The Mighty 258 > The Engine

complete rebuild

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Demigawd:
i just pulled my motor out a couple days ago and droped it in a shop the be bored 40 over and im going to have it 0 decked when i get my new pistons and crank shaft in doing a master rebuild i'm also switching to a 4.0 head and im quite cruious what i can mill off the head without the pistons meeting the valves so i can reach maximum compression since ill be converting it to run off of E85

Sunny:
If you are having it bored out anyways, you'll need new pistons so why not just put 'pop up' pistons in?
I'd also probably put a stroker rotating assembly in there too if it's already apart.
Milling the head is possible, although I don't know the specifics on an AMC/Jeep head.
Keep in mind, if you are going for the max you can mill off, you'll need new pushrods, depending on how much you can take off... [Although you may need new pushrods anyways with a 4.0 head]

You will also need to adjust/buy a new carb for E85, aswell as a timing and timing curve adjustment.
E85 can support higher compression than regular gas but it uses more gas to do it.
You need about 30% more fuel than you would gasoline [one of the major reasons you'll need to completely rip apart your carb to convert it]

If you are leaving the motor basically stock, I wouldn't bother with E85.. It's more of a pain in the assembly to set up than it's worth, and you'll be burning about 30% more gas.

If you are going to port the heads/new cam/etc then maybe, just for the boost in compression.. but you can get a lot more compression than stock on pump gas.

Demigawd:
can u get a stoker assembly for the 4.2? i know u can stroke the 4.0 with the 4.2 crank and Domed pistons there already about flat of i zero deck it i wont have to worry about domes hitting the valves ive been looking for a new carb online that was my next question about what CFM do i need? im sure a 2 barrel would do it for low rpm since im getting a cam to optimize for low rmp torque and i know it takes 30% more gas but it will still be cheaper since gas is more than 50% more expensive than e85 at the pump and with a higher compression ratio u can get more power from the e85 yea it will take more gas but its cheaper so *shrug* when i have the money ill probably switch to a EFI with it set up to where i can adjust the timing and the air fuel ratio with a couple nob on the dash so i can switch from gas to e85 and i know the compression ratio would be a factor cause i id have to run high grade gas or i could put a turbo on it for when im running e85 my head just bursting with ideas

IowaEagle:
IIRC the 258 is a stroked 232??

priya:
We took 20 thousands off my head and 10 thousandths off the deck of the block for a total of 30 thousands and about 1/2 point extra in compression.  My mechanic husband says you can easily take 30 thousands off the head without problem.  Long before you have a problem with pistons hitting the valves if you take too much off the head you'll collapse the valve lifters and they'll become like solid lifters.  Any competent machine shop should be able to advise you on the maximum you can remove before you encounter problems and how much compression increase that'll give you.

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