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1986 Comanche 2.8 To 4.2 Scrambler Motor Swap Question

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What motor mounts would i need to put a inline 6 into my 2.8 powerd comanche?

I'm planning on using the engine trans and transfercase out of the scrambler.

The engine is the 4.2 and it has a TF 999 trans and the dana 300 transfercase.


Thanks for the help. :cheers:


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They shortened the ft belt drive system when they designed the 4.0L

Look at a 4.0L Cherokee/Comanche

Even making the lower pulley part of the harmonic balancer, the ultra short serpentine setup barely squeals by.

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The drivetrain only had 40k miles when it was replaced with a 454 and it is free. That is why i want to put it in.

Fab work aint a problem i know i will need to make a new trans mount and custom driveshafts.

My truck is bone stock besides enough fender cutting to fit 32's.

These are real rare arround here and if you do find one its so beat and rusted out that it aint worth scrap.

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40k mile 30 year old engine?

I'd at the least want to change some seals in that engine anyway.


Rear main seal at the very least.


The ft pulley drive system HAS to be swapped for an XJ set up.

Serpentine belt, XJ ps pump, reverse rotation waterpump, XJ fan, idler pulleys, etc etc.

Your Alternator should fit the Renix mount,

so long as the wires reach.


Hopefully none of the brackets interfere with the fuel pump mounting (4.0L's don't have mechanical fuel pumps)


The best harmonic balancer you could use for it would be from YJ with a serpentine belt drive.

YJ's came both ways.

A spacer will move the serpentine pulley where it needs to be on that long nose crankshaft.


No way will that CJ ft drive setup fit.

There just isn't room firewall to radiator for that long engine + the old school drive set up.


If you use 87-90 4.0L Renix style belt drive parts,

they will all work with your carburated engine.


^that is if the intake/carb & air cleaner clears the hood.


It probably won't.


They completely repackaged that engine to fit XJ/MJs.


If it doesn't,

a low profile air filter might squeak you by,

if not, start cutting the hood for clearance,

or swap to one of the two 4.0L fuel injecton setups that do fit

(I would go HO, head and all in this case)


Speaking of hoods,

You'll need a 4.0L hood,

radiator support, radiator, hood release setup, etc,

and have to chop some sheet metal behind the hood to fit it all.


85 4cyl/V6 & 87 4.0L are completely different.

The old hood latch & radiator had to be moved to fit the 4.0L too.


Some say the firewall is different 86-87 for the change to 4.0L

but others have said it didn't really change much.


The swap has been done,

It's just not an easy one.


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The thing is that its almost impossible to find a 3.4 here in alaska.

The reason that i got this drivetrain for free was so i could put my truck back on the road.

If i aint gunna put the motor in then i can't have it a good friend of mine is giving it to me and helping me put it in.

He also has the original axles from the scrambler and he just told me i could have them to.

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I'm 6-7 hours south of wasilla.

I understand that it is a big project. I got 8 months to do this swap.

I REALLY appreciate the input and all that.

My income is low but i have ability and time to do this thats why i am going to use this drivetrain.


Thank's :cheers:


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My last suggestion would be to have a 4.0L XJ parts car close by when you're ready for this swap.

There's dozens of small parts needed to get even a 4.0L into an 84-86 XJ/MJ

Nevermind a CJ 258.


In this instance,

a parts car would pay for itself in fuel costs back and forth to the junk yard alone.




I just saw you mentioned the trans mount.

904/999's were used off and on through the years on XJ/MJ's,

so the mount itself should be an off the shelf item.


84-86 4cyl auto's used them if you need an example for the parts store guy.

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I am in the process of a 4.0l swap in a 86 mj and so far it has been big job. I had to cut the lower firewall out of the mj and section in a firewall out of a xj. I have a donor xj for my swap and I would be lost without it. I also ran into problems with the transmission cross member as it dose t have the same mounting points on the mj. I am also going to have to shorten the rear driveshaft and move the transfer case linkage as the inline 6 engine and trans assembly are way longer than the factory turbo diesel 4 cyl. Good luck with your swap!

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Listen to these guys, They know what they're talking about. Sometimes you get your mind set on a project and you lose sight of reality. I recently received a 1999 7.3 diesel and I had big plans for dropping it into a '88 MJ, until I had them side by side and got a good look at it. Reality set in. No problem. Got a '77 J20 with a tired 360. Now that's a little more realistic. :thwak: :wrench:

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  • 2 weeks later...

I will probably just get an ax-15 and put that behind it.

I have no problem re working the trans tunnel and firewall.

I have acsess to a 4.0 xj that was sunk in the ocean so the engine and trans are trash but i can get all the mounts the hood and the rad. support.

I have no problem cutting a hole in the hood for the carb and airfilter.

This truck will be a DD/weekend wheeler.

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