FAX: (541) 258-6968

1929-1939 Engine Oil Leaks

By Stephen Kassis

I like to joke with friends that the way I check the oil level in my old Chevy engines is to see if they are leaking. As long as it is leaking, it must have oil in it! Unfortunately, the old Chevy engines leak and no matter how diligent we are about sealing up a new engine, it seems that over time it will start to leak. A small amount of leaking is to be expected if you are driving the car on a regular basis and this is not unusual or worrisome. However, serious oil leaks can be a symptom of a larger problem.

A regular question that comes to me (several times per month) is "My front timing cover (or rear main) is leaking, do you have a seal for it?" Until 1935, Chevy engines did not have a front timing cover seal. The 1935-36 timing cover seal was just a cork ring. The rear main bearing seal on the Chevy Six did not appear until 1940. The answer to the problem is that something is either plugged or worn out. We will discuss the problems and solutions that are available to you.

The front timing cover on 1929-1934 engines has an oil slinger on the crankshaft snout to keep oil from splashing out. Otherwise, the oil should drain back into the pan through a small hole at the bottom of the front main bearing. The rear main bearing cap has a deep groove in it that catches excess oil and returns it, through a hole in the cap, to the oil pan.

If the front timing cover or rear main bearing is seriously leaking on a 1929-1934 engine, the cause is usually one of two things:

  1. The oil return hole in the front or rear main bearing has become clogged and will not allow oil to flow back into the oil pan fast enough. Early engines were run on non-detergent oil. This oil was designed to "stick" impurities to the side walls and valleys of the engine to keep the bad stuff out of the bearings. Over time, this would cause "sludge" to build up in the engine. This black sticky substance would be everywhere that oil traveled in the motor. Sludge can, over time, close up the return hole to the oil pan in the front main bearing and cause oil to flow out of the engine. If you pull the oil pan off and find that the return hole is plugged, this would actually be good news. That is because the alternative problem is worse (see #2 below).

The rear main bearing cap also has a check ball that prevents oil from running out of the engine when the vehicle is on a steep incline. This ball can get stuck if sludge has made it sticky. Pull the rear main bearing cap, remove the bearing shell and clean the cap with a strong solvent. I use Berryman's B-12 Chemtool aerosol carburetor cleaner for this purpose. It is very effective in cutting sludge quickly. When you have it good and clean check the ball to make sure it will rock back and forth without sticking.

  1. The most common problem (and of course the worst) is that the main bearings are worn and need to be adjusted. The main bearing clearances on the early Babbitt engines should be .001" to .003", although clearances of .0015" to .002" are ideal. If your bearings are beyond .003", they must be tightened up or replaced. If you have shims still in the main bearing caps, remove them equally from both sides and re-check. Check all of the mains at the same time. If you cannot get the tolerances below .003" it is probably time to rebuild the engine.


Tech Videos

1947-55 1st Series Chevy
Pickup Door Weatherstrip


1967-72 Pickup
1967-72 Chevy Pickup
Vent Window Restoration


R-blox and Dynamat Installation
DIY Sound
Dampening Installation


Chevrolet Pickup Door Hinge Repair
Chevrolet Pickup
Door Hinge Restoration


Chevrolet Pickup Door Hinge Repair
1929-1932 Chevrolet
Hood Removal