That's Great! Have you driven it yet? You're going to love the extra hp!
Hard to say, talk to the guys at morris4x4 or quadratec. I personally wanted one that matched the color scheme under the hood.
The header won't work with the 4.0 head. Your hesitation is a lack of fuel. The accelerator pump on the Weber doesn't feed enough gas. I have the same problem and will eventually add FI. You may also want to add an HP cam. I am using Comp Cam 235 grind and love it!Well crud, I wish I had seen this thread and knew about this solution before pulling the trigger on my own recent concoction. I have been disappointed in the results.
To summarize what I did: I've got a 4.2L ATK reman, installed about 10k miles ago. It also has a 2bbl Weber carb. I just added a DUI HEI, Hedman 6-in-1 headers, a Flowmaster Super 50, and a 2.5" pipe width all the way back.
My specific areas of disappointment in the solution:
I didn't find any power increase; instead, I've got a possible power decrease. If I hold throttle steady (driving down the road) and then accelerate (passing someone) there can be a momentary pause, followed by a little surging... The guys at my shop said that the increased pipe size could have released too much backpressure, causing this problem. Your thoughts? Was 2.5" too big?
Second issue = too loud. My neighbor 3 doors down says she can tell when I'm going for a ride in the jeep. WTF? From my research, the Flowmaster Super 50 appeared to be the quietest muffler beyond stock. Someone in the CJ-8 forums piped in again about the 2.5" being "like a sewer pipe" for the 258, increasing sound... Again, thoughts?
So I'm thinking that I should have done the 4.0L head swap. I suppose its not too late. However, it would SUK if the near $1k I just invested in headers, exhaust, install, etc are totally wasted. I've tried to absorb as much from this thread as possible, however I get lost in details.
Optimally, I can just slap the 4.0 head on, reusing my existing carb, headers, and 2.5" exhaust system and be good-to-go. But things arent generally optimal... Anyone care to set me straight?
(Other relevant info: T-5, 3.31 TracLoc)
Well crud, I wish I had seen this thread and knew about this solution before pulling the trigger on my own recent concoction. I have been disappointed in the results.
To summarize what I did: I've got a 4.2L ATK reman, installed about 10k miles ago. It also has a 2bbl Weber carb. I just added a DUI HEI, Hedman 6-in-1 headers, a Flowmaster Super 50, and a 2.5" pipe width all the way back.
My specific areas of disappointment in the solution:
I didn't find any power increase; instead, I've got a possible power decrease. If I hold throttle steady (driving down the road) and then accelerate (passing someone) there can be a momentary pause, followed by a little surging... The guys at my shop said that the increased pipe size could have released too much backpressure, causing this problem. Your thoughts? Was 2.5" too big?
Second issue = too loud. My neighbor 3 doors down says she can tell when I'm going for a ride in the jeep. WTF? From my research, the Flowmaster Super 50 appeared to be the quietest muffler beyond stock. Someone in the CJ-8 forums piped in again about the 2.5" being "like a sewer pipe" for the 258, increasing sound... Again, thoughts?
So I'm thinking that I should have done the 4.0L head swap. I suppose its not too late. However, it would SUK if the near $1k I just invested in headers, exhaust, install, etc are totally wasted. I've tried to absorb as much from this thread as possible, however I get lost in details.
Optimally, I can just slap the 4.0 head on, reusing my existing carb, headers, and 2.5" exhaust system and be good-to-go. But things arent generally optimal... Anyone care to set me straight?
(Other relevant info: T-5, 3.31 TracLoc)
Like the others have said, you'll need to get a manifold for a 4.0, but the good news is you can buy a brand new one from ebay for around $75 delivered, and it's actually a good flowing tubular header instead of restrictive cast iron. People go out and spend a fortune on expensive exhaust because the manufaturers promise all kinds of increased hp, but the truth is, exhaust does very little on an engine that size because they just don't flow that much, especially in the RPM range that 258's run. You can use the intake and webber, but you'll have to do a little modding to get the intake to clear the manifold. I had to dent one of the exhaust manifold tubes a little, but not enough to restrict flow. You'll also have to fabricate some larger keeper washers to bridge the gap between the intake and exhaust manifolds where the center bolts hold both. The stock 258 thick round washers diameter is too small.
