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.

The first step would be to check the common gasket leaks, like the valve cover, engine side cover or oil pan. If you locate a leak in these areas, wipe down the area and run the engine to see a fresh oil path. After determining the offending gasket, try tightening fasteners to see if this eliminates the problem. If not, a new gasket would be next. On the valve cover, clean the head with a solvent so you can adhere to a clean surface. I use silicone, but any gasket sealer will work. Put gasket sealer on the bottom of the valve cover gasket and also on the head. Install the gasket, moving it slightly back and forth while pressing down to get a good seal. Let it set up until completely dry. Use a thin coating of grease on the top of the gasket to seal but allow easy removal for valve work in the future.

The oil pan gasket is a little more difficult. To assist in installation, make a couple of guide pins out of two 1/4-20 bolts about 1-1/2″ long. Cut off the heads of the bolts and use a grinder to round the top. Once the mounting surface is clean, check the new gaskets for fit. Most engines have a different bolt pattern on each side, so make sure these are correct before gluing. All mounting holes must line up or you may have an incorrect gasket set. Use gasket sealer on the top surface of the side gaskets and press into place. Use the guide pins in a offset pattern to make sure the bolt holes line up.

Early engine oil pan gaskets come with straight pieces of cork for the front and rear of the oil pan. To help with installation, coil up these gaskets in a plastic aerosol paint cap and let set overnight. This will give a natural curve to the gasket and help keep it in place on the front & rear main caps for easier installation. These gaskets should be installed after the side gaskets as they sit on top of the side gaskets. Put a small dab of sealer where the end gasket mates to the side gasket. Once the end gaskets are in place, you are ready to install the oil pan.

Just like the valve cover, put a light coat of grease on the lower gasket surface and the mating surfaces of the oil pan. This will help if you need to drop the oil pan later for major repairs. The guide pins will be a great help to align the pan for a smooth installation.

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 crankshaft seal. The 1935-36 timing cover seal was just a cork ring. The rear main bearing rope 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 early 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 early engines, 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. Use Berryman’s B-12 Chemtool aerosol carburetor cleaner for this purpose. It is very effective in cutting sludge quickly. When you have it clean check the ball to make sure it will rock back and forth without sticking.

  1. The most common leak 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″, with clearances of .0015″ to .002″ being 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.

Related Parts