Rear Main Leak

Proverbial Rear Main Leak

  1. After 30 plus years of experience with Model A & B engines, the most common problem is a rear main oil leak. This problem can be very simple to correct or it can turn into a costly time-consuming job. What is important is a correct diagnosis. You do not want to rebuild an engine if you have a bad oil filler cap. Do not assume the worst until you have checked out the problem thoroughly.
  2. After wiping clean the flywheel housing, check to see if oil is leaking from out of the cotter pin hole at the bottom of the flywheel housing. If oil does not leak out of the cotter pin hole, you have a leak some place else. Some suggestions could be the oil pan, timing case cover, front seal, valve cover, or oil pipe to mention a few. Most oil leaks drip off of the cotter pin. You need to determine that oil is leaking out of the cotter pin hole not off of the cotter pin to be a rear main leak. If oil is leaking out of the cotter pin hole go to step 2.
  3. Remove the oil filler cap and check to see that the oil filler pipe is not restricted or plugged. With the oil filler cap removed, take the car for a drive. If the leak stopped, the oil filler cap is bad. There are tabs bent over under the top of the cap to stop the cap from seating all the way down. The stops could be bent or missing on your cap. This will seal crankcase pressures in and will force oil out. Replace the oil filler cap. This is a common problem, which can be overlooked and is easy to fix.
  4. Hold a plastic food storage bag around the oil filler pipe and bring the engine to high idle. The bag should flutter. If does not and fills up with air, you have excessive blow-by in the crankcase. This can be caused by bad rings, pistons, cylinders or possibly valves and guides. Excessive pressures in the crankcase will cause oil to leak out of the rear main and any other places not sealed.
  5. If above is all good, now remove half-moon inspection cover on the flywheel housing, three hex head screws, and chock the wheels. Check to see if inside of flywheel is wet with oil. If flywheel is wet inside this means oil is leaking out of the rear main cap and/or the cap to the block. Start engine, then with flashlight look for oil leak. EXHAUST IS HOT AND MUST BE VENTED.
  • Check inside flywheel housing with half moon cover removed.
  • With engine running:
  1. Dripping or running down right side of engine around outside of rear main cap: Probably the gasket between engine and flywheel housing. Remove engine and replace gasket. We seal around camshaft hole with RTV, no longer use a gasket.
  2. Dripping between rear main cap and block at shims: Cap must fit flat to engine block. Rear seal part A6335 must be filed flat with block. I use a thin film of RTV on both sides of shim to prevent leakage.
  3. Dripping outside of rear main block could be oil running up rear main bolts. I use RTV under heads of the rear main bolts and on threads of the two upper bolts that secure flywheel housing to block. These leaks are generally slow drips and are not common.
  4. Dripping or running out of back of rear main cap into flywheel is common. This is a rear main leak. If you have not seen oil leak at this time, take the car for a drive, immediately chock the wheels and check again. If rear main leaks, remove oil pan.
  5. Check oil tube on rear main cap that it is not missing, broken or cut off. The end of this pipe must be in oil in bottom of sump. If pipe is not in oil, pressures in crankcase will blow oil out rear main. If pipe is screwed into cap too far this will restrict oil flow back to sump and cause rear main leak. The oil galley plug in rear main should be installed again to stop crankcase pressures from pushing oil out rear main. Next remove rear main cap. Oil galley in cap must be open and clean so oil can flow back to sump. With cap removed from engine you can now inspect Babbitt for wear, cracks, chips, etc. The last quarter inch of the bearing must be full diameter of Babbitt, no cuts, grooves or chips. Shims must fit tight to crankshaft. Plastigage can be used to check bearing clearance at this time with engine still in car. I suggest using a jack to apply pressure upward on crankshaft. This will seat the crankshaft up into bearings in block to give you an accurate measurement. Desired clearance is .001 to .0015.

By now you should have diagnosed the problem. I hope that your problem was easy to fix. I have seen newly rebuilt engines leak and many times it was just the oil filler cap. Do not assume the worst until you have checked out the problem thoroughly.