P.S. Just checking in. My 258 is still running perfect with the 4.0 head. I decided I wanted even more hp last year and started in on a 4.7L stroker, but I got side tracked by a lot of things including a big boat project so the 4.7 went on the back burner. Currently I have a 4.0 block sitting on the stand freshly bored .030 over. I have a new set of pistons with coated skirts, a Comp cam, and a high volume oil pump. What's missing is I still have to canibalize my good running 258 for the crank and rods. I hate doing that but I haven't been able to locate a good pre 1981 258 crank and rods for a reasonable price. Going from 110hp to 150 with the 4.0 was great. I can't wait to have 250 hp with the stroker! 35" tires will be next on the list since I'll have the power to push them.
on the stroker build. 
Like the others have said, you'll need to get a manifold for a 4.0, but the good news is you can buy a brand new one from ebay for around $75 delivered, and it's actually a good flowing tubular header instead of restrictive cast iron. People go out and spend a fortune on expensive exhaust because the manufaturers promise all kinds of increased hp, but the truth is, exhaust does very little on an engine that size because they just don't flow that much, especially in the RPM range that 258's run. You can use the intake and webber, but you'll have to do a little modding to get the intake to clear the manifold. I had to dent one of the exhaust manifold tubes a little, but not enough to restrict flow. You'll also have to fabricate some larger keeper washers to bridge the gap between the intake and exhaust manifolds where the center bolts hold both. The stock 258 thick round washers diameter is too small.
P.S. Just checking in. My 258 is still running perfect with the 4.0 head. I decided I wanted even more hp last year and started in on a 4.7L stroker, but I got side tracked by a lot of things including a big boat project so the 4.7 went on the back burner. Currently I have a 4.0 block sitting on the stand freshly bored .030 over. I have a new set of pistons with coated skirts, a Comp cam, and a high volume oil pump. What's missing is I still have to canibalize my good running 258 for the crank and rods. I hate doing that but I haven't been able to locate a good pre 1981 258 crank and rods for a reasonable price. Going from 110hp to 150 with the 4.0 was great. I can't wait to have 250 hp with the stroker! 35" tires will be next on the list since I'll have the power to push them.
Upgraded to 4.88 and I have 600 miles on it no problems.Back to the original topic of this thread.
How many miles has everyone put on their motors since the 4.0 head swap, and has anyone had any problems?
So far I have roughly 12,000 miles on mine and all I've had to do to it is change the oil. The engine still runs perfect, and my valve train is still running quiet as can be.
Not wanting to bring up an old argument, but with all the bad info that was floated a while back by someone who had never actually done this swap, but because of his posts, he may have scared others away from doing it, this bears being mentioned.
All of the doom and gloom about the valve train(pushrods etc...) between the 4.2 and 4.0 not working together has been totally proven wrong.
If you're thinking of swapping a 4.0 head to your 4.2(258) don't be scared off by some ill informed previous posts that say things like the valve train in not compatible. Other than some minor mods to the water jackets that are easy to do yourself, the 4.0 head matches up perfectly with the 4.2 block, lifters, and cam. I'm using the 4.0 push rods and they were the perfect length.
Also, though this is a totally different subject, if you choose to swap in an HEI distributor because of its one wire simplicity and ease of maintenance and tuning, don't be scared off by those that say the hardened HEI distributor gear will eat your soft AMC cam gear. That issue only applies to installing an HEI on an AMC V8. My HEI distributor gear and AMC Inline Six cam gear still look like new after all of the miles I've put on them.
To date, quite a few of us have put a lot of miles on our AMC inline 6 cylinders with both of these swaps, and not one post that I've read said anything about any of us who have actually done these mods having either of the problems that we were all warned about by one former member who had no experience doing either swap.
It really pays to learn from the experience of those who have actually done something, rather than listening to those that dabble in theory or read something somewhere. All of us who have made these upgrades are still available today, and are more than willing to answer any questions about our experience for those that are thinking of doing this cheap, easy and very worth while upgrade.
Happy Swapping!
