Rear Main Seal Replacement On Mopar Magnum V8

After 130,000 miles I developed an oil leak from the rear main seal. Enough to use oil and leave drops on the driveway. Decided to tackle the project myself to save a bunch of money. I also decided to replace the oil pump since I was under there as it’s only $22 at the local parts house. At that price, its not worth checking the specs on the old one and trying to rebuild it.

Well to start, if you need the room, jack the truck up and support it on jack stands and all that safety stuff. I suggest laying a large piece of cardboard under the engine to save your driveway/garage floor. Drain the oil and then replace the oil pan drain bolt. REMOVE THE OIL DIPSTICK!!

First step is to remove the transmission to motor support struts.

You may have to drop the exhaust y-pipe to make enough room to get the oil pan out. To do so, remove the 2 bolts on each clamp. If these have not been taken apart recently, you will probably need to use some penetrating lube and let it soak for awhile.

Next step, at least if you have an automatic transmission, is to remove the starter and the inpection cover on the front of the transmission housing. The starter uses a bolt on the top and a stud/nut on the bottom. The inspection cover has 3 bolts (in addition to the strut bolts and starter holding it in place).

Remove the oil pan bolts. There are a lot of them and they are fine thread. This is where an air ratchet comes in mighty handy. The inspection cover is inside a lip on the pan (I found out the hard way after unbolting the pan, that is why the cover is still attached with the pan down). Using a rubber mallet or screwdriver between the block and pan, drop the oil pan. The rear will want to fall completely and the front will hang up on the cross-member. According to my Haynes manual, I should have used an engine hoist to hold the motor and removed the engine mount through bolts and lifted the engine up 3-4″. This would have made removing and reinstalling the pan much easier. I did not have access to one, so it’s a bit harder getting the pan in and out.

Work the pan out from under the truck and be careful not to damage the oil pump pick-up tube. This is where the cardboard comes in handy. Not only is there a bit of oil still in the pan, but every part of the block starts to drip oil everywhere.

It’s now time to remove the oil pump. It is bolted to the main bearing cap so it needs to come off anyway. You might as well replace it since it is off. Carefully remove the 2 mounting bolts on the oil pump flange. Do NOT remove the oil pump cover (hex shape). Check to see if there a paper gasket stuck anywhere. My truck did not have a gasket. Also, there is a large column of oil above the pump, being held there by the pump. Once you break loose the first bolt, another 1-3 cups of oil starts to come out, be ready with a bucket.

Clean the oil pan with solvent and inspect the mounting flange for warping/damage. Mine had a bit of rust on the inner surface the was more or less removed with the solvent and wiping. Clean the gasket surface also. Also, clean the inspection plate and straighten any kinks and warping.

If you are replacing the oil pump, remove the pick-up tube from the old oil pump, unless you will be installing a new one of those as well. It is simply threaded in.

Now it is time to remove the rear main bearing cap. There are 2 bolts holding it in place. It is a large cap, not like the usual style. You may need to pry it with a screwdriver. It is fairly heavy also. Use caution when removing it since the oil pump driveshaft runs through it. You want to inspect that driveshaft for wear and replace if needed. Clean the cap and remove the lower half of the main seal. Do NOT damage the main bearing! Install a half of the new seal into the slot. Lightly oil the new seal prior to installing it. The side with the paint stripe goes towards the rear of the truck.

You might as well check the teeth on the flexplate and anything else you can see in there.

New oil pump. Thoroughly clean the pick-up tube and thread it onto the new pump, if installing a new pump. Be sure it is properly threaded to avoid sucking in air at that location. Prime the pump by turning the pump and pouring oil in it until it comes out the other end. Do this whether or not you are replacing the pump.

To replace the upper main seal, you will probably have to use a screwdriver and small hammer to get the seal (ends marked with red arrows) to slide around the crank. It is a rigid piece, so it will follow the crank shape as it is removed. Only use the screwdriver to start the seal moving, then switch over to a needle nose pliers to pull it the rest of the way. You do not want to scratch the crank. Do not damage the oil pump driveshaft (also marked with red arrow). Lightly oil the new seal half and with the paint stripe side facing the rear of the truck, carefully insert it into the seal hole. Do not cut or shave the seal on the engine block as an oil leak may result. Use your finger to apply pressure on the seal, pushing against the crank to avoid this. Push it all the way around until it is flush with the block on both ends.

You may need to loosen the bearing caps immediately in front of the rear one to help relieve the pressure from the crank on the seal. I had to loosen the two caps closest to the rear main. If you do this, you need to re-torque ALL the caps when you are done.

Installation is the reverse of removal. Reinstall the main cap, after applying a drop of Loc-Tite on either side of the cap surface, between the bearings and seal. Also apply some RTV to the notches where the oil pan gasket fits. Wipe the crank surface and lightly oil the parts afterwards (you want good lube/oil in there not any of the dirty stuff). If you loosened any of the other caps, start at the center and torque them all, working outwards, in three steps to spec (85 lb-ft for me). Install the primed oil pump, making sure the driveshaft is correctly aligned. Torque to spec (30 lb-ft for me). Apply RTV to the corners where the main cap contacts the block and where the front of the pan hits the timing cover. Using gasket adhesive, install a new gasket on the oil pan, if you didn’t raise the engine. If you raised the engine, install the gasket to the block. Install the oil pan. If you didn’t raise the motor, it can be difficult. This part took the longest of the reassembly for me.

Reinstall the inspection cover, the starter, the transmission to motor struts and the exhaust y-pipes, if removed or bolts loosened. Reinstall the oil dipstick, fill with oil and check for immediate leaks. Start motor and be sure oil pressure comes up and that there are no leaks.

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