Big Block Oil Leak Insights

[Gary C.]

The old 427 in the 67 sure leaks a lot more oil than I would like. Makes a mess. Any insights to possible sources would be appreciated.

I am sure the rear main seal leaks. What has me most puzzled is a significant amount of oil accumulates on the right rear of the block. The upper right rear flange that receives the transmission (the area where the casting number would be if it was on the right) is saturated with oil. The rear of the head is clean and dry. I have a little blow by evident around the valve cover oil filler cap and the grommet in the right valve cover, but cannot find any real leaks around the valve cover gasket. Around the distributor looks pretty clean too.

Any other areas to check would be appreciated.

Thanks

Gary

[Joe L.]

Re: Big Block Oil Leak Insights

The old 427 in the 67 sure leaks a lot more oil than I would like. Makes a mess. Any insights to possible sources would be appreciated.

I am sure the rear main seal leaks. What has me most puzzled is a significant amount of oil accumulates on the right rear of the block. The upper right rear flange that receives the transmission (the area where the casting number would be if it was on the right) is saturated with oil. The rear of the head is clean and dry. I have a little blow by evident around the valve cover oil filler cap and the grommet in the right valve cover, but cannot find any real leaks around the valve cover gasket. Around the distributor looks pretty clean too.

Any other areas to check would be appreciated.

Thanks

Gary

Gary------


The most common source of leaks on a big block (or, a small block, for that matter) is the valve cover gaskets. I have found over the years that a lot of times, even when I became convinced that the problem was not the valve cover gaskets, the problem ended up being the valve cover gaskets. These valve cover gaskets seem to leak "in strange and mysterious ways". So, even though it might seem like the valve cover gaskets are not the problem, I would not rule them out.

Another common leak source for the area you describe is the intake manifold end seals. This is a VERY common leak area. GM finally solved it by eliminating the end seals and using RTV, formed-in-place gasketing. I HIGHLY RECOMMEND that RTV gasketing be used for the end seals. I also recommend that the RTV contact surfaces on the intake manifold and block end rails be SCRUPULOUSLY clean and, then, treated with Permatex Kleen-N-Prime, or, better yet, an aerospace RTV primer, prior to application of the RTV. I recommend only the use of Permatex Ultra Copper as the RTV sealer. Ultra Copper is not only highly resistant to oils and heat, but it also EXPANDS when contacted by oil which improves the seal.

Another possible, but less likely, source in the area you describe is a leak from the head gasket. Even if the head gasket is otherwise ok, there can still be an oil leak. Some folks think that head gaskets only leak from the combustion seal or the water passage seals. It's true that those are the most common types of head gasket leaks, but they can also leak oil and, sometimes, this will consist of an EXTERNAL leak.

[Edward J.]

Re: Big Block Oil Leak Insights

Gary, Joe is right on the money with the common leak areas, but my approach is to first clean the engine block first, if you have a lift or a set of ramps it makes it easier to clean. after spending some time and you cleaned the areas, take to car for a 15 minute drive and look at the areas you saw the oil, leaks will aways throw you for a lope, that can start at the top and work there way to just about any point on the engine,do only what is needed.

I do use brake cleaner solvent to clean areas, if you have a nicely detailed engine compartment then maybe some simple green and a few rags, brake cleaner evaporates quickly but is not easy on fresh orange engine panit.If you use the brake cleaner solvent, keep off the top of driveway and out of your eyes.card board under car will help.

[Terry M.]

Re: Big Block Oil Leak Insights

The cynic in me wants to tell you all big blocks leak -- that's what seven rocker arm cover bolts are for.

Seriously though, after you clean the back of the block, like Ed has described get some foot powder and spray it on the back of the head and cylinder case back there. Then take the car for a drive. You should be able to see the traces of the leak in the powder. You will have to clean the area again, but you are in for cleaning that area a couple of times anyway.

