boring 258 block for a 4.0 head

[captspillane]

You can bolt the 4.0 right up to everything. The trick is deciding to keep the 4.0 serpantine belt or not. I personally want all my cars, 4.0 or 4.2, to eventually be serpantine. I hate squeaking V belts and that problem doesn't exist with the serpantine. If you use the 4.0 belts you'll need to keep the original 4.0 harmonic dampner, AC pump, and power steering pump. That power steering pump is compatible with the Eagle steering hardware.

Realize that a 4.0 block doesn't have a fuel pump hole. That's the biggest difference between the two. If you bolt on a carb to a 4.0 you'll need a low pressure electric fuel pump to feed the carb.

You'll always need an Eagle oil pan and oil pickup tube, even if you're using a 4.2 from a Spirit or Jeep. The 4.0 is the same way. The Eagle stuff goes right unto any inline 6 1970 to 2006.

Some people will tell you that 4.2 exhaust and intake manifolds don't work on a 4.0. I disagree. The problem seems to be the bolt hole in the front of the manifold doesn't line up anymore. The gasket surface itself looks like it lines up fine. In my CJ8 I have a 4.2 cast exhaust manifold with a Hesco 4.0 intake. That bolt that used to go to the 4.2 intake was just moved and some work with the grinder trimmed the power steering bracket. Its using 4.2 V belt accessories. My Spirit had the opposite. It had a 4.0 exhaust manifold with a 4.2 intake manifold. I don't see why a 4.2 intake and exhaust won't work on a 4.2 head except for moving that same hole and trimming the bracket like my CJ8 has.

Any inline 6 AMC flexplate or flywheel is compatible with any inline 6, 1971 to 2006. (1970 hardware works too for manual transmissions but not automatic). It just bolts on and the trans just bolts on exactly the same. It's only the V8's that have a slight difference. Those flexplates and flywheels look exactly the same and bolt on exactly the same, but they're balanced differently and will vibrate severly if you accidently use one on an inline 6.

I agree you should never overbore any more than necessary. The only reason to overbore to .60 is if the engine has already had a great deal of mileage on a previous .30 overbore. Realize that AMC blocks accept new rings quite well even if they have 200+ miles and you need a ridge reamer to take off the ridge on the bore. It's up to you if you want the peace of mind or not. Realistically most AMC engines would function well with just new rings and no overbore during periodic rebuilds.

[captspillane]

My 1/2" head has the plastic valve cover.

Sounds to me like you think you have 1/2" bolts but actually have 7/16" bolts, especially since your Eagle is a 1982. They look so darn close. Try putting a tap on them and you'll know for sure. I confuse my 4.0 head bolts all the time as 4.2 bolts.

An even easier test is to take the accessory head bolt and take the nut at the top and try it on the threads at the bottom. If the threads are exactly the same top and bottom its 7/16". If its different its 1/2" bottom and 7/16" top. Those are the only two possible combinations since 4.0 head bolts still used 7/16" threads at the top compatible with all the old 4.2 hardware. A 4.2 would have at least one of those double ended head bolts at the front of the driver side for attaching a Vbelt pulley and a 4.0 would have at least three of them for the wiring harness plastic box to snap to.

I'm pretty sure all plastic valve cover engines are the lightened 1981 to early 1987 blocks with 7/16 headbolts. I guess its plausible that someone used a 1988 short block and the 1982 head with the holes slightly enlarged, but it's much more likely you're just wrong about your own head bolts.

[vangremlin]

Any inline 6 AMC flexplate or flywheel is compatible with any inline 6, 1971 to 2006. (1970 hardware works too for manual transmissions but not automatic). It just bolts on and the trans just bolts on exactly the same. It's only the V8's that have a slight difference. Those flexplates and flywheels look exactly the same and bolt on exactly the same, but they're balanced differently and will vibrate severly if you accidently use one on an inline 6.

My experience is that you should not interchange the bolts that hold a flexplate on with the bolts that hold a flywheel on.  Keep flexplate bolts with flexplate, flywheel bolts with flywheel.

[captspillane]

Flexplate bolts are about a half inch shorter than flywheel bolts. Keep the bolts and flexplate or flywheel with the transmission it originally came with as you swap engines.

In a related note, AW4 flexplates "work" with a TF998. They work for about a week until the 4 mm difference in backspacing causes your torque converter to push on the transmission pump, breaking it. I've seen two people on forums do that by mistake. If you use a 4.0 long block don't be tempted to leave the 4.0 flexplate on.

[mudkicker715]

Flexplate bolts are about a half inch shorter than flywheel bolts. Keep the bolts and flexplate or flywheel with the transmission it originally came with as you swap engines.

In a related note, AW4 flexplates "work" with a TF998. They work for about a week until the 4 mm difference in backspacing causes your torque converter to push on the transmission pump, breaking it. I've seen two people on forums do that by mistake. If you use a 4.0 long block don't be tempted to leave the 4.0 flexplate on.

Worked fine for me.

[captspillane]

All Automatic XJ's have AW4 transmissions. Some Grand Cherokees and Wranglers had a 4.0 in front of a compatible flexplate from a transmission that wasn't an AW4. Its the same flexplate to look for if you're trying to do a fancy CPS modification on a TF998.

It's just the AW4 flexplate that absolutely will damage your transmission if you drive with it for too long. If you know you have an XJ flexplate in front of your Eagle TF998 get ready for the transmission to give out.

