blown 256

[jeepindavis]

I have an -84 CJ7 with a blown 256. I ran her low on oil - I didn't keep up with the leaks and blew a crank bearing. I pulled the pan and put in an oversized bearing and it ran for a few weeks. Now I have a 4.0 with a decent lower end and am considering pulling the crank from it. Anyone ever tried this or know anything about it? Are the cranks interchangable?

Thanks for any help and/or ideas.

 

[ThisGuyUKnow]

I have an -84 CJ7 with a blown 256. I ran her low on oil - I didn't keep up with the leaks and blew a crank bearing. I pulled the pan and put in an oversized bearing and it ran for a few weeks. Now I have a 4.0 with a decent lower end and am considering pulling the crank from it. Anyone ever tried this or know anything about it? Are the cranks interchangable?

Thanks for any help and/or ideas.

Are you saying you want to put the 4.0 crank into the AMC 258 i6 / 4.2l ? Normally people use the AMC 258 i6 / 4.2l in the 4.0 to make a stroker. I know you use the crank and rods from the AMC 258 i6 / 4.2l and I think you have to do some work to the head. there is alot of info on making a 4.0 stroker out here on the interweb.

 

[jeepindavis]

Ben,
No, my situation is I have actually a pair of 4.0's - one is locked up so the bottom end is no good. It had a bearing knock before it locked. I also have another 4.0 but the top end is really rusty (rods etc.). It came from a junk yard and was purchased to replace the locked up engine but the manifold was cracked and when we pulled the manifolds we saw all the rods were rusty and didn't think it was worth the labor to put in. I thought I could put the two together and make one (I have a Cherokee too - whose engine is still good but a spare is never bad to have). So I have a CJ with this worn crank and thought if the bottom end of the one 4.0 is still good, maybe I can just pull it and put it into the 256 and make a runner. I don't have anything other than the engine from the donor XJ so making an injected CJ isn't a great option yet (no wire harness, computer etc). Maybe down the line...
So my main question is will the crank in a 4.0 drop into a 256 and what other swaps will I have to do to make it work? Do I need the pistons out of the 4.0 etc...

 

[Cheepjeep85]

not without other modifications, its not going to work right with the AMC 258 i6 / 4.2l rods, and 4.0 crank they are a different stroke, so your looking into tearing the whole motor down any way. i would look into either fixing the 4.0 and putting it in as a whole, buying a crank kit Crankshaft Kits Amc/Jeep From RPM Machine or rebuilding the AMC 258 i6 / 4.2l . the problem with putting just a crank kit in a motor though is all of the metal shavings that go along with spinning a bearing or just plain destroying one, once all of that metal goes through the motor, the cam bearings, other mains, and rod bearings wind up in bad shape as well. if the bearing spun, then you really should have the mains line bored, to make sure the bottom end is straight and true.

you should tear into the 4.0 and see exactly what you have there, it may not be as bad as you think. your AMC 258 i6 / 4.2l intake will bolt to the 4.0 head with a 4.0 header or factory exhaust manifold, and you will still be able to keep it carbureted.

 

[IOPort51]

No way your getting out of this fast and cheep.

 

[Cheepjeep85]

Sorry to say but IO is right.

DROOOID2

 

[ThisGuyUKnow]

Ben,
No, my situation is I have actually a pair of 4.0's - one is locked up so the bottom end is no good. It had a bearing knock before it locked. I also have another 4.0 but the top end is really rusty (rods etc.). It came from a junk yard and was purchased to replace the locked up engine but the manifold was cracked and when we pulled the manifolds we saw all the rods were rusty and didn't think it was worth the labor to put in. I thought I could put the two together and make one (I have a Cherokee too - whose engine is still good but a spare is never bad to have). So I have a CJ with this worn crank and thought if the bottom end of the one 4.0 is still good, maybe I can just pull it and put it into the 256 and make a runner. I don't have anything other than the engine from the donor XJ so making an injected CJ isn't a great option yet (no wire harness, computer etc). Maybe down the line...
So my main question is will the crank in a 4.0 drop into a 256 and what other swaps will I have to do to make it work? Do I need the pistons out of the 4.0 etc...

It took me a minute to figure out who The Ben you were talking to was. That is the name of my CJ7 not my name lol

if you are looking at a rebuild I would highly suggest looking into the strocker forums online. From what I have gathered if you are looking at a complete rebuild anyway then stroking it will nit cost too teribly much more. If I were staring that down I would go stroker but that's my personal preference

 

[fzsk4p]

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