around the place it's not lifted and it's not going to be it works perfect with what I'm doing" I'd like more power wouldn't we all I pull stumps , logs, tractors out of the field and hay into the field for the animals . One more thing it has an NV 4500 under it it was a hold my beer moment and I'm keeping it it's NOT !! for sale ..

 

[ytradio]

The way you say that you are going to use your YJ, I personally would not go out of my way to get the 4.0 head. If you have access to one and it will save you money over rebuilding the 258 head fine, but the performance gain will be at higher rpm not low end torque. I am in the process of rebuilding my 89 and I have studied quite a bit of torque and horsepower curves on different builds, and unless you are planning on winding the rpm’ s up. I feel it is just money wasted. I am reconditioning the crank with new main and rod bearings, probably end up boring 10 over with new pistons, rings and pins. New rod bolts, plane the 258 head for flatness, and do a valve job, maybe new valves not sure if needed.

 

[Stoopid Jeep]

Time to rebuild my 258 /4.2 and I'm wondering those of you here who here who have done it what would you do again what would you do different what would you not bother with?
It's a 1988 YJ ...probably going to do a 4.0 head swap possibly an RV cam but I don't want to kill my fuel economy although right now I'm only getting 16 MPG . ... I'd like more power wouldn't we all I pull stumps , logs, tractors out of the field and hay into the field for the animals ...

I've never built a 4.2 but I have probably built enough motors to weigh in on this.
First thing is make sure you calculate your compression ratio and quench before you do anything. Quench is one of the most important factors that dictates how your new engine will run. (it is also one the most commonly forgotten). make a plan before you have the block machined and know what thickness head gasket you're going to use.
You can get by with inexpensive cast pistons, they will run just fine in your application, guaranteed.
Never cheap out on bearings, timing sets, oil pump, or rings. Buy the best available.
Same deal with gaskets, stick to Felpro, Mahle, or Mopar. Don't cheap out on gaskets.
There have been libraries written on cam selection.
Just make sure you match the cam specs to the compression ratio and the RPM range that you will actually be driving. Nothing worse than driving a motor with too much cam. Ignore the peak numbers and focus on useable power range under 4000 Rip'ems.
Let's be honest, how much time do we actually spin our motors 6000prms? Personally, my engine has never seen 4500rpms, so any power gains over that point effectively are unuseable, and worse it hurts the low end power.

Make a clean organized work space.
Measure EVERYTHING!
If you can't measure it yourself, have the machine shop assemble it.
Have fun, I love building motors.

 

[Gilaguy23]

What to leave out? crap rebuild kits off ebay or scamazon. Thats my $.02

 

[Smellypiratehooker]

I was recently in the same position with the daughters jeep and I ended up sticking with the 4.2. Now we already had the 4.0 HO head and header with a Motorcraft 2100 carb just for background.
Things I can tell you about the 4.2;
It dumps its torque so low that putting it into a jeep that had a 2.5 you feel like you've lowered the gear ratio one notch. Our example had 33" and 4:56 gearing. It needed 1st gear and fell on its face in 5th. The 2.5 was in perfect operating order. A simple swap to a 4.2 (before HO head) made it so 1st gear was an option and it would pull 5th for days. Keeping in mind I believe the ax15 trans 1st gear is actually higher the the ax5!. The 4.2 runs like a diesel as its all low end and nothing past it. I like that, some don't. All things similar, the 4.2 CAN achieve better fuel mileage then the 4.0. Before you jump in with one being injection and the other not, I did say all things similar! Just like the Chevy 305, the 4.2 has tiny pistons that shroud the valves. AKA hurt breathing, which means less air/fuel in. We beat the 2.5 milage significantly with the 4.2 with and without the 4.0 head swap. Now for the bad parts.....the 4.2 isn't cheap to rebuild. Machine work is all the same but parts not so much. And there ain't much of them. Even piston selection is tough, I ended up using pistons ment for the smaller 60's amc inline and decking the block to achieve 0 deck. And paid much more for them. Add maybe 500 extra for parts compared to the 4.0 depending on what you are doing. If you are looking for an HP build go for a 4.0 or Chevy. All three end up being the same weight which was the cap I set for any swap. The 4.0 can make power for sure, but the small block with obviously make more and probably do it with better mileage. Infact I kind of regret not going a 305 virtec build only because the rebuild would have been cheaper and I would have had the option of much more power if ever needed. 305 vortec were getting 18-19+ mpg in full size 4x4 trucks. Let's get back on track......so if you want a stockish build the 4.2 is a great engine. Ours has about 9.3:1 compression, stock cam with HO head and a sniper injection now after rebuild. We honestly don't need any more power as the torque is nasty. We've managed 17-18 mpg and its still breaking in. It will spin the 33" tires with 4:56 gears if you so choose on asphalt. With nothing more then a bump in compression, an HO head and attention to detail in the build. If you need more then that, certainly go the 4.0 or the small block.