It’s a fact of life — all small-block Chevys leak oil. If you want to stop the leaks on the older two-piece rear main seal engines, it would probably be best to dip the whole engine in resin. It must be written in stone that small-blocks leak, so our approach with this story is to dive into that problem with ideas, gaskets, and seals that will attempt to convert the torrent of oil to something more like a trickle. We’ll focus mainly on oil leaks, which for small-block street engines mean the rear main seal, oil pan, valve covers, as well as touching on a related area like using a PCV valve system.
Rear Main Seals
We’ll start from the bottom of the engine and work our way upward with first attention paid to the rear main seal. The traditional small-block from 1955 through 1986 used a two-piece rear main seal. Then, in 1986, Chevy converted to roller cams which meant a change to a one-piece rear main seal. This at least mitigated some of the common oil leak issues. For the purpose of this article, we’ll concentrate on the two-piece rear main seal.
The most important point to make when talking about a two-piece rear main seal is to avoid installing it backwards. The seal is designed to use internal crankcase pressure to push outward on the lip of the seal to minimize leaks. This means the lip must always face in, toward the inside of the engine. It’s very easy to accidentally invert the seal. This is especially critical when working with the rear main cap because when you’re working with it on the bench, it’s upside down. So always double-check to make sure the lip is toward the crankcase.
Offsetting the ends of the seal by positioning the parting lines of the seal away from the parting line of the main cap to the block is another way to minimize leaks. Combine this technique with a small amount of anaerobic sealer on the flat portion of the rear main cap to prevent oil from finding a leak path around the rear main seal.
Oil Pan Gasket
With the rear main seal in place, we can move to the oil pan gasket as the next great leak path. The original small-block oil pan gasket was a four-piece affair with rubber end gaskets combined with a pair of cork rail gaskets. If the engine builder is careful and attends to the details, these gaskets are more than capable of sealing a small-block. We prefer to use studs to keep the gasket lined up, with particular attention paid to the corners where the rail gaskets meet the end seals. This is where the gasket will generally leak if not installed with a shot of RTV in each corner.
In the mid-1980s, when Chevrolet upgraded to a one-piece rear main seal, it also brought about the one-piece oil pan gasket. If installed properly, the one-piece gasket will help with those nagging pan gasket leaks. People have complained about seepage from the middle of the pan rail. The solution appears to be to use studs instead of bolts and include a small dab of RTV around each oil pan bolt hole to prevent oil from migrating through the oversize holes in the gasket.
Several companies are making one-piece pan gaskets for the earlier small-block Chevy, and this does offer a few significant advantages, especially if the engine is taken apart regularly. To help with pan rigidity, Chevrolet Performance offers a pair of steel reinforcing oil pan rail supports that can be added to a typical two-piece rear main seal small-block.
Thick versus Thin
When it comes to pan gaskets, the two-piece small-block rear main seal engines are not all the same. Up to around 1974, Chevrolet used a thin front seal between the timing cover and the oil pan. From that date and later, Chevy added a much thicker front seal, which predicated a change to the oil pan. This causes much confusion when enthusiasts are swapping parts. Most aftermarket oil pans are designed to use the thicker front pan seal.
We’ve listed the dimensions of the seals plus the oil pan to illustrate the difference. The important thing to remember is that a thin seal must be used with an oil pan designed for a thin seal, and the same is true for thick seals and pans. A thick seal will not seal properly if used on a thin seal pan and a thin seal will obviously not come close if used on a thick seal pan. The front timing covers remain the same.
Front Seal Thickness
Seal Spec Years
Thin 1/4-inch 1955 – 1974
Thick 3/8-inch 1975 – Later
Most aftermarket oil pans are designed to use the thick seal
Valve Cover Gaskets
Small-block valve cover gaskets also seem to get more than their share of attention and perhaps, rightly so. The classic four-bolt flange has always suffered from a large span between the clamp points, which has led to the implementation of load spreaders and, of course, cast valve covers with much greater rigidity. As for gaskets, the big change was when manufacturers began placing steel strips in between the cork gasket surfaces to reinforce them.
An example of this is Fel-Pro’s P/N:1604 gasket. These gaskets are also thicker than the normal cork gasket, offering the ability to accommodate slight imperfections in the sealing surfaces of the cylinder head or valve cover. This steel reinforcement also helps prevent the gasket from pushing out when clamped down.