Be sure your PCV system is properly functioning -- the checks are in the FSM. A malfunctioning PCV system will allow the crankcase to become pressurized and you will get oil all over the place.

[Steve D.]

Re: Big Block Oil Leak Insights

63 Convertible

Joe

Your comment is timely for me as I have an intake manifold gasket leak in the rear, right behind the oil pressure gauge fitting. ( I checked the archives and didn't get any hits on RTV). I can see where the leak is, since the oil surges from a particular spot when the engine is revved. My question is this: is the RTV used only at the ends, or all the way around the perimeter of the manifold?

Before I remove the manifold, I want to make sure it is not just that the manifold was not torqued properly. I read your comments re: proper torquing procedure in a recent post, but my problem is that I can't get my torque wrench on some of the bolts. I can get a socket on each of the four (2 on either side) interior bolts, but due to the bolt angle, I can not get the torque wrench into the head of the socket due to interference with the side of the manifold (Manifold PN 3844461). Same issue with the second bolt from the front on the driver side, due to interference with the upset for the radiator hose connection. Is a special shallow socket with a small drive torque wrench required?

Steve

[Joe L.]

Re: Big Block Oil Leak Insights

63 Convertible

Joe

Your comment is timely for me as I have an intake manifold gasket leak in the rear, right behind the oil pressure gauge fitting. ( I checked the archives and didn't get any hits on RTV). I can see where the leak is, since the oil surges from a particular spot when the engine is revved. My question is this: is the RTV used only at the ends, or all the way around the perimeter of the manifold?

Before I remove the manifold, I want to make sure it is not just that the manifold was not torqued properly. I read your comments re: proper torquing procedure in a recent post, but my problem is that I can't get my torque wrench on some of the bolts. I can get a socket on each of the four (2 on either side) interior bolts, but due to the bolt angle, I can not get the torque wrench into the head of the socket due to interference with the side of the manifold (Manifold PN 3844461). Same issue with the second bolt from the front on the driver side, due to interference with the upset for the radiator hose connection. Is a special shallow socket with a small drive torque wrench required?

Steve

Steve-----


Primarily, the RTV is used in place of the end seals, extending it so it covers the "tabs" on the side gaskets (the part of the side gaskets that normally overlaps the end seals). I also use a very light film of it around the water passages on both sides of the side gaskets. I do not use it anyplace else on the intake sealing system.

To torque the bolts you describe, you need a torque adapter. This is a short wrench with a 9/16" box end and a square drive end. You can also use a crow's foot wrench, but they usually are too big to fit in the manifold recesses around the bolt.

[Steve D.]

Re: Big Block Oil Leak Insights

I have tried both the crow's foot and a 90 deg box end with ratchet connection, and neither one fits in a workable manner. I will look for for another box end adapter with a thinner profile.

Thanks.

[Ridge K.]

Re: Big Block Oil Leak Insights

Joe's advice is spot-on.
Next month I will celebrate by 40th year of continuously driving big block Chevrolets (both 396s, and 427s).
My "parts store driver" for the past ten years has been the original SS396 Chevelle I've attached a photo of.

As Joe mentioned usually the oil leak will be tracked down to either a valve cover, or, the rear of the intake manifold.
My 396 has not spilled one drop of oil since sealing in the manner Joe Lucia describes, ...about 10 years ago.
A correctly functioning PVC is also a key component, especially if one is heavy footed.
My cars prove the myth about big blocks consistently leaking is in fact, just a myth. You only need to take the proper sealing steps.

Ridge

(two vintage big blocks in the garage)

[John H.]

Re: Big Block Oil Leak Insights

I have tried both the crow's foot and a 90 deg box end with ratchet connection, and neither one fits in a workable manner. I will look for for another box end adapter with a thinner profile.

Thanks.

Steve -

Just put a box-end wrench on one of the other bolts that you've done with a torque wrench, and do the center four bolts to the same "feel"; it's not that critical.