Author Topic: My 258 is wore out, swap ?s MN  (Read 2504 times)

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Offline wampus_cat

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My 258 is wore out, swap ?s MN
« on: September 14, 2011, 10:29:28 AM »
So after replacing the valve cover and changing the oil (the oil my car came with was like black gear grease), I discovered that my ol' 258 has terrible blow-by.  126000 miles?  advertised as freshly rebuilt motor.  I think someone turned back the odometer because it looks more like 226k.  I took the head off, at least a .020 ridge on 3 cylinders. Took the motor out, dirt scored bearings and crankshaft, sloppy timing chain... sigh.... I feel like a sucker. 
So the long and short of it, this motor would need a full rebuild including bored cylinders, ground crankshaft, master kit, etc...  Sounding pretty expensive ($900 ish I think).   I don't see being able to work that into the budget anytime soon..
There is not exactly to many 258s kicking around at the local salvage yards, I found one 1.5 hours away with unknown miles for $300 if I pull it, said it ran when it came in. 
So my question is, do I have any choices for other engines to swap in?  I read up on 4.0 head swaps, but is it feaseable to just put a 4.0 jeep motor in?  do any other motors bolt up to the same bellhousing, and fab a new front axle mount?
Or is anyone around west-central MN with a good running 258 for me?

Offline ammachine390

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Re: My 258 is wore out, swap ?s MN
« Reply #1 on: September 14, 2011, 12:03:06 PM »
A lot of 4.2 engines seem to have the problem with blowby gumming up the air filter. My engine has it too, and also only has 120,000 miles on it. I believe someone on here said it had to do with the way AMC designed their pcv systems, that they were not able to handle the the amount of blowby. I figured that my piston rings were really worn out, however, after doing a compression test, everything seemed normal for a 30 year old engine. So you may want to try a compression test.
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Offline amcinstaller

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Re: My 258 is wore out, swap ?s MN
« Reply #2 on: September 14, 2011, 01:13:34 PM »
there are a couple easily overcame hurdles with puttin in a 4.0 but it possible and i think the eaglepedia has an awesome writeup on it, but i havent looked lately
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Offline DaemonForce

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Re: My 258 is wore out, swap ?s MN
« Reply #3 on: September 14, 2011, 02:21:10 PM »
I killed my blowback problem by using the right PCV valves. Engine is clean but now I get condensation in the oil again. Ah well.

I'm thinking of rebuilding my spare 258 engine for when I finally need it but my compression is spot on so I'm not too worried right now.

$900 for a rebuild? Gaskets are $40, not sure about new rings, I could probably do the rebore myself and I intend to use a new crank. Probably will run me up closer to $400 if I'm lucky. Just do what you have to and it should run great.
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Offline carguy87

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Re: My 258 is wore out, swap ?s MN
« Reply #4 on: September 14, 2011, 09:05:01 PM »
Be careful with boring your own cylinders, take your time and lots of measurements to fit each cylinder to the piston you will be putting in that hole.  As for your blowby, check your compression in all 6 cylinders, if they are all around 110 or better, you should be able to use the ENVALVE to give you some more time before your rebuild.  It works to replace the garbage pcv valve system that these engines came with and flows lots better.

Offline vangremlin

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Re: My 258 is wore out, swap ?s MN
« Reply #5 on: September 14, 2011, 09:30:05 PM »
I bought a 258 out of a Jeep that I saw advertised on craigslist.  The guy was going to be swapping in a new motor, I got to hear this one run before he pulled it and its worked out well.  Bought it for $300.  I thought about a 4.0 upgrade but at the time I didn't want to put in any more money than I had to as I'd aleady rebuilt the tranny and the transfer case.
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Offline wampus_cat

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Re: My 258 is wore out, swap ?s MN
« Reply #6 on: September 17, 2011, 08:28:35 PM »
Thanks for the advice.  The eaglepedia has a good write up on 4.0 conversion, which I was leaning towards, until I happened upon a 258 on craigslist..  ONLY 50 BUCKS!  Unfortunatley, I was not able to hear it run, but even if it runs decent I can't go wrong, as it came with the motor mounts (one of mine is broken), 2 alternators, carb with the kickdown linkage (kickdown linkage was also missing on mine).  What luck


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