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building a 4.2l

building a 4.2l
1

1986cj7

Senior Jeeper
Posts
940
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Location
southeast missouri
Vehicle(s)
1986 cj7 258, t-176 4-speed, d-300, moser one piece axle in back in amc 20,G2 4.10 gears front and back, detroit truetac front and back,G2 4340 chromoly front axles in a d-30, 4" rough country lift, 33" tires
I'm in the process of gathering parts for the rebuild of my AMC 258 i6 / 4.2l . One question is:
How much is to much on overbore is .060 to much is .030 more realalistic?

What cams have others used? I'm looking at k68-204-4 comp cam kit from summit cam is a 260h grind. 212 .050 lift. All and any opinions are welcome thanks.:)

Jason
 
I would go with 20 or 30 over, 60 is a little to much IMO although a lot of guys do it. I am doing my rebuild this summer and I will go 30 over but it comes down to personal preference and block condition.
 
I would go with the least amount you have to take off. what are your plans for this motor?
 
coldair said:
I would go with the least amount you have to take off. what are your plans for this motor?
Click to expand...

Why go anymore than you need to? If she cleans up at .030 why go more. Then if you need to rebuild again, you're not looking for a block. :D

If its cubic inches you're after, drop in a 401.
 
I agree if this the first time this engine has been rebuilt you souldn't need to go any more then a 30 over bore.
 
this time, less is more.:cool:
 
73CJ said:
Why go anymore than you need to? If she cleans up at .030 why go more. Then if you need to rebuild again, you're not looking for a block. :D

If its cubic inches you're after, drop in a 401.
Click to expand...

Did I say that wrong? The least amount that needs to be machined off the better. That sound better?
 
Thanks for the opinions I though .030 would be plenty, just trying to get the most out of the motor with out going overboard. 73cj your right about that, a 401:drool: is what I dream about.:D Coldair the motor is going back into my cj.:)
 
After calling all the salvage yards in my area the only head is a 97-98 Cherokee for $100. Don't know the cast #. I don't think my intake will fit on this new of a head, will it? If not I guess it's on to ebay.
 
Well I found a 96 head casting #0630 at a local salvage yard for $100 so I'm off to pick it up.:) Coldair I've read your build of your motor and see you used the same head. We're you able to use your AMC 258 i6 / 4.2l intake, and if so what alterations to It did you have to do. I will be using a carb for now. Thanks in advance.:)
 
1986cj7 said:
Well I found a 96 head casting #0630 at a local salvage yard for $100 so I'm off to pick it up.:) Coldair I've read your build of your motor and see you used the same head. We're you able to use your AMC 258 i6 / 4.2l intake, and if so what alterations to It did you have to do. I will be using a carb for now. Thanks in advance.:)
Click to expand...
The intake i had to grind for clearance for the 4.0 exhaust manifold. You will also have to drill and tap where your temp sensor goes mine didnt have it or you could use a port on your intake manifold.
 
coldair said:
The intake i had to grind for clearance for the 4.0 exhaust manifold. You will also have to drill and tap where your temp sensor goes mine didnt have it or you could use a port on your intake manifold.
Click to expand...
I've read and reread your motor build thread and one thing I'd like to ask you is did you have pre-detonation with 10:1 compression? I've run my build specs and my compression will be 9.4:1 thats if they only have to take off .005 on the block to square it up. My dynamic compression looks to be 8.1:1. I'm going to run a carb and have read that carbs are a little more forgiving on higher compression ratios. I like the idea of little higher compression but don't want to run into detonation problems. I've read that dynamic compression should be at 7-7.7 for 87-89 octane and 7.7-8.4 for 89-91. Any insight would be helpful. I've been reading on the net till my eyes crossed:bang: and not sure what is true. Sorry for the long post. Thanks in advance.:)
 
1986cj7 said:
I've read and reread your motor build thread and one thing I'd like to ask you is did you have pre-detonation with 10:1 compression? I've run my build specs and my compression will be 9.4:1 thats if they only have to take off .005 on the block to square it up. My dynamic compression looks to be 8.1:1. I'm going to run a carb and have read that carbs are a little more forgiving on higher compression ratios. I like the idea of little higher compression but don't want to run into detonation problems. I've read that dynamic compression should be at 7-7.7 for 87-89 octane and 7.7-8.4 for 89-91. Any insight would be helpful. I've been reading on the net till my eyes crossed:bang: and not sure what is true. Sorry for the long post. Thanks in advance.:)
Click to expand...
Im not sure yet with the Pre-detonation i dont have the exhaust installed just open manifold. With 87 octane it runs good I havnt run it that much though. I did clean up all the sharp edges in the combustion chamber to make sure it wouldnt PING. I will have to figure my dynamic compression again i cant find all my notes. I know in my build i said they took .010 off the deck of the block that is wrong they took .020 off the deck.
 
Well I was going to get a comp cam kit but have read that the comp cam springs will not fit into a 4.0l head :confused: is this true, its listed as being able to fit a 4.0l head. Do anyone know if this is true? If so looks like I will be buying the cam separate and getting the rest separate as well instead of buying the kit. Thanks for the help as always.:)
 
Yes the springs are wrong I had to buy mopar racing springs. They are spendy because u have to buy springs, retainers, valve locks all that cost me $300ish
 
coldair said:
Yes the springs are wrong I had to buy mopar racing springs. They are spendy because u have to buy springs, retainers, valve locks all that cost me $300ish
Click to expand...
D***:censored: that's about the price of the comp cam kit. Also what valves did you use? I got the other part # from your build thread.
 
I will have to see if I can run down what brand and the part numbers for the valves. They are high flow valves. On top of that I had a 3 angle valve job done also.
 
Consider a full 4.0l swap. Pick up a good running xj. ,swap and part out the xj. My swap emded up to be free. Just sayin.

Sent from my Galaxy S3 with Taptalk 2
 
deadfeat said:
Consider a full 4.0l swap. Pick up a good running xj. ,swap and part out the xj. My swap emded up to be free. Just sayin.

Sent from my Galaxy S3 with Taptalk 2
Click to expand...
That is a consideration. But after all the reading and research I've done, I've read so much back and forth about building a stroker, or a AMC 258 i6 / 4.2l -4.0l head swap it makes my head hurt. I think I will be better served to build my AMC 258 i6 / 4.2l with a 4.0l head. All the benefits of the longer stroke AMC 258 i6 / 4.2l and the benefits of the 4.0 top end.:) Also I can upgrade to fi in future. I'm having to do my build in baby steps.
 
For what you are using the motor for a little built up AMC 258 i6 / 4.2l will the ticket. Plus you will know what's done to the motor and not wondering how many miles a used 4.0 actually has.
 
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