Built 258?? to much?

Built 258?? to much?


Full Time Jeeper
St.Cloud FL
83 CJ-7.. 4" YJ springs front rear, J-10 44 front/AMC 20 rear,6 cy auto dana 300 35' bfg Done and gone!!

Ask me about my head light upgrade harness!!!
This is for my YJ (cj is on hold) I know this is a CJ forum but the AMC 258 i6 / 4.2l is the same.

So I picked up a motor last night thinking it was a 4.0 to build a stroker for my YJ with and come to find out it is a AMC 258 i6 / 4.2l . It was my fault for not properly reading the post correctly but thats anotehr story. Heres my question, the PO gave me the build sheet from the motor and WOW..

AMC 258 i6 / 4.2l block bored 60 over, KB 2227 pistons, comp 68-235-4 xtreme cam, balanced crank, every thing was turned and polished and resized. Clear water 4.0 7120 conversion head, block machined for ARP head bolts and studs .017 mopar head gasket. plus a bunch more. When I inputted all that info into a compression calculator its showing a little over 10:1 and roughly 4.66 cui. The only thing that seems wrong with the motor is a bad freeze plug and the PO says when he was running it, it was burning 1 quart of oil every 1000 miles. He thinks either a ring did not seat right or the shop who did the head work put the AMC 258 i6 / 4.2l valve seals on the 4.0 head causing them to leak around the valves. I got a ridiculously great deal on the motor so Im pretty stoked.

Am I close on the compression ratio? I know theres quite a few things that alter it but most places are showing the same numbers. Most 4.6 strokers thow out about 250hp, and reason this one should be any different? Im hoping to come up with either a offy or clifford intake for it.
I would suspect your compression ratio might even be higher than the 10:1 you came up with....probably a 10.5 or closer to 11 believe it or not.

I dont think the AMC 258 i6 / 4.2l valve seals will even come close to fitting properly on the 4.0...a quick inspection will tell you.
It would suck if it's a bad set of rings, but from the sounds of that build, it would be worth it to take care of the issue. Re-hone the cylinder wall and start over with a fresh set.

Your hp should be pretty close to what you are guessing, probably higher. I was right at 300hp on my build with high compression pistons, 262 duration cam and 11:1 compression.

.060 is a lot of material taken off the block....
We need more info on the pistons what CC is the dish, what CC the heads come out to. what head gasket they used ( I mean did they use a 4.0 head gasket or did they use a AMC 258 i6 / 4.2l head gasket ) When i fugured the comp ratio mine came out to 9:1
As for the burning oil Im thinking it might be due to the rings. Eather they didnt seat right, or didnt install them correctly, As for Clear water knows what they are doing dought they installed AMC 258 i6 / 4.2l valve seals on a 4.0 head
My AMC 258 i6 / 4.2l I built ran it through Comp Cams cam selector with what they said it should be running around 225 hp with 289 Torgue.
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DANG you JIM LOL beat me to it.
the gasket is .017 mopar. I think he had the shop do some extra valve work to the head. He said he remembers the shop guy saying he bought the gasket set for the AMC 258 i6 / 4.2l and that the seals would work. He put about 7000 miles on the motor after rebuild driving it to a few places to wheel and said it didnt smoke and ran great.
The AMC 258 i6 / 4.2l head gasket will make it have a higher comp ratio. How much did you end up paying for said motor?
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One thing that may happen with the high compression ratio you may have to run high octane fuel 91+ would be the only thing I would be worried about.:)
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