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Oil Filtration for Jeep 4.2L, 4.0L and Inline Six Stroker Motors

4.0L swaps into earlier CJ and YJ Wrangler 2.5L four 4.2L inline six chassis are popular.

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.

     *Note: When selecting a 4.0L long block core or complete engine, I recommend use of a 1991-up engine for its improved cylinder head and other upgrades. The 4.0L inline six was available in the Jeep XJ Cherokee, Jeep YJ Wrangler, Jeep TJ Wrangler and the Jeep ZJ and WJ Grand Cherokee models.

1991-up 4.0L block or stroker core with earlier YJ Wrangler style oil filter.

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).

     Note: The engine block passageways on 4.0L engines do not have a provision for a bypass check valve. These engines all require an oil filter with the built-in bypass valve. The 4.0L engines use an oil filter connector that must match the filter thread type and filter mounting style.

     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!

  Caution: The intent of the bypass valve is to protect an engine with a clogged filter or excess resistance in the filtration system. However, bypass oil is not filtered and can harm internally lubricated parts. This is one more reason to change the oil and oil filter on a regular basis.

Bypass mechanism on 1987 2.5L four...Pre-1987 4,2L inline sixes use a similar bypass check valve.

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.

2.5L oil filter faces downward like 4.2L engines.

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.

1987-90 4.2L YJ Wrangler inline six oil filter

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.

    Important caution: The "emergency" oil bypass system protects the full-flow lubrication system from oil starvation. Whether in-the-block or in-the-filter, the bypass is a last gasp response for the poorly maintained engine. Bypassing oil, uncleaned of impurities, will contaminate bearings and cause premature parts failure. This is the reason for periodic fresh oil and oil filter changes! 

     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:

1) Jeep CJ 258-4.2L used through 1986 has an in-block oil by-pass system with a spring, valve and retainer. The OEM Mopar J3242397 filter mounts directly to the block. This filter does not contain a bypass valve. It has a U.S. thread with a 13/16"-16 pitch connector. The filter unit offers no bypass protection and relies upon the bypass valve on the engine block.

     Note: If you install a stock 4.0L or a stroker 4.0L block into a CJ chassis, the easiest approach is a 4.0L oil filter with an in-filter bypass valve. Look closely at the 4.0L block—there’s no place for the earlier 4.2L bypass spring, valve and retainer to fit in this block! Use the correct oil filter and filter connector.

     Caution: Similar in many ways to the 4.2L block, the 4.0L uses an in-the-filter bypass valve, which relies on an oil filter with anti-drainback and bypass valving built into the filter. (The 4.0L block passageways will not accommodate an in-the-block bypass setup.) There are different oil filter part numbers for Jeep CJs versus YJ Wrangler applications. Oil filter connector part numbers are also different; connectors vary in length and thread size/pitch (including U.S. versus metric threads).

     Warning: Some 4.0L oil filter adapter stands have a filter connector with a built-in check valve. This helps prevent oil drainback from the upside-down oil filter mounting position. If the adapter stand uses this type connector, keep it to prevent oil drainback and dry filter start-ups! Use the oil filter listed for this engine application.

     Note: As required by your Jeep model's chassis and motor mount layout, use either a direct-to-block oil filter mounting method or the horizontally mounted filter adapter. Avoid the straight upside-down oil filter adapter unless upside-down is the only fitment option.

2) YJ Wrangler 4.2L built from 1987-90, the filter still mounts to the block, but note that the by-pass is now an integral part of the oil filter. The filter connector and oil filter are different than the CJ 4.2L type. The YJ Wrangler 4.2L does not use the in-block bypass spring, valve and retainer.

1987-90 YJ Wrangler 4.2L oil filter mounts to block

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. 

3) 1991-93 4.0L YJ Wrangler—When looking at the part numbers in the schematic, note that “MJXJ” is the Cherokee and Comanche pickup. “YJ” is the Wrangler. The YJ Wrangler gets the 4.0L in 1991. Bypass valving is in the oil filter, not in the block.

     Note: In the 1991-93 parts catalog's picture, the generic illustration can be confusing. It shows a 4.0L XJ/MJ and ZJ style aluminum adapter stand. In other references, the YJ Wrangler 4.0L does not use the stand-off adapter until 1994. (See parts #18 through #20A, distinguishing YJ from XJ/MJ and ZJ.) When working with a 1991-93 YJ Wrangler 4.0L engine or core, confirm the filter mounting method. Use the correct filter connector and oil filter.

