PROVERBIAL REAR MAIN LEAK
After 30 plus years of experience with the 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
costly time consuming job. What is important is a correct
diagnoses. You do not want to rebuild engine if you have a
bad oil filler cap. Do not assume the worst until you have
checked out the problem thoroughly.
1. After wiping clean flywheel housing.
Check that oil is leaking from out of cotter pin hole at bottom
of flywheel housing. If oil does not leak out of cotter
pin hole you have a leak some place else, suggestions oil pan,
timing case cover, front seal, valve cover, 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 cotter pin hole go to step 2.
2. Remove oil filler cap check to see that
oil filler pipe is not restricted or plugged. With oil
filler cap removed from car take car for a drive. If leak
stopped the oil filler cap is bad. There are tabs bent over under
top of cap to stop 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 oil
filler cap. This is a common problem, overlooked and easy
3. Hold a plastic food storage bag around
oil filler pipe and bring engine to high idle. Bag should
flutter if does not and fills up with air, you have excessive
blow by in crankcase. This can be caused by bad rings,
pistons, cylinders or possibly valves and guides. Excessive
pressures in crankcase will cause oil to leak out rear main and
any other place not sealed.
4. If above is all good now remove half moon
inspection cover on flywheel housing, three hex head
screws. Chock wheels. Check to see if inside of
flywheel is wet with oil. If flywheel is wet inside
this means oil is leaking out rear main cap and or cap to block.
Start engine, with flash light look for oil leak. EXHAUST
IS HOT AND MUST BE VENTED.
POSSIBLE PLACES FOR OIL LEAK.
Checking inside flywheel housing with half moon
A. Dripping or running down right side of
engine around outside of rear main cap. Probably gasket
between engine and flywheel housing. Remove engine and
replace gasket. We seal around camshaft hole with
RTV no longer use a gasket.
B. 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.
C. Dripping out side 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.
D. 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 car
for a drive, immediately chock wheels and check again. If
rear main leaks. Remove oil pan.
E. 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, ECT. 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
a jack be used to apply pressure upward on crankshaft.
This will seat crankshaft up into bearings in block to give you
an accurate measurement. Desired clearance is .001 to
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 till you have checked
out the problem thoroughly.
ANTIQUE ENGINE REBUILDING
SKOKIE, IL 60077