351w Rear Main Seal

[Shep69 149]

Can anyone  help me out with some  info on how to replace the rear main seal on my 351w. I know its a 2 piece seal and I have heard that loosening  the crank main bolts can help but i have never attempted this so any tips or advice welcome.

 

[1969_Mach1 333]

What you heard is correct.  Make certain its the rear main and not the oil pan.  Aftermarket oil pans are known for not fitting very well in that area.

When installing the new rear main seal halves, offset the ends a little from the parting line of the cap and block, 3/8" or so.  Put a small dab of your favorite sealer on the ends of the rear main seal halves.  And sometimes oil will seep between where the cap meets the block.  Wipe a very very thin film of sealer in those locations as well.  I also put some sealer between the rear main seal half and the cap.  Since it's in the car you cannot get sealer between the rear main seal half and the block.

[Shep69 149]

33 minutes ago, 1969_Mach1 said:

What you heard is correct.  Make certain its the rear main and not the oil pan.  Aftermarket oil pans are known for not fitting very well in that area.

When installing the new rear main seal halves, offset the ends a little from the parting line of the cap and block, 3/8" or so.  Put a small dab of your favorite sealer on the ends of the rear main seal halves.  And sometimes oil will seep between where the cap meets the block.  Wipe a very very thin film of sealer in those locations as well.  I also put some sealer between the rear main seal half and the cap.  Since it's in the car you cannot get sealer between the rear main seal half and the block.

I have already fitted a new one piece gasket to the oil pan so I’m thinking it’s the rear main. Cheers for the info Mach 1.

[RPM 1,190]

I also have tried the one piece gasket w/o luck. The first one"fit" so poorly at the rear main I didn't even try to install it. The second fit better and didn't leak for a while.

I met and talked with a fellow Mustanger who had my exact issues. I told him I thought my leak was from the rear main. He too originally blamed the rear main,  but said he put a dab of rtv at the front and rear corners where the pan goes over the mains.  This trick solved his oil leak. 

I haven't touched my car in 2 weeks,  but plan on trying the rtv as the next fix. 

[1969_Mach1 333]

You might certainly have a rear main seal that is leaking.  But, I also use a one piece oil pan gasket, Fel-Pro Permadry Plus gasket.  There are a few things to keep in mind with one piece oil pan gaskets.  First, small block Ford oil pans had two different radius' in the corners where the side rails meet the rear main cap.  Older pans, like ours have a sharper radius in those locations.  Second, there are one piece pan gaskets for pans with flat rails (late model) and with ribbed rails (older models like ours).  Third, when you have the correct one piece oil pan gasket, you still need a bead of silicone sealer around the rear main cap, the timing cover, and the corners wherer the pan rails meet the ends, like RPM mentioned.  I learned that the hard way.  Called Fel Pro, and they said it's okay to use silicone sealer with the Permadry Plus one piece oil pan gasket. 

[Shep69 149]

3 hours ago, 1969_Mach1 said:

You might certainly have a rear main seal that is leaking.  But, I also use a one piece oil pan gasket, Fel-Pro Permadry Plus gasket.  There are a few things to keep in mind with one piece oil pan gaskets.  First, small block Ford oil pans had two different radius' in the corners where the side rails meet the rear main cap.  Older pans, like ours have a sharper radius in those locations.  Second, there are one piece pan gaskets for pans with flat rails (late model) and with ribbed rails (older models like ours).  Third, when you have the correct one piece oil pan gasket, you still need a bead of silicone sealer around the rear main cap, the timing cover, and the corners wherer the pan rails meet the ends, like RPM mentioned.  I learned that the hard way.  Called Fel Pro, and they said it's okay to use silicone sealer with the Permadry Plus one piece oil pan gasket. 

I did use sealer with the one piece oil pan gasket.  I’m not sure what you mean by ribbed rails . The gasket fitted well when I installed it about 2 years ago but it is possible I have the wrong one. 

[1969_Mach1 333]

If you look at later model 302 and 351W oil pans, 80's, 90's, that have flat side rails.  Older oil pans like these classic cars use have ribs between each bolt hole on the side rails.  Sometimes somebody will try to install a late model one piece gasket for flat side rails on an older oil pan with ribbed side rails. It will fit, but might leak.

If you installed it 2 years ago and now a leak has started.  Look close and try to determine if its the oil pan gasket or rear main seal.  It's not too easy on Fords because of the plate between the engine and trans.  Silicone sealer is kind of a strange thing.  It's for sealing oil leaks, yet, oil slowly deteriorates it.