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So what year 4.0 can I use to replace my 91 4.0?


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The 91 I have is blown out (see other thread). I want to replace it with another 4.0. I can find lots of 4.0's newer than 1991. Other than the pilot bushing, will a later engine from a manual or automatic bolt into my 91 AX-15? Will the engine from say a 96-98 ZJ work? 97-98 TJ? 97-98 XJ? Does auto or manual matter? I want to keep using my 1991 fuel system and intake/exhaust manifold.

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What exactly is blown about your engine? if you already have all the parts for the top end it wouldn't cost much more to go through the bottom end unless it has major damage

 

Previous owner drove it without water and a blown headgasket for a good while. All 6 cylinders scored and likely bent rods from hydrolocking. Stock engine is getting pulled and none of the longblock will be recycled. Getting a complete replacement and then trying to bolt on my 1991 accessories, intake, and exhaust. Any idea if the engines I mentioned could be used?

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Obviously 1991 would be your best option, but later years will certainly work. Anything up to 98 would be simple.

 

As you may know, the pilot tip size of the AX-15 and bore size in the crank weren't changed until 92. 91 is kinda a crappy year to have.

 

Putting a newer trans(92+) behind an older 4.0(87-91) is easily fixed with the 74 CJ 304 pilot bushing. I am not aware of a bushing that will easily allow you to put an older trans behind a newer motor.

 

I (without much difficulty other than dealing with the wrong bushing) put a 91 4.0 in a 98 XJ that exploded the #6 piston. That 91 motor is purring along in the 98 just fine. The cam was changed around 96 I believe, but it shouldn't change anything. Just make sure you hang onto all your 91 sensors. You will definitely need your 91 CPS, and you may need to put your 91 distributor in the replacement motor.

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Obviously 1991 would be your best option, but later years will certainly work. Anything up to 98 would be simple.

 

As you may know, the pilot tip size of the AX-15 and bore size in the crank weren't changed until 92. 91 is kinda a crappy year to have.

 

Putting a newer trans(92+) behind an older 4.0(87-91) is easily fixed with the 74 CJ 304 pilot bushing. I am not aware of a bushing that will easily allow you to put an older trans behind a newer motor.

 

I (without much difficulty other than dealing with the wrong bushing) put a 91 4.0 in a 98 XJ that exploded the #6 piston. That 91 motor is purring along in the 98 just fine. The cam was changed around 96 I believe, but it shouldn't change anything. Just make sure you hang onto all your 91 sensors. You will definitely need your 91 CPS, and you may need to put your 91 distributor in the replacement motor.

 

Thanks! So I found this on Dino Savva's site in regard to what I can do to put a newer engine to my 1991 AX-15:

 

'89.5-'91 AX15:

 

"OEM pilot bearing 33004041 for '87-'91 4.0 engine or stroker

Advance Adapters pilot bushing 716120 for '92-'06 4.0 engine"

 

Now I don't know anyone that has bought this Advanced Adapters pilot bushing but it seems like that is all I need to make it work.

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You would think.. but no. See, in 92, they didn't just change the size of the pilot tip. They also changed the size of the bore in the crank that the bushing gets pressed into.

 

Pilot bushing dimensions :

 

I.D. O.D.

87-91 0.586" 0.872"

92-06 0.75" 1"

 

The 87-91 motors also had a brass sleeve that the bearing itself was in, so the crank bore on those is actually bigger than the 92-06 motors. Can't find a dimension on the actual bore, though.

 

Basically, if you get a 92+ motor, you'll need to get a bearing or bushing with an OD of 1", and an ID of 0.586".

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Your head ports should line up just fine.

 

CPS isn't an issue. It bolts to the transmission and reads pickups off the flywheel. A newer flywheels should work fine with no issues, at least the 91 fly in my brother's 98 is working great. Just be sure to remove the CPS before pulling the motor and reinstalling it after the motor is in to ensure you don't break it. New ones are pricey... Especially since 91 used its own plug. Plug and sensor were changed in 91 with the HO system, then they changed the plug again in 92 for some reason.

 

The 0331 head casting (99 1/2+) won't have the threaded hole in the back for the gauge's temp sender. This head is also prone to cracking. You should look for a 91-early 99.

 

I'm not sure how long the dizzy from 91 was used, but it was definitely changed in 97. I think every sensor was changed in 97, but you'll want to hang on to all your 91 sensors anyway in case you need to use them.

 

That pilot bushing you put up may work, but it sits further back in the crank than the OEM location. There's essentially two bores in the crank, you could use either one. Personally I'd use the deeper one if possibly, but using the one further back should be fine.

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Your head ports should line up just fine.

 

CPS isn't an issue. It bolts to the transmission and reads pickups off the flywheel. A newer flywheels should work fine with no issues, at least the 91 fly in my brother's 98 is working great. Just be sure to remove the CPS before pulling the motor and reinstalling it after the motor is in to ensure you don't break it. New ones are pricey... Especially since 91 used its own plug. Plug and sensor were changed in 91 with the HO system, then they changed the plug again in 92 for some reason.

