This must be an inherent weakness in the 385 series (429 and 460) engine designs. Maybe 2,000 miles on this 460 engine build and the rear main seal is leaking. Ugh! I replaced the original at 90,000 miles myself from underneath by removing the engine pan (a real pain), removing the rear main bearing cap, drilling a tiny hole in the end of the upper seal half, threading in a small screw, and pulling out the upper seal half. At about age 37 I swore I'd never due this again. Now at age 79, what do I do besides buy oil and let it leak?
And you're sure it's the seal? You do know that ONE of the bellhousing bolts goes straight into the lifter valley and HAS to have sealant on it or it will leak? Absolutely certain it's not the end of the intake (another common place) or a valve cover (not so common)? What brand of seal did you use, and did you offset the ends? Oil or grease on the lip when installing? 460's are not know to be leak prone.
I have heard some brands of rear main seals have too much crush and cause leaks. How to cut rear main seal for proper crush. I actually used Enginetech S3502 viton rear main seal on my son's 466 and no leaks. Like Phil said, make sure you offset the seal about 3/8-5/8" from the rear main cap's parting line and add a dab of silicone to each end prior to installation
I'll add to what Todd said that you also need to put a touch of silicone on the CAP mating surfaces, to prevent oil from seeping out between the cap and block.
I do not think it a valve cover gasket. I did not know about the bellhousing bolt. I will also recheck the intake manifold. I did not do the engine rebuild so do not know any of the rear main seal details.
There's at least 5 other potential culprits: 1) Intake manifold china wall gasket/seal. 2) Oil pressure sending unit. 3) Camshaft tunnel end plug. 4 & 5) Rear oil galley plugs, at about the 10 and 2 o'clock position of the camshaft tunnel end plug. Are you using the correct PCV valve? With the engine running, check to see if there is a slight to "0" vacuum reading within the crankcase/valve cover area.
It doesn't leave any oil on the outside of the bell - it lets it seep out and down the back of the block.
There's some risk in shaving off some of the replacement seal material when you thread in the upper half of the seal. As Phil notes, a very light application of RTV between the seal and edge of main cap is what we always did at the shop I worked. You don't let this cure and need to torque the cap before that happens. Oil leaks = comeback in the auto biz....so we were very careful to seal up our engines. I've had both valve cover and china wall leaks that I was certain were rear main. If the engine is not too dirty, you can use a clean dry paper towel and dab it around the leak path from a valve cover or intake seal to rule those out. Pull that bell housing bolt out and see if you have oil on the threads...it should resemble spark plug threads when a cylinder is using oil.
