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Experience with a Rebuilt 258

Experience with a Rebuilt 258
Neuner

Neuner

Old Time Jeeper
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Vehicle(s)
"Oscar":1985 CJ7, 4.0 '98 OBDII, T177, RE 4.5" lift w/ CV Jnts, Dana 300, Dana 30 and AMC 20, 33" BFG KO2s, Raptor Lined Interior and Rustoleum BBQ exterior.
My AMC 258 i6 / 4.2l is tired and in need of a rebuild. I've tossed around all sorts of options but come back to wanting to keep it the same as it is. I've read and heard all about the different swaps, FI head swap, etc but I'm leaning more towards just getting it rebuilt as is because I like it that way.

Appears the AMC 258 i6 / 4.2l is notorious with blow by especially from the breather. Looking to get feedback from those who've had theirs honed/bored with new pistons/rings and valve seals and whether or not they still experience this? Was it worth it?
 
I've rebuilt a few Jeep engines, including the AMC 258 i6 / 4.2l . Not really difficult at all, just time consuming. I wouldn't recommend tearing into anything until you've done a compression test. You can pick up a decent compression tester at any parts store, mine was around $40. A good investment and beats spending $1k+ on a rebuild which you may not need.

Depending on condition and miles, you should should be between 120-AMC 150 psi. Any cylinders that read low, add a tablespoon or two of motor oil and try again. If the compression comes up, it means your rings are worn. If it is still low, it means your valves are likely worn.

Another thing to check is your emissions / vacuum system, if they aren't working right they can cause all kinds of problems including blow by. I see your profile lists a Weber 32/36, how is your vacuum system arranged? When I had my 85 CJ7 with the 32/36, I followed this vacuum diagram to the letter. I still ran the canister, EGR and heated intake manifold. Jeep ran great, even on my old engine.

I don't take credit for the diagram, I can't recall where I got it.
 
Thanks but I've already researched, read all of that, etc, etc. Didn't want this to turn into a thread about whether or not I need it done, to what extent or how much it costs but what your experience has been with the engine afterwards.

Does the engine still show any signs of weakness, blow-by? How has it held up over time? If in the situation again, would you still rebuild it or dream of something else? I know it varies depending on the condition of the engine but did you have a noticeable improvement in power?

Searches only tell of the person's analysis, symptoms, cost, or process of the build but never whether or not they were happy with the results.
 
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A rebuild engine when done by someone who knows what they are doing. Will be the same as a new engine.
LG
 
I would not rebuild a AMC 258 i6 / 4.2l to bone stock. I was VERY happy with the 4.0 head conversion.

It will STILL be a AMC 258 i6 / 4.2l ...but, quality of machine work helps with blow-by. Staying in the lower rpm range (where this motor was designed to work) will keep that to a minimum.

I believe that the old Hastings ductile iron rings work well, maybe better than moly, unless your shop does the light-tight torque plate type of hone job.

But I personally was the most happy with that simple head swap Bang for Buck there for sure :D There's a cylinder head shop in Tampa that makes a $330 bolt-on for the AMC 258 i6 / 4.2l .
 
Interesting. Thanks Gutthans.
 
If you watch cl a low mileage 4.0 will show up eventually. It's a very good motor and if you don't want to fi it( it's not hard I make the conversion harness for people all the time) you can keep running the carb on it.

To be honest I priced out a rebuild on a 6cyl and as much as the shop wanted I could buy a crate stroker for 0-500 more. So if I get to that point every I would just go that route.

I just got a motor with under 85k for 200 bucks on cl. Considering that I have personally seen 4.0s with over 200k on the clock I don't mind trading the mileage for saving a few thousand dollars.
 
I'm running a 1978 AMC 258 i6 / 4.2l with the factory main bearings. seven main bearings make for a pretty strong lower end on the AMC 258 i6 / 4.2l /4.0 I only broke torque on the #7 cap to replace the main seal and that bearing looked really good. You can replace the main seal on these, both sides, without pulling the crank. I honed the cylinders and replaced the rings and rod bearings. I have a little bit of blow by but nothing to make me say "I wish I dropped a grand on a rebuild". I used a rebuilt cylinder head I found on CL. I had my cam reground and installed new lifters. My cam bearings are factory. The motor runs great.
 