 

The 0331 head casting (99 1/2+) won't have the threaded hole in the back for the gauge's temp sender. This head is also prone to cracking. You should look for a 91-early 99.

 

I'm not sure how long the dizzy from 91 was used, but it was definitely changed in 97. I think every sensor was changed in 97, but you'll want to hang on to all your 91 sensors anyway in case you need to use them.

 

That pilot bushing you put up may work, but it sits further back in the crank than the OEM location. There's essentially two bores in the crank, you could use either one. Personally I'd use the deeper one if possibly, but using the one further back should be fine.

 

Thanks! And I assume what you meant in your last sentence was ....but using the one closer to the end of the crank should be fine."

 

So If I use the bigger one I posted up what would be the downside? I have read of some people having trouble with mating the engine to the transmission when using the one I posted up. Would I just be able to take some material off of it with a grinder? I am really unclear on what role this bushing plays.

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Really the only issue with using the bushing that sits further back, is it's awfully close to the splined part of the input shaft. I imagine that's the issue people have run into.

 

The pilot bushing/bearing keeps the input shaft centered when you push in the clutch. With the clutch engaged(pedal up), the motor and input shaft are spinning at the same speed. But when you push in the clutch, they're no longer spinning together, so the input shaft has to have something to ride in.

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Now I'm about to throw a wrench into the gears...if my 91 has an external slave cylinder then does that possibly mean that the transmission isn't a 1991 transmission? Is there a date stamp on the transmission or anything that I can check? I would be ecstatic if it turns out to be a 92+ AX-15. I guess I could also find a 92+ AX-15 with my replacement engine...

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I'll snap a picture and post it up but I am pretty sure I saw a long black plastic slave-cylinder hanging off the transmission. Would this fact change anything with my pilot bushing predicament? It could mean either the person put on later model bellhousing or later model transmission?

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You said in another post that other then hard starting and steam in the exhaust the engine ran and revved fine,when you bought the truck you talked about it starting in under a crank and you didn't say anything about it running bad,smoking,knocking ,ect.... Now all of a sudden you think the engine is blown,since you already got all the parts why don't you replace the headgasket and see what happens

 

If you worried about a bent rod,which is unlikely just roll the engine over and make sure all the pistons have the same deck height.

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If you have an external slave, then yes, either someone put the external stuff onto the original transmission, or it's not the original trans. There's really no way of knowing for sure what pilot tip size you have without yanking the trans or motor.

 

4.0s are pretty tough. Your motor may just need a little TLC as mnkyboy suggested. Since you think it's dead anyway, it can't hurt to take it apart in the truck and poke around.

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You said in another post that other then hard starting and steam in the exhaust the engine ran and revved fine,when you bought the truck you talked about it starting in under a crank and you didn't say anything about it running bad,smoking,knocking ,ect.... Now all of a sudden you think the engine is blown,since you already got all the parts why don't you replace the headgasket and see what happens

 

If you worried about a bent rod,which is unlikely just roll the engine over and make sure all the pistons have the same deck height.

 

It ran and revved but the previous owner only ran and revved it for a couple seconds because it had no water... When I put water in it to prepare to drive it home the actual problem showed its face. Thats when the hard starting and steam got crazy. The number 2 piston is destroyed and has around .125" of gap with the wall (piston can rock in the bore). Engine is toast - end of story.

 

Now I am curious to find out what year my transmission is...perhaps its a later year...are they date coded?

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If you have an external slave, then yes, either someone put the external stuff onto the original transmission, or it's not the original trans. There's really no way of knowing for sure what pilot tip size you have without yanking the trans or motor.

 

4.0s are pretty tough. Your motor may just need a little TLC as mnkyboy suggested. Since you think it's dead anyway, it can't hurt to take it apart in the truck and poke around.

 

Yeah, the head is off and the number 2 piston is nasty. Number 2 is so scored it would take a decent overbore to fix. The rest are pretty scored. They are tough as nails so long as you don't drive it without water for an extended period. To his defense the previous owner didn't seem like an avid jeeper or even seem like he knew much about automobiles.

 

The thought about the external slave just hit me so now I want to figure out what year my transmission is...frankly i'd love to know what year the engine is...the head didn't have 12 point bolt heads which means someones either been in there before...

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Not to hijack, but my 91 also has an external slave AX15, build date 11/90. Don't know if previous owner changed it I sort of doubt it, seems to be alot of work for a guy that just did regular upkeep.

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I was totally wrong. I looked late at night :oops: and mistook the electronic speed output sensor as the slave cylinder. I in fact have an internal slave. My head says "91" on it so I assume the block and crank match along with the transmission. I will probably be getting a 97/98 engine and either buying or making a pilot bushing on a lathe.

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