1987-2006 Jeep 4.0L Inline Six - Inside The Inline

Head gasket thickness seems like one easy way to adjust CR, but compressed gasket thickness can be difficult to determine exactly. In the high-performance gasket realm, the compressed thickness is listed and the manufacturers stand by those numbers. The standard gaskets are less controllable. The range runs from about 0.040 to 0.055 inch on the commonly available 4.0L gaskets. A rule of thumb for the 4.0L stroker is that with a 4-inch gasket bore, an 0.010-inch change in head gasket thickness equals a change of about 0.180:1 in CR.

9. We used the 707 casting 258 connecting rods (5.875-inch length), really the only ones commonly available for the 258. We found that you can buy reconditioned rods via various outlets. We also replaced the stock bolts with aircraft-grade bolts from ARP.

10. From '96, 4.0L engines used a main bearing girdle for more overall strength. This is something that can be added to any earlier 4.0L by replacing the main bearing cap bolts and getting the girdle...

...The longer throws on the 258 crank, however, will let the rods interfere with the girdle. The solution is simple-add a couple of hardened washers to space the girdle down a quarter inch or so.

You can get a rough idea of what your compressed dimension will be by measuring the thickness of the gasket body (not the flame ring around the bore or sealing strips imprinted on the body) and deduct about 0.005 inch. This applies to the gaskets advertised as "permanent torque" or "no-retorque." Cheaper gaskets that require retorquing can compress two to three times that much and are less predictable. Bottom line, you cannot say with certainty that X gasket has Y thickness unless it's a performance gasket that has a listed specification.

Quench is the "Great Equalizer" when it comes to a high compression ratio. The quench area is the area between the highest flat spot on the piston and the lowest part of the roof of the combustion chamber. Less quench distance induces more turbulence in the combustion, which promotes better fuel mixing and fewer hot spots-all of which reduce pinging. Many experts think that 0.040 to 0.045 inch of quench is ideal, but without making other changes, decking the block for zero deck height (removing about 0.045 inch) will increase the static compression ratio to about 9.7:1 with the standard combustion chamber, an 0.052-inch head gasket and largest available 4.0L piston dish volume (18.83 cc). If you put in a 218-degree cam (intake at 0.050 inch, 36 ABDC), the dynamic ratio is about 8.56:1, and most experts consulted agree that this engine would run on midgrade, maybe even regular, with timing and EFI adjusted to optimal power settings.

Sample Compression Ratios From Various Parts Combos

Calculated, not observed. The volume dimensions are averages, not absolutes. The only way to know for sure is to test your parts.

*Note: This piston dish is commonly listed as 17.5cc. The number shown is our measured amount, including ring land volume (about 1.3cc)
** The Golen Engine. Piston dish estimated from advertised CR.
SP= Sealed Power, KB =Keith Black Silvolite, OE= Original Equipment
Bold text indicates the setup used for the UNO engine.