I will have a quality shop perform the rebuild. I will research and make sure they do it right and I will pay extra for it but it doesn't sound like I'd be as happy as I would hope after spending the dough on just a straight rebuild.

Got me thinking now on the alternatives, thanks for the input.
 
My Jeep's PO had my AMC 258 i6 / 4.2l rebuilt 3000 miles before he sold it to me. We both believe the original motor had 150k miles on it at the time. He did it because he had chronic low oil pressure.

I don't know if the cyl were over bored, or if sleeves are in there.... The only upgrade I can detect is a Clifford aluminum valve cover. I have put about 2000 miles on it, for a total of 5k on the rebuild with no issues. Oh, there is also a weber carb on it: two detectable mods.....

My Limited experience is positive.

When you say your engine is tired, what do you mean? Compression? excessive blow by? I don't notice a lot of blow by at 5k on my rebuild.....

-Jon
 
I bought a CJ5 with a AMC 258 i6 / 4.2l that had a rod knock for cheap when I got it home I pulled the motor and put it on a stand. After pulling the pan I found that the # 6 rod had spun the bearing. So I took the whole thing apart and took it all down to the machine shop. They check everything and ended up boring the block 40 over resizing the crank and rods 20 over checking the rod for straightness and installing them on the new pistons. And they installed new cam bearings and freeze plugs and did a valve job on the head. Then I got it back with all the bearings, rings, cam, lifters, timing chain and gasket set. I put it back on my stand and put it all back together with a new oil pump and clutch kit. It was alot of fun and ran great for over 100,000 miles. If you want to stick with the AMC 258 i6 / 4.2l I would recommend rebuilding it.
 
Even if it doesn't have to have it done, how much can be bored over before changes to headers, etc need to be changed and would I gain anything?
 
Neuner said:
Even if it doesn't have to have it done, how much can be bored over before changes to headers, etc need to be changed and would I gain anything?
Click to expand...
Boring a motor will not change anything but the size of the pistons.
 
Neuner said:
Even if it doesn't have to have it done, how much can be bored over before changes to headers, etc need to be changed and would I gain anything?
Click to expand...

are you talking about porting the head?
 
Neuner said:
Even if it doesn't have to have it done, how much can be bored over before changes to headers, etc need to be changed and would I gain anything?
Click to expand...

One has nut'n to do with the other.:eek:
I have bored many AMC 258 i6 / 4.2l 's out to .070 over with no issue.
IMO-Your dead cyl is either a crack in the head, of a leak in the headgasket.
Have you re-torqued the headbolts?? I would do this at the very least--
Pull the head and have it 'hot-tanked and magnafluxed(penetrate inspection).
Till you do that--It's all guess'n.
LG
 
Lumpy Grits said:
One has nut'n to do with the other.:eek:
I have bored many AMC 258 i6 / 4.2l 's out to .070 over with no issue.
IMO-Your dead cyl is either a crack in the head, of a leak in the headgasket.
Have you re-torqued the headbolts?? I would do this at the very least--
Pull the head and have it 'hot-tanked and magnafluxed(penetrate inspection).
Till you do that--It's all guess'n.
LG
Click to expand...

WTF? Who said anything about a dead cylinder or a cracked head?
 
Neuner said:
WTF? Who said anything about a dead cylinder or a cracked head?
Click to expand...


SORRY:o
Had your post confused with another along the same lines.
LG
 
ThisGuyUKnow said:
If you watch cl a low mileage 4.0 will show up eventually. It's a very good motor and if you don't want to fi it( it's not hard I make the conversion harness for people all the time) you can keep running the carb on it.

To be honest I priced out a rebuild on a 6cyl and as much as the shop wanted I could buy a crate stroker for 0-500 more. So if I get to that point every I would just go that route.

I just got a motor with under 85k for 200 bucks on cl. Considering that I have personally seen 4.0s with over 200k on the clock I don't mind trading the mileage for saving a few thousand dollars.
Click to expand...

Starting to consider this option. Need to research it more and what it would take. Of course I'd like to keep it FI but not sure how much work goes into that especially with the wiring and electronic setup. Also, the PO clamped shut the O2 ports on my exhaust. Just put the T177 and new clutch in so I hope it mates up with it's bellhousing?
 
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