The 4.0L swap into an earlier Jeep CJ or YJ Wrangler 2.5L four or 4.2L inline six chassis has become popular. So is the stroker build-up that uses the 4.0L inline six engine block.
Jeep 4.0L and stroker inline six buildups have become popular. 4.0L conversions into the earlier four-cylinder and 232-258-4.2L six-cylinder Jeep CJ and YJ chassis are also common. Whether you retrofit a stock 4.0L or build a stroker inline six, you will be using a 4.0L cylinder block and head.
Oil filtration and the oil filter bypass system are concerns when installing a 1987-up inline six-cylinder block into a Jeep CJ or pre-’87 full-size J-truck chassis. This also applies to the stroker inline six retrofits, which use a 1987-2006* 4.0L Jeep inline six “long block” core.
Mopar™ Reman long block is a 1991-up 4.0L design (specifically a mid-’90s application). This engine is being retrofitted into a 1987 YJ Wrangler originally equipped with the 2.5L inline four. Note downward facing oil filter like the 1987-90 4.2L inline-six engine. This profile would be identical for a stroker 4.5L, 4.6L or 4.7L buildup.
Filter and By-Pass Designs
The 4.2L and 4.0L inline Jeep sixes built from 1987-up use a different oiling scheme than pre-1987 232 and 258/4.2L sixes. Oil filter types differ between engines.
AMC/Jeep used two distinctly different oil filtration and bypass approaches. View these engines as 1) those with in-block bypass valves like the 232 and 258/4.2L inline sixes built through 1986 and 2) engines that do not have a bypass valve in the block built from 1987-up. The latter group includes the 1987-90 4.2L blocks and all 4.0L engine blocks (1987-2006).
The bypass valve plays a critical part in full-pressure oiling systems. In a full-flow system, the oil moves from the oil pump directly through the oil filter, then to the lubricated engine parts. This provides optimal protection for the engine.
If the oil filter becomes clogged or flow resistance at the oil filter is too great, the bypass valve opens and allows oil to flow directly into the oiling passageways—without being filtered first!
This is the bypass mechanism on a 1987 2.5L Jeep four cylinder engine…Pre-1987 4.2L inline sixes use a similar bypass valve. This is an in-block bypass method. Over the years, several connector sizes were used.
The oil filters on 232 and 258/4.2L inline sixes built through 1986 do not contain a built-in bypass valve. These filters angle downward, similar to this 1987 2.5L Jeep YJ Wrangler four-cylinder application.
If the oil filter clogs on a pre-1987 AMC-Jeep inline six, the in-block, spring loaded bypass valve will redirect oil flow. The oil “bypasses” into a passageway in the engine block.
1987-up 4.2L and all 4.0L inline AMC/Jeep sixes have the bypass mechanism built into the oil filter. This is a common, modern method of filtering; the correct filter contains the bypass provision.
This downward angled 1987-90 4.2L oil filter has a built-in bypass valve. The filter attaches directly to a 4.2L engine block. Jeep CJ-era 232 or 258/4.2L inline six filters mount the same way. This filter, however, has metric 20mm X 1.5mm threads and a connector unique to 1987-90 4.2L inline six engines. The CJ-era filters are U.S. thread.
The 1987-up block design does not have an in-block bypass system. Oil pump flow goes directly into the oil filter, flows through the (unclogged) filter, then enters the engine’s lubrication passages as filtered oil. When a clogged oil filter “bypasses”, the unfiltered oil flows from the oil pump directly into the engine’s oil galleries.
This article offers several Mopar PDF parts illustrations with schematics for 4.2L and 4.0L engine bypass methods and filtration designs. Click on the links for various engine applications, open the PDF and “zoom in” for details. Here is a summary:
This is a 1987-90 style YJ Wrangler oil filter installed on a 4.0L engine block. In 1987, the 4.2L inline six abandons the in-block bypass valve and opts for an oil filter with built-in bypass valve. The metric thread 05012968AA filter shown (same fit as the Mopar #33004195) has a built-in bypass valve.
Mopar 05281090 oil filter is common on 1991-2006 Jeep 4.0L inline sixes. This quality filter handles a stroker motor’s needs, too. The filter has built-in bypass valving.
When working with a stock 4.0L or a stroker motor, always replace the O-rings on the adapter bolt. At mileage, Jeep 4.0L inline sixes will develop leaks at these O-rings.
For CJ Jeep owners who acquire a 4.0L block with a stand-off oil filter mount, this filter position may not work with the pre-1987 Jeep CJ motor mounts. Even as an OEM approach on later Jeep models, this mounting position is not convenient for oil changes. A spillage mess can occur during routine service!
Use the Right Filter and Filter Adapter
CJ retrofit installations should consider mounting the filter to the engine block, preferably with a U.S. thread oil filter. The 1987-up 4.2L and 4.0L oil filters provide built-in bypass valving and can mount directly to the block with the correct connector. (You will remove the adapter locating pin if you eliminate the adapter and fit the filter to the block.)
This mounting method looks much like a CJ 258/4.2L or 1987-90 YJ 4.2L approach. The filter points downward; however, you must use an oil filter with internal bypass valving.
1987-90 4.0L and 4.2L engines each use the same oil filter (the Mopar 33004195 or equivalent). These blocks do not have an in-block bypass valve*. This filter fits on 20mm X 1.5mm threads. The filter has a built-in bypass valve.
If your Jeep is a 1991-up model, the oil filter goes back to U.S. thread type. 1991-up 4.0L engines use the Mopar 05281090 oil filter or equivalent. This filter has a built-in bypass valve. Thread size is 3/4-inch, 16 threads per inch.
Many 4.0L conversion engines and stroker motor cores come from the 1991-up XJ Cherokee or a ZJ/WJ Grand Cherokee 4.0L six cylinder model. These engines use the stand-off filter adapter and the common U.S. thread Mopar 05281090 oil filter mounted either upright (upside-down) or horizontally on the adapter stand.
The 4.0L inline six engine block does not have a provision for an in-block bypass valve system. Here, the needed height for the oil filter connector is determined with a depth gauge. This block is the same as a “stroker six” core and requires an oil filter with built-in bypass valve. The installation is a retrofit into a 1987 Jeep YJ Wrangler.
On this 4.0L (or stroker six) engine block, the oil filter connector did not stand out far enough. A pre-1987 bypass valve retainer has been modified to act as a spacer. The older (pre-1991) connector will accept the 1987-90 #33004195 metric oil filter with built-in bypass valve.
This Mopar Remanufactured 4.0L inline six comes with a spring roll pin for positioning an oil filter adapter stand. (If the engine uses an oil filter adapter stand, a pin must be installed.) At right is a spacer fabricated from an earlier Jeep engine’s bypass retainer that will index on this roll pin. Making a spacer is not necessary if you find the correct connector for the filter.
The older style metric connector fits through the retainer. Modified retainer acts as a spacer and uses the 4.0L adapter locating pin to keep the assembly from rotating. Red Loctite 271 on block-to-connector threads is added insurance. Apply Loctite to connector threads only; keep Loctite away from the oil passages!
Relationship of the locating pin, fabricated spacer/retainer and a 1987-90 oil filter connector is shown here. Oil filter has built-in bypass valve, required for a 4.0L or stroker engine block. There is no bypass valve provision in the 4.0L or “stroker” block.
If your Jeep model’s engine bay and motor mounts require use of a stand-off filter adapter, apply the 1991-up mounting method if possible. This provides the more common U.S.-thread filter and connector option. Shown is a ’99 XJ Cherokee 4.0L that takes the 05281090 Mopar oil filter. The filter mounts horizontally on the adapter stand.