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Thread: rebuilding 258 head.....

  1. #1
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    Default rebuilding 258 head.....

    well as fate would have it, i finally got the newly installed MC2100 carb adjusted just right and the jeep was running perfectly.....this morning on the way to work i dropped a valve and limped the jeep back home.....i haven't taken the valve cover off yet to determine the extent of the damage, but i imagine i either broke a rocker arm or push rod......my question is how hard is it to rebuild the 258 head?......are the parts expensive?......i am assuming the valves are OK but won't know for sure until i get in there and see......i am just trying to get a feel for what i will be up against if i want to rebuild the head.....thanks...
    1988 YJ Wrangler (4.2L) 5 spd.
    Sahara Edition
    Lancaster, SC

  2. #2
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    Default

    If I were you and ya had the money I would buy the 4.0 head and put it on. Supposidly the carb intake bolts right to it, and you gain like 75 hp for it. I dunno, something to look into. Never rebuilt a 4.2 head though...

    aerojt
    aerojt
    2001 Jeep Cherokee XJ Limited
    4.0, AW4, NP242, Rusty's CAI, Dynomax Catback, NGK Plugs

  3. #3
    next to go postal Jays89YJ's Avatar
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    Default

    It's easy and the parts are not expensive. IMHO, do a complete long block rebuild. You'll ruin the bottom end fast if you just rebuild the head. I have many posts on here saying what I did to my 258. I was running more hp than a FI 4.0L and more torque.

    Edit: Found one for ya:
    I rebuilt my '89 258 at around 126K miles. I had some blow-by and after starting it would knock for a few seconds.

    I rebuilt my 4.2L myself, but went with a 4bbl carb.

    4.2L long block overhaul:

    - bored .030" over
    - stepped up Elgin camshaft (smooth idle)
    - high performance lifters, springs, valves, cylinders, rods, etc. etc.
    - 390cfm Holley 4bbl carb w/ electric choke
    - Clifford Research 4bbl intake manifold
    - Borla SS header w/o O2 sensor
    - Mallory Unilite Distributor
    - Mallory 8mm wires
    - Valve cover breather
    - Resurfaced flywheel
    - New Clutch & bearings
    - New alternator
    - New starter
    - New fuel pump (mechanical)
    - 2.5" cat back exhaust system
    - K&N air filter
    - high performance fuel filters (2). One before & after fuel pump. (Holley told me to do this because of the constant tweaking I had to do. It didn't help at all.

    Note:
    Do not go with a Holley carb. Go with a Q-jet carb or MC2100 off of an old 360 V8. They are much better offroad because they handle inclines & declines very well. I liked the performance of a 4bbl carb much, much better than my old 2bbl. Plus you will get an increase in gas mileage... Unless you keep your foot in it! The Holley was a PITA to keep in tune. If the temp, humidity, or weather changed at all, the carb needed tweaking (air : fuel mixture).
    I highly recommend replacing your motor mounts.

    My 4.2L really moved very well. It had lots of torque and plenty of power. The redline jumped up quite a bit. If you beat on it, keep an eye on your rear main seal.

    After rebuild go easy with it. Change the oil @ 500mi, 1500mi & 3000mi. Then every 3000mi.

  4. #4
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    Default

    thanks for the replies and suggestions.....i found a local shop that will rebuild the head for me for $150.....i can do the removal and installation myself......i think i'll take the chance with this and if i do end up messing up the bottom end i'll probably consider a 4.3L conversersion.....i don't want to swap tranny's or gears, so i won't go to a V8....all that power with the stock tranny and gears is just asking for trouble.....
    1988 YJ Wrangler (4.2L) 5 spd.
    Sahara Edition
    Lancaster, SC

  5. #5
    Registered TSM_06LJ's Avatar
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    Where you having ANY problems before you did the swap? I just got my 2100 installed along with TFI ignition and my Jeep is like brand new. I can accelerate up hills, and it starts incredibly fast, like fuel injected fast.

    Anyway, I was wondering cause after reading this I'm kind of woried about something going wrong.

    *knock on wood*
    06LJ Rubicon. LOADS of stuff.

  6. #6
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    Default

    the only trouble with the valve train i had was a lifter tick that would come and go.....once in a while the tick would turn into a knock that would last for a few seconds.....this well could have been warning signs that something was about to break....as far as power, my jeep is stock with 235/75 tires and i could go up moderate hills without any problem.....during the short time that i had the MC2100 running before the valve train problem the 258 felt like it had more power....it definetly had faster pick-up and acceleration than the carter.....i have my timing advanced as far as i can without any spark knocks.....this helps alot with acceleration also....
    1988 YJ Wrangler (4.2L) 5 spd.
    Sahara Edition
    Lancaster, SC

  7. #7
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    Definately look into the 4.0HO head swap. It is a substantial improvement and a direct bolt on. I bought my head from Clearwater Cylinder Head in FL completely rebuilt and modified for the 258 (couple water jackets welded shut). IIRC it was ~$350 with no core charge.

    Here it is:http://cgi.ebay.com/ebaymotors/ws/eB...category=33617

    HTH

    MP
    '89 YJ - 4.2HO MPI - York OBA - NV3550 - D300 4:1 - 4.88 - D30/shaved CHP 9" - ARB's - 3-link front/4-link rear on Bilstein 9100's

  8. #8
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    On the 4.0 head swap-can I bolt up my factory intake/exhaust manifold with a carbuertor?? Or do you have to change to 4.0 manifolds?

    aerojt
    aerojt
    2001 Jeep Cherokee XJ Limited
    4.0, AW4, NP242, Rusty's CAI, Dynomax Catback, NGK Plugs

  9. #9
    Awesome D'Mann's Avatar
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    From what I understand the intake is the same, but you need a 4.0 exhaust manifold. Good news is that now any 4.0 aftermarket bolt up exhausts will work now!!!
    Ever wonder what it was like to drive an H2?
    Me neither.
    Well once. But I was drunk, does that count?

  10. #10
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    the intake doesn't bolt up but the modification should be easy. intake lines up on dowel pins. These pins are further from the ports on the 4.0 than the 4.2 because the 4.0 intake ports are raised for better flow. You need to grind the bottom of the intake so the intake rests on these pins.

    read this http://www.monsterslayer.com/jeep/40Head/40Head.htm

    '89 YJ, 4.2L, ax-15, B&M shifter, 1" BL, TJ flares, '99 intake, '91 EFI, 5.0L explorer injectors, GenRight tank & stretch, 35" ProComp Xtreme muds, 52" chevy springs rear, 44044 Ranchos front.....

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