There are also several different compositions of gasket materials, including silicon gaskets from both Fel-Pro and Victor-Reinz that are exceptionally nice. The gaskets also include torque-limiting reinforcements to prevent over-tightening, which can damage the gasket.
Moving forward, there’s a small hole in the front of a small-block Chevy located just above the mechanical fuel pump area. In many cases, this hole is drilled and tapped to intersect with the fuel pump pushrod. When changing mechanical fuel pumps, you can use a long bolt threaded into this hole to retain the fuel pump pushrod in place. Once the new pump has been installed, don’t forget to remove the long bolt and replace it with a shorter bolt with a small amount of thread sealer to prevent leaks. We’ve seen freshly-rebuilt engines where the builder overlooked this bolt which caused a substantial leak.
Intake Manifold Gaskets
For the intake manifold, most intake gasket kits include a pair of rubber or cork end seals to span the gap between the manifold and the china walls on the block. These are called china walls because their shape resembles the Great Wall of China. Most modern small-block builders ignore these seals and instead use a bead of RTV that does a great sealing job. We like to place a small dab of RTV underneath the intake manifold tab at each corner, that ties in with the bead of RTV at the china wall.
Remaining with the intake manifold, it’s not uncommon to see oil seeping out of the center four intake bolts and staining the intake manifold. All it takes is a small amount of thread sealer on these bolts since they are open at the bottom into the lifter valley. The only time this will occur, however, is if there is excessive crankcase pressure. One clue your engine may have a problem with crankcase pressure is if your dipstick continually is pushed out of its tube. This indicates excessive crankcase pressure sufficient to move the dipstick.
Keeping Pressure In Check
This leads us to PCV valves. Testing performed by Matt Wagner from M/E Wagner Performance has revealed that a malfunctioning PCV valve can often be a major contributor to engine leaks. His testing shows that shaking and rattling a PCV valve is not a conclusive way to evaluate whether a PCV valve is operative or not and that often these valves pull nearly nothing from the engine.
When this occurs, pressure builds in the crankcase and eventually pushes oil out from the least restrictive seal in the system, like a rear main, pan gasket, valve cover, or dipstick tube. This is simple physics with pressure being relieved through the least restrictive pathway.
As an example, we recently assembled a big-block Chevy and mistakenly installed the bottom half rear main seal backwards. On the dyno, this produced a massive oil leak, but deadlines pushed us to complete the test. Westech’s Steve Brule installed a vacuum pump on the engine, and after sealing, the engine quickly stopped leaking.
The outside atmospheric pressure kept the majority of the oil in the engine against the vacuum created by the pump. The pump pulled roughly 10 inHg of vacuum during the test. After the test, we repaired the seal and the engine is drip-free so far, but this reveals how much even a small amount of crankcase pressure can contribute to leaks.
M/E Wagner builds a fully adjustable, two-stage PCV valve that allows the user to adjust the flow through the PCV valve separately at idle and at highway cruise vacuum levels. We won’t go into all the details on this valve, but it does work and they have multiple examples of where users have minimized or eliminated their engine leaks altogether by using this valve.
This billet aluminum valve offers enough flow that it is fully capable of pulling liquid oil right out of a stock baffled valve cover. While this will help to reduce oil leaks, it can contribute to pulling an excessive amount of oil through the intake system. M/E Wagner recommends some type of oil separator as the best way to minimize oil usage. Our friend Eric Rosendahl found an oil separator on Amazon that both looks cool and does the trick of removing liquid from the vapor stream before it enters the engine.
For street engines running aftermarket breathers, excessive pressure can build up in the engine if the breathers are restrictive. We’ve seen aluminum aftermarket breathers that look cool, but are probably better at minimizing venting than anything else.
The ultimate way to eliminate leaks in a street small-block is to run a vacuum pump. If reducing pressure with a PCV valve system will help, then creating an actual vacuum inside the crankcase will all but eliminate the most annoying drips. The downside of course is the cost and complexity, since the system generally includes not only a pump and brackets but also lines, fittings, and a pressure relief valve. However, that’s not a quick and simple fix, so not really within the purview of this article.
Many enthusiasts believe that a small-block Chevrolet will always leak oil no matter what you do. That may be the case, but our approach will always be, that for every leak there is a drip-less solution.