boring 258 block for a 4.0 head

[Canoe]

I've read that if you're going to torque the 4.0 head to its 110 lbs., and you're having a tired 258 block bored as part of its rebuild to take this head, you have to bore with a boring deck torqued to 110 so the newly bored cylinders won't go out of round when the 4.0 head is torqued to 110 lbs..

Question: for this newly bored block, until you get a 4.0 head, can you run it with the stock 4.2 head torqued to 75 lbs?

[mudkicker715]

i never done this with my 2  strokers. really do not understand what your saying. however i know i went past 75 on both. my first is standard bore. the other is bored 60 over. the first is a 7120 head the other is a iirc 2126 head. however theses utilize the 4.0 block.

[Canoe]

From what I've read, the 4.0 head wants 110 lbs..
But, you can't take a standard 258 block, re-bored or stock, and torque the 4.0 head to it's 110 lbs..This much torque will distort the stock 258 block, causing the cylinders to be out of round.

To get around this, a boring deck is torqued in place of the head, at 110 lbs, so the boring is done with the "head"/block torqued to 110 lbs.

My question: if this is done, will it run with a stock head at stock torque?

[Canoe]

But, maybe you can answer this for me. If I'm going to get my stock 258 block bored 030 over, how does this compare to a stroker of a 258 block, both in spec and in cost?
Can it run with a stock 258 head for now, and go to the 4.0 HO head later?

[mudkicker715]

i replied on another thread of yours

[carnuck]

You have to run the 1/2" head bolts to get 110 ft lbs. The early/mid '80s 4.2L ran 7/16" which will shear off before 110 ft lbs. If you are building a stroker, then you want the boring plate on the block when it's machined. Not really needed on a stock rebuild.

[captspillane]

My "stroker" is an overbored 258 with 4.0 head. Usually when people say stroker they mean a 4.0 block with a 4.2 crank. The 258 block can be overbored to 4.5 but a 242 block can be overbored to 4.9. I personally don't mind calling a 4.5 258 block with 4.0 head a stroker because it performs exactly the same as a 4.5 stroker made from a 242 block. In the end it's really a combination of 258 crank and 4.0 head that makes the difference, so I personally don't mind using "stroker" to describe a 258 block as long as a 4.0 head is on it. It's just easy to confuse people because not everyone uses the word the same way.

On my engine I have 7/16" head bolts. I like that better because they are reuseable. The larger 1/2" head bolts are meant to be torque to yield. "Torque to yield" essentially means that the bolt is designed to fatigue and be damaged the very first time you tighten them in an effort to make a more consistant pressure across the head gasket. That was the reason for the higher torque numbers. You're actually supposed to break the bolt a little bit as you put them on. Since you have to buy an expensive set of head bolts each time you rebuild the engine with the larger size so I plan on sticking with the smaller size. I only replace them if they are high mileage.

You'll need an adaptor as you bolt the 4.0 head on to the 258 block with the 258 head bolts. For this I use two old 4.0 head bolts. About half of the driver side head bolts on every 4.0 are 7/16 threads at the top and 1/2 threads at the bottom. I ground the hex head off the shank of the bolt, leaving both sets of threads intact. I flipped the head bolts upside down and screwed them into the block. The gasket and head slid perfectly down the shank of the larger head bolts into perfect position. I snugged up all the other head bolts before removing my homemade adaptors and filling those holes with head bolts too. It works much better than the spacers Hesco sells and it makes for a much more reliable head gasket placement. The best part is that the adaptors themselves are made from junk you can usually take home for free when you pull your donor 4.0 head at the junkyard.

[Canoe]

You have to run the 1/2" head bolts to get 110 ft lbs. The early/mid '80s 4.2L ran 7/16" which will shear off before 110 ft lbs.

Mine's an '86 Eagle, but the head bolts are 1/2", making me suspect it's not the original block.

[Canoe]

... The 258 block can be overbored to 4.5

I'm stuck on the road, so adding in the adaptor to put a 4.0 block onto my trans along with finding a 4.0 block... I already have the 258 block sitting here.


A 258 block:
stock, is 4.2
over bored 030 is ______?
over bored ___ is 4.5

So I tell the shop I want it over-bored by ____ to ____ to get 4.5?

Stock cam or otherwise?

[captspillane]

I think the 1/2" head bolts started in late '87 at the same time they switched to the aluminum valve cover eventually used with the 4.0. Both of my '87 Eagles still have 7/16" head bolts and the plastic valve cover. I do not know if those late 4.2 blocks use torque-to-yield bolts. I bet a service manual from 1988 or a 1989 YJ would tell you if those bolts needed to be torqued to the bigger 4.0 value or the smaller 4.2 value. That is worth looking into if you plan on using the old head bolts.

My engine is either .30 or .60. It was rebuilt professionally about 6 months and 400 miles before I purchased my Spirit. The original owner kept calling it a 4.5L. I'm about to install it into my SX4 because the Spirit is getting a V8. I can't tell you the exact cubic inches that correspond to both overbores, or the liter amount down to three decimal places. I would like to know that myself.

[Canoe]

I won't be reusing the old head bolts. Not worth the risk with all I'm stuck spending.

My 1/2" head has the plastic valve cover. But, one of the mechanics here was taught by an old AMC mechanic and had an Eagle for a few years. He said that the platic valve cover was designed to run without a gasket: completely clean valve cover intent surface and the bonding surface of the head with brake cleaner, then apply a 3/8" bead of RTV silicone gasket maker, set in place, press lightly so RTV coats entire bonding surface, let sit until cured, then torque. Claims this never leaks oil, but that the oil filler cap always will...

[carnuck]

RTV grey will work for quite awhile, but "the Right Stuff" works AWESOME on non-cracked plastic cover to seal it up.

[carnuck]

... The 258 block can be overbored to 4.5

I'm stuck on the road, so adding in the adaptor to put a 4.0 block onto my trans along with finding a 4.0 block... I already have the 258 block sitting here.


A 258 block:
stock, is 4.2
over bored 030 is ______?
over bored ___ is 4.5

So I tell the shop I want it over-bored by ____ to ____ to get 4.5?

Stock cam or otherwise?

No adapter for 4.0L to 4.2L trans or front axle. Change the oil pan from the 4.0L to the one from the 4.2L, clean the axle mount holes in the block before installing. Not all 4.2Ls can be bored (need to check for core shift) and 1/2" headbolts started coming back in early '87 model year according to the Jeep guys I've talked with.

For the cam, if you plan EFI later, use the aftermarket performance one or the ping monster will haunt you.

[Canoe]

No adapter for 4.0L to 4.2L trans or front axle. Change the oil pan from the 4.0L to the one from the 4.2L, clean the axle mount holes in the block before installing.

Good to know.
So I could drop a 4.0 right in onto the auto-trans, using the 258 intake for the carb, using the 4.0 exhaust manifold patched into the 258 exhaust.
Understand only issue should be mounting the power steering pump?

Not all 4.2Ls can be bored (need to check for core shift)

Now you're scaring me. What is core shift?

For the cam, if you plan EFI later, use the aftermarket performance one or the ping monster will haunt you.

No EFI planned.

[eaglefreek]

Only overbore enough to get a good bore. Not much is gained by overboring more than needed.