Engine rebuild help needed!

[Beaulac]

I have an 83 CJ7 , auto Transmission , weber 38deg carb, and 33" MTR's. Just pulled the original AMC 258 i6 / 4.2l to have it rebuilt. I have read a lot about the 4.0L head swap, and need to make a call as to whether this is the way to go. My rebulider seems to think a comp cam and the stock head will be as good for less cash. Even with the 33" tires the speedo is off only by 10% - cannot find the stamp as the rear end has many miles on it to determine gear ratio. Any suggestions? Stock and re-gear or swap the head

 

[Old Dog]

Even with the 33" tires the speedo is off only by 10% - cannot find the stamp as the rear end has many miles on it to determine gear ratio. Any suggestions? Stock and re-gear or swap the head

Do your rearends (both) a favor and pull the covers and change the gear oil. While you have the cover off the gear ratio will be stamped on the ring gear as two numbers, divide them to get the ratio.
For us to recommend a gear ratio we need to know what you are going to use the jeep for.

 

[LongHorn]

Do your rearends (both) a favor and pull the covers and change the gear oil. While you have the cover off the gear ratio will be stamped on the ring gear as two numbers, divide them to get the ratio.
For us to recommend a gear ratio we need to know what you are going to use the jeep for.

X2on that!

 

[LongHorn]

I have an 83 CJ7 , auto Transmission , weber 38deg carb, and 33" MTR's. Just pulled the original AMC 258 i6 / 4.2l to have it rebuilt. I have read a lot about the 4.0L head swap, and need to make a call as to whether this is the way to go. My rebulider seems to think a comp cam and the stock head will be as good for less cash. Even with the 33" tires the speedo is off only by 10% - cannot find the stamp as the rear end has many miles on it to determine gear ratio. Any suggestions? Stock and re-gear or swap the head

On the motor, when I did the wifes rig I rebuilt the AMC 258 i6 / 4.2l with better parts.
Better cam, more compression, 4 barrel intake, headers, free flow exhaust, MSD ignition.
It runs great, has better acceleration, even gets better mileage when driven lightly.

 

[LongHorn]

Oh yeah, on the wifes rig.
She has run 33" & 35".
She has 3.92"s with a Dana 300 with T-18 .
It would need more gear if she didn't have the T-18 .

 

[Beaulac]

The jeep is a weekend warrior.... nothing too tough.. trails etc, but lots of highway to get to places to play. I'll check the gears, but was wondering if a towing cam and AMC 258 i6 / 4.2l head would be comparable to the 4.0l head swap.

 

[LongHorn]

The jeep is a weekend warrior.... nothing too tough.. trails etc, but lots of highway to get to places to play. I'll check the gears, but was wondering if a towing cam and AMC 258 i6 / 4.2l head would be comparable to the 4.0l head swap.

What do you expect from the head swap?
It's not like your going to have 4.0L power without the F.I. set up.
It's more like porting your head.

 

[Old Dog]

Put a small 4X4 comp cam, the 4.0 head with efi and never look back.

 

[Old Dog]

As for gears I would say 3:92 or 4:10 maybe as far as 4:56.

 

[Beaulac]

Thanks... all this helps. I'll get the gear ratio, and price out both Engine options

 

[IOPort51]

The 4.0 head is a good deal, especially if you have a cracked head on the AMC 258 i6 / 4.2l . It happens.
You will need the head, headers or a 4.0 exhaust, new push rods and some gaskets. It does not get you 40 more HP but it is noticeable. The 4.0 head is designed much better than the AMC 258 i6 / 4.2l . Do yourself a favor and do the TFI ignition upgrade if you have not already. I would go for the RV cam for the torque. If your engine builder is suggesting a competition cam for a AMC 258 i6 / 4.2l , it may be time to start talking to a new engine builder; hi RPM’s are not the AMC 258 i6 / 4.2l ’s long suit. With an auto Transmission I would suggest you have some ungodly low number in the axles like maybe a 2.73:1 but I could be mistaken.

 

[Beaulac]

My stock Gear ratio is 3.31. With the Auto T-999 Transmission I am thinking a 3.92. 4.10 might be a bit low

 

[Cheepjeep85]

Put a small 4X4 comp cam, the 4.0 head with efi and never look back.

x2 do this