4) The XJ Cherokee and MJ Comanche pickup truck introduced the 4.0L inline six in 1987. These engines use a stand-off filter adapter. (Oil filters on the Jeep MJ, XJ, ZJ or WJ models do not mount against the block.) 1987-90 MJ and XJ 4.0L oil filters have 20mm X 1.5mm metric threads. 1991-2006 4.0L inline sixes use a U.S. thread oil filter.

5) 1994-up Jeep YJ Wrangler oil filtration joins other 4.0L applications with a stand-off filter adapter; on stand-off adapters, note whether the filter mounts horizontally or upright (upside-down). The adapter stand indexes on a locating pin in the engine block; the position is clear.

Mopar 05281090 oil filter

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.

     Note: From 1997-2006, the Wrangler is a “TJ”. “ZJ” and “WJ” are the Grand Cherokee models.

6) On any of the stand-off oil filter adapters, install new O-ring seals on the mounting bolt to the block. Make sure you tighten the mounting bolt securely*. Get the Mopar seal kit for your 4.0L engine or core application and adapter design. Verify kit and O-ring part numbers for your 4.0L block application.

Typical Mopar O-ring kit for 4.0L adapter bolt

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.

     *Note: Listed "factory" torque figures for the adapter bolt are all over the board. Some call for 50 ft-lbs, others indicate 75 ft-lbs. These bolts are always very tight coming off. Secure tightly enough to hold the stand and a filter in place while providing a leak-proof seal at the O-ring. At mileage, this is a common leak point for the 4.0L engine, easily remedied with an O-ring seal kit.

  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.

     Note: The common 4.0L Wrangler oil filter connector is Chrysler/Mopar part number 53007563. The filter is the popular 1991-up 4.0L Mopar 05281090. This filter is U.S. 3/4"-16 thread.

     Caution: Pre-1987 AMC/Jeep inline sixes have the in-block bypass valve. They also use a 13/16"-16 U.S. thread oil filter connector fitting. The oil filter or connector for these earlier 258/4.2L sixes should not be installed on a 1987-up Jeep inline six block—there is no bypass valve in the CJ-era filter!

     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. 

     *Caution: The 1987-90 4.0L Jeep inline sixes use an upright (upside down) oil filter layout. The metric connector accepts the 33004195 oil filter with built-in bypass valve. This is a briefly used oil filter adapter that requires the 1987-90 metric oil filter connector with 20mm X 1.5mm metric oil filter threads.

     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. 

     Caution: If you install a 4.0L or a stroker derivative inline six into a pre-1987 Jeep, make sure the oil filter has a built-in bypass valve with the correctly threaded connector!

4.0L engine block also used for stroker inline six build-ups.

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.

     Reminder for CJ and 1987-90 YJ Wrangler owners: Oil filter connector threads must stand out far enough for the filter to mount safely...The YJ Wrangler 4.2L six uses the Mopar 33003538 oil filter connector and metric oil filter with built-in bypass. If you use this approach, make sure the connector seats properly at the block and provides enough filter-end thread exposure to safely mount the #33004195 metric oil filter. If you prefer using the U.S.-thread 1991-up YJ Wrangler oil filter connector (Mopar #53007563), this will mount the filter directly to the block like the YJ Wrangler 4.2L application. The 53007563 connector has U.S. threads and takes the 05281090 Mopar oil filter.

Roll pin plus custom made spacer for oil filter connector

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. 

4.0L engine with locating dowel for the oil filter adapter stand

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.

     Caution: Any time you install a connector fitting, make sure it will seat squarely and securely at the engine block or adapter stand. Also be certain that there is enough thread exposure for the oil filter to attach securely. Torque parts properly; you don't want the oil filter to come loose!

Modified retainer, roll pin and connector tightened to specification

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!

     Note: Use of a spacer is only necessary if you cannot find the right connector for the oil filter. See the factory parts schematic PDFs for details. Avoid parts fabricating unless you must for your application.

1987-90 style oil filter custom-fitted to 4.0L engine block

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.

     Note: Use of the 1991-up 4.0L oil filter connector (Mopar #53007563) should eliminate the need for custom fabricating a spacer as shown in the hybrid approach (above). Do not use a spacer unless required.

Mopar 05281090 oil filter mounted horizontally 

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.

     When seeking a 4.0L inline six engine core, the 1991-up engines are preferred. They offer an improved cylinder head design and other upgrades. If possible, get the OEM 4.0L oil filter stand and hardware with your 4.0L engine core. If this does not fit your Jeep Wrangler or earlier CJ chassis properly, consider the alternatives discussed within this article...

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