Thread: 258 build

  1. #1
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    258 build

    I've had a couple of other threads pertaining to my 74 cj with a 258 but now I am considering a different route and want to get opinions. I talked to a local machine shop and he said decking the block would be the easiest solution to raise compression as there is no piston options but he had done it before and ran into some valve clearance issues. 74 is 8:1 compression. I'd like to be in the neighborhood of 9:1 to 10:1. 82-90 is 9.2:1. Here is the most cost effective options I see. 1. Find a head from a 82-86 and use it on my 74 along with pushrods for 82-86. 2. Find complete rebuildable 82-86 engine. 3. Deck the block on my 74 and use pushrods from 82-86 engine.? Anyone have any experience with any of these??

  2. #2
    superj's Avatar
    superj is offline Senior Member
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    Re: 258 build

    I cannot help
    no more jeeps at this time. next time it will be a cj with a v8

  3. #3
    69jeepcj's Avatar
    69jeepcj is offline Senior Member
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    Re: 258 build

    I cant help either, I cant build engines. I have read that decking the block will raise the compression and or dif thickness head gasket. thats about it for my usefulness on this subject, lol. good luck

    JP magazine did a article on building a 258 that made 600+ horses, they called it insane inline, or insane 6, something like that. Im sure they spent insane money on it but it could give you some ideas.

  4. #4
    frhrwa's Avatar
    frhrwa is offline Senior Member
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    Re: 258 build

    why not put a 4.0L head on it..
    I want to die in my sleep like my grandfather,
    not screaming in terror like the passengers in his
    JEEP!...

  5. #5
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    What does a 4.0 head do? Will it raise compression and be an easy swap? Also will I be able to use my 4.2 intake and header?

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    flatie46's Avatar
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    Re: 258 build

    You can put the engine at TDC on no. 1 ( both valves shut) , put some clay in the cylinder on top of the piston. Install a gasket, A used one will be ok I think. Then install the head with that plug out. Torque it down to what it will run at. Remove the head and measure the clay with a depth gauge, that'll tell you how much clearance you have. Now, I have read about this but have never done it. I suggest you read up on it in case I've left something out or maybe anthor forum member has done this. I've got a 4stroke dirt bike I'm getting ready to have to do this to, as this piston will have to be cut a bit for more valve relief. It'll be a first time for me. Like they say, you get experiance right after you need it. It takes very little shaved from a block or cyl head to raise compression. If it were me I'd shave the head instead of the block, if something happened I'd rather replace the head than the block.

  7. #7
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    Good call on the head rather than the block. Thats what I originally asked the machine shop about and he said block I wasn't sure if he misheard me or if there's an Advantage one way or another other than as you said replacing a head would be cheaper I would think than a block but I'm no machinist. I would think even if I did run into clearance issue I could use shorter pushrods but again I'm no machinist.

  8. #8
    frhrwa's Avatar
    frhrwa is offline Senior Member
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    Re: 258 build

    there are all kinds of responses to head swap in the forum, they can explain it better than I can.. I understand its a direct bolt up, one hole is made a little bigger and that's not tough.. you can use a 4.0L intake, injection, and if you like yours, I believe it said use it.. the exhaust they recommend is the one with the expandable section made into it.. stops the cracking like the earlier exhaust manifolds were know for.. gains horsepower on top end. someone said you lose a little low end torque, well, gearing, transfer case gears, etc. put you right back in to low end torque.. I can't believe you'd lose enough to worry about over the gain in everything else.
    I want to die in my sleep like my grandfather,
    not screaming in terror like the passengers in his
    JEEP!...

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