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Installing Rear Main Seal on 4.0L Jeep Inline Six


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Sooner or later, every AMC/Jeep inline six cylinder engine will require a rear main seal replacement. Forum member JayDLogan and I had this Q&A exchange just prior to the start of the forums. This should benefit others and will continue here at the forum:

Jason: I am installing a new rear main seal in my 1999 jeep wrangler 4.0L. I am a bit confused on where to put the sealant. I have attached a few photos. I am using mopar anaerobic sealant, is this correct? The sealant should not be on the bearing cap mating surfaces, right?  My understanding is to use sealant along the full length of the chamfered edges, on both ends of the bearing cap. In addition, apply sealant to the upper and lower tabs on the lower rear main seal. The factory manual is very vague. Some sources say not to put sealant on the upper and lower tabs of the lower seal or any sealant in seal seat in the bearing cap. I am concerned that if I put sealant on the seal tabs, that it will prevent a proper seal. Can you clarify what is required and what NOT to seal with a detailed diagram or picture if possible? Also, is it wise to replace the oil pump and pick up tube while I am doing the rear main seal? The jeep wrangler is a 1999 with approximately 120km on it!



Photo at left is definitely wrong use of sealant. Second photo with anaerobic sealant looks slightly better but still needs sealant at chamfer edges. Do not apply sealant at the seal's lips! Cap would need installation with this sealant still pliant...See the comments and details below...

Moses: Jason, see the attached PDF scan below. This is the official Mopar procedure. My rules-of-thumb are to 1) avoid creating a gap between the cap and block from using too much sealant, and 2) use sealant wherever oil can “wick out” between the cap and block. The illustration is very helpful. A sealant bead at each cap chamfer to the block is required. You do not want an oil leak or seepage between the cap and block.

4.0L Rear Main Seal Installation.pdf

As long as the residual film of sealant will flatten and not interfere with cap torque and mate-up of the cap to block, you’re fine. Note that Loctite 518 is the recommended sealant…The seal and tabs are of oil-resistant material that seals well on a clean surface. These materials even swell slightly in the presence of oil. There’s no need to use excessive sealant.

Follow the Mopar recommendation for these modern sealing materials…Presumably you’re using the one-piece oil pan gasket, which works very well and is way easier to install than the old 4-piece sets!

AMC/Jeep oil pumps put out high pressure and volume. Unless you have experienced low or erratic oil pressure, your pump is likely fine until you completely rebuild the engine. Most often, the damage to oil pumps is from debris, not “wear”, as the hard steel impellers run in a constant oil flow, which prevents rapid pump wear. If the engine has sludge and the pickup screen appears clogged and suspect, I would replace both the pump and oil screen assembly. I use a Melling high-volume replacement pump.

Jason: So are you saying to only put sealant on the chamfer edges of the bearing cap (metal surface)? Should I put sealant below and above the seal tabs? Is it required to put sealant where the seals contact each other?

Moses: At the chamfer is a must and also a thin film with anaerobic sealant across the flat face between the cap and block would be acceptable. My personal rule is to avoid excess sealant, which will slough and migrate into the engine’s oil pump pickup screen. Avoid stacking sealant or allowing it to harden before installing the cap. There must be complete crush and an “interference fit” between the cap and block.

I do use RTV sealant at junctions between seals and gaskets. Imagine any mating point or contact surface that could provide a seepage or wicking point. Oil pan gasket-to-seal contact points are always an issue; however, later AMC/Jeep engines use a one-piece pan gasket to avoid this issue.

Don’t “gob” sealant where it will only squeeze out, slough and float inside the engine. Thin, even beads work well on seals and gaskets that crush during assembly. The illustration shows the key sealant area for wicking; however, use your judgment to eliminate seepage elsewhere.

Again, the main seals are designed to seat against metal castings. They crush into the grooves during assembly. A thin film of sealant was traditionally added to the outer seal edges where they seat into the main cap and block. Today’s materials do not require this approach, although a thin film of anaerobic 518 sealant on the outer edges of the two-piece main seal would be insurance—and will not cause an issue if the cap gets installed before sealant sets up.  Trust this helps...Moses

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