[mudkicker715]

All Automatic XJ's have AW4 transmissions. Some Grand Cherokees and Wranglers had a 4.0 in front of a compatible flexplate from a transmission that wasn't an AW4. Its the same flexplate to look for if you're trying to do a fancy CPS modification on a TF998.

It's just the AW4 flexplate that absolutely will damage your transmission if you drive with it for too long. If you know you have an XJ flexplate in front of your Eagle TF998 get ready for the transmission to give out.

i drove for about 4 years with no damaging results on that setup.

[carnuck]

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.

4.2L intake needs minor mods to bolt to 4.0L head. Just renotching where it attaches to the head so the intake ports are covered by the carbed intake.

PS bolts on and you have the choice between 4.2L stuff (swap all 4.2L accessories including balancer) or use the 4.0L accessories (leave the EFI intake and make/buy an adapter for your carb to it)

There is no fuel pump or spot on the 4.0L block so electric pump is needed.

Core shift is when they poured the motor block, the sand they used for the sleeves moved during pouring, leaving thin spots (sonic check of the block is needed when boring over 0.030)

[carnuck]

My 1/2" head has the plastic valve cover.

Sounds to me like you think you have 1/2" bolts but actually have 7/16" bolts, especially since your Eagle is a 1982. They look so darn close. Try putting a tap on them and you'll know for sure. I confuse my 4.0 head bolts all the time as 4.2 bolts.

An even easier test is to take the accessory head bolt and take the nut at the top and try it on the threads at the bottom. If the threads are exactly the same top and bottom its 7/16". If its different its 1/2" bottom and 7/16" top. Those are the only two possible combinations since 4.0 head bolts still used 7/16" threads at the top compatible with all the old 4.2 hardware. A 4.2 would have at least one of those double ended head bolts at the front of the driver side for attaching a Vbelt pulley and a 4.0 would have at least three of them for the wiring harness plastic box to snap to.

I'm pretty sure all plastic valve cover engines are the lightened 1981 to early 1987 blocks with 7/16 headbolts. I guess its plausible that someone used a 1988 short block and the 1982 head with the holes slightly enlarged, but it's much more likely you're just wrong about your own head bolts.

I use an open end wrench to see what size the bolts are.

[carnuck]

All Automatic XJ's have AW4 transmissions. Some Grand Cherokees and Wranglers had a 4.0 in front of a compatible flexplate from a transmission that wasn't an AW4. Its the same flexplate to look for if you're trying to do a fancy CPS modification on a TF998.

It's just the AW4 flexplate that absolutely will damage your transmission if you drive with it for too long. If you know you have an XJ flexplate in front of your Eagle TF998 get ready for the transmission to give out.

All 4.0L XJs are AW4. 4 cyl and 2.8 V6 are 904 till '88

[Draekon]

... 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?

Stock bore is 4.235 liters
Overbored .030 gives 4.303 liters
Overbored .060 gives 4.371 liters

In order to get 4.5 liters, you need to overbore your motor by .120, which is quite a lot for most motors.  You would have to get the block sonic checked to make sure there is enough material to do it.

[captspillane]

I use an open end wrench to see what size the bolts are.

7/16" is the tap size of the threads, not the nut hex head size. Both 1/2" and 7/16" head bolts use the same size hex head.

[captspillane]

Stock bore is 4.235 liters
Overbored .030 gives 4.303 liters
Overbored .060 gives 4.371 liters

In order to get 4.5 liters, you need to overbore your motor by .120, which is quite a lot for most motors.  You would have to get the block sonic checked to make sure there is enough material to do it.

What formulas and given values did you use to calculate that?

What are the numbers for stroker combinations using 4.0 blocks and 258 cranks with no overbore, .30 and .60 overbore on the 4.0 block?

[Draekon]

Stock bore is 4.235 liters
Overbored .030 gives 4.303 liters
Overbored .060 gives 4.371 liters

In order to get 4.5 liters, you need to overbore your motor by .120, which is quite a lot for most motors.  You would have to get the block sonic checked to make sure there is enough material to do it.

What formulas and given values did you use to calculate that?

What are the numbers for stroker combinations using 4.0 blocks and 258 cranks with no overbore, .30 and .60 overbore on the 4.0 block?

A stock 258 has a bore of 3.75", and a stroke of 3.90"

Engine size is just the displacement created by the pistons in the cylinders, so you just need  to calculate the volume of a cylinder (or 6 in the case of a I6)

So for a stock motor:
Volume of a cylinder is height*pi*radius^2, or height*pi/4 *diameter^2
Bore is equivalent to diameter, so we have 3.9 *3.14159/4 * 3.75^2 = 43.074 cu in per cylinder.
Take this number and multiply it by the 6 cylinders, and you get 258.445 cu in total displacement.

You can then convert that number to liters by using a conversion factor, or by going to google and typing in "258 cubic inches to liters".

If you want to calculate the volume if you bore the motor, you just add however much you are boring the cylinders out to the original bore.  For example, if you bore the motor .030, you are increasing the bore of the cylinder to 3.780".

If you increase the bore by .120", making it 3.870", you get a 4.5L motor.

[profeagle]

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.

No worries, 1/2" bolts started with the 86 model. Same as the one that I have which is original engine !