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Cylinder Head FAQ

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John
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Cylinder Head FAQ

Post by John » June 3rd, 2008, 4:05 pm

Jeep heads are identified by a casting number on top of the head above the #4 exhaust port, on the manifold side.

The cylinder heads

The stock I6 cylinder head is cast iron and weighs about 60lb. The valve head diameter is 1.91" intake/1.50" exhaust. Casting numbers are as follows:

Year............Casting No
1987-90......2686
1991-95......7120
1996-98......0630
1999-06......0331

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1bolt
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Re: cylinder head FAQs

Post by 1bolt » September 13th, 2008, 8:56 pm

Renix heads (can't remember casting number) have the lower ports and presumably less flow (don't recall actual flow numbers though)
7120 (91 to 95 I believe) this head somehow got the reputation as the "best" head... I can only guess because it's popular for the 4.2 head swap because some misinformed early swappers claimed the later 0630 head didn't have a water temp sender boss (sometimes is does sometimes is doesn't).
0630, 96-99 (98 on a ZJ I believe) this head is supposed to be identical to the 7120, I haven't flowed my 7120 yet but I believe it will flow the same. This head supposedly doesn't have a water temp sender location, but it does on 4 out of the 5 that I've seen... there's a small bolt in it depending on what model Jeep its in.
0331 2000+ (99+ on Grand Cherokees) this head is prone to cracking in its first few years of casting. It has Ports that are higher on the head and the intakes flow better than the intakes of a 0630 (and I suspect the 7120 as well but I will confirm that at some point).
The exhausts on the 0331 should be better for torque (perhaps worse for high RPM horse power, perhaps no difference), they are smaller, shapped much better, and the floor has been raised a lot while the "roof" was raised slightly making for a higher velocity port, more exhaust "scavenging" and thus better low end torque. How much is anyones guess as no ones done a dyno shootout of Jeep heads (unless HESCO has but they would never make the sheets public).

I feel the need to point out that the last part about the 0331 will be controversial, because it flies in the face of every assumption about 4.0 heads on the net. It's a crack prone head so it really doesn't matter most people aren't going to be brave enough to take a chance for a few extra foot pounds, if it means possibly having to do it all over again.

The exhaust port floor on previous heads was extremely lazy, the area that Jeep filled up on the 0331 (making the port smaller) was this lazy area, and actually makes the velocities higher in the port, while flowing slightly less volume. A good pocket porting and valve job would (In my opinion) make the 0331 flow significantly better than any previous 4.0 exhaust. I believe there's a WHOLE LOT left on the table because this port is smaller and has more meat to work with in terms of shaping.
Last edited by 1bolt on November 5th, 2008, 7:33 am, edited 1 time in total.
Reason: updated make and model year info and Temp sender boss info for the 0630
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Re: 7120 Heads on ebay.

Post by dwg86 » April 10th, 2009, 2:35 pm

The 258 uses a 11/32 valve with a 7 degree lock. the 4.0 uses an 8mm valve with an 8 degree lock. Also the 4.0 valve has a round groove in the valve for the valve lock.

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Re: Cylinder Head FAQ

Post by Muad'Dib » August 27th, 2009, 9:58 am

For every .010 you shave, you lose 1.4cc's from the chambers.
Or for every .007 you shave off the head, you reduce the chamber volume by 1cc.

Taken from the following thread.. thanks to John, 5-90 and the rest for thier contributions.

http://www.jeepstrokers.com/forum/viewt ... f=15&t=997
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Re: Cylinder Head FAQ

Post by SilverXJ » May 21st, 2010, 8:04 am


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