What's the Secret? - Impala Tech
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post #1 of 7 (permalink) Old 10-23-2019, 01:43 PM Thread Starter
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What's the Secret?

Since finishing the engine & transmission swap in my '68, I have been dealing with a very annoying issue: oil leaks. Nothing major, just a drip here and there but even this is enough to set me off since I don't like seeing fluid on the garage floor especially since everything under the hood is brand, spankin' new.

The first place where this happens is the outside, rear of both valve covers. The oil of course leaks onto the exhaust, which then has me diving for the fire extinguisher every time I open the hood. Haha, joke's on me.

The second place where this happens is the front and rear of the oil pan. A few drops will leak out when I shut the engine off (it doesn't leak when running as far as I can tell) and this ends up on the floor.

Know what both the oil pan and valve covers have in common? The fancy, reusable Fel-Pro rubber gaskets with the steel inserts that say you don't need silicone. I have since added silicone to the valve covers which helped. The oil pan will just have to wait until later this winter.

So what is the secret here? Is there an old-school engine builders trick to sealing up a BBC? I'm having to re-torque all fasteners every few times I drive the car, which seems a little unreasonable especially since I am using lock washers everywhere also. Pic attached. Note the blue silicone, spreader tabs, AND lock washers. Thanks for any replies.
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1968 Impala SS
496 Stroker
T56 Conversion
3.73 Posi-Trac
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post #2 of 7 (permalink) Old 10-23-2019, 04:02 PM
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Factory noticed the same problem so in 1996 they redesigned the BBC to use a machined flat head surface on the valve cover with an O-ring to seal die cast aluminum valve covers (more rigid than stamped steel) to seal the oil leak at the valve covers. They also went to a one piece rear main seal at the back of the motor and a die cast aluminum timing chain cover to prevent drips from the oil pan.

With a Mark IV engine you have all of the drips in the engine design that the engineers included when the motor was first designed in 1961 (the SBC was still using a road draft tube that dumped excess oil on the road and your garage floor when the new and improved BBC was on the drawing board). So oil leaks were not high on their priority list.

The EPA didn't exist before 1963 and it's first edict was for car manufactures to introduce a Positive Crankcase Ventilation system that kept the oil out of the environment. I not only remember the black streak of oil stained pavement visible on old photographs of heavily traveled paved roads, but living in Florida since 1957 remember the first rains of summer: when that oil created a infinitely long skid pad. The deep south equivalent of driving on ice.

You can glue the valve covers down with RTV and that will stop a lot of it, but you have to be careful as to the amount you use as RTV on the inside will break off eventually and clog the oil pump pick-up. Better to do as they do at the fire station and put a drip pan under your engine.


I kept three of these drip pans under the engine on my cars that I kept inside my seven bay garage (because if left out side they would evaporate in moonlight despite my having a six foot chain link fence topped with barbed wire). This was because the EPA actually has an investigator that goes around to car garages to see how you handled "hazardous wastes" like engine oil; and the fine for oil on the floor was a lot more than drip pans.

Chevy designed the BBC mark II (aka the 427 Mystery engine) that debuted on February 22, 1963 at Dayton beginning in 1961. The young Cadillac Engine engineer was transfered to a special development program and tasked with beating the Pontaic 421 using a 409 block but keeping in mind the engine's tooling had to be paid for by sales of the engine a medium duty to heavy duty truck (diesel engines under 700 cubes didn't exist then). This engine became the Mark II, basically a 409 stroked to 427 cubes with a modern Mark IV porcupine head on top of the old Mark I block. Ford introduced their 427 Cammer (SOHC) FE engine in 1963 and Chrysler offered a little head improvement to their already existing 426 wedge head raised block stroker engine that they called a Hemi in the 1964; so Pontiac's 421 super duty motor's days were already numbered. Chevy banned factory support for all racing in late 1963 after a number of high speed wrecks due to these new engines pushing stock bodied cars beyond their ability to stay on the track.

The Mark III was a new truck engine based upon the tooling that GM bought from Packard motors to cast a 5.000 bore center block capable of supporting five hundred cubes with out Siamesing the bores. Though there were twenty Mark III BBC blocks cast for evaluation the tooling modifications would cost too much to please the bean counters. The Mark III plus the Beach boys 409 song kept the Mark IV engine out of production until late in 1965. The first six months production run was cast with overly thick cylinder walls because even though the 1965 production run had already started the executive board at GM couldn't make up their mind on building a 409 with the new porcupine heads or the "long" stroke high torque 396 engine. The small bore long stroke torque motor won out and we lost the opportunity for a 500 cube BBC in 1966, getting a new and improved 427 instead.

Big Dave

Last edited by Big Dave; 10-23-2019 at 04:37 PM.
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post #3 of 7 (permalink) Old 10-23-2019, 08:26 PM
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Sounds like he's saying your **** out of luck. :-D lol. (sorry, couldn't resist)

I feel your pain on the oil drippage, it bugs the crap outta me too. I've never had a big-block though so I'm no help here.

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post #4 of 7 (permalink) Old 10-23-2019, 10:07 PM
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Best thing the OP has going is the Corvair valve cover hold down clamps. They spread the force out evenly and were developed by Chevrolet to slow down the massive oil leak that was a Corvair engine. They did leak from the valve covers until the hold downs were developed but nothing helped the push rod tubes seal after the silicon rubber O-rings hardened. Just like with a Harley (another engine developed in the late fifties).

Is your Harley leaking? No, then it is out of oil!

Our expectations have changed with time (Curse you Japanese car companies for raising our expectations in the first place). Now everyone expects these dinosaurs to be house broken as well as well behaved. They never were.

Big Dave
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post #5 of 7 (permalink) Old 10-24-2019, 07:44 AM Thread Starter
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Thanks for all the replies. I think with some RTV I can get the oil pan to seal up right, but that's a little more involved than the valve covers. The weezy old 396 I took out of the car last winter didn't leak a single drop of oil (probably burned it though), so I know there's hope. I knew going with a Mark IV big block I would be in for a little more upkeep compared to say an LS conversion. Time to suck it up and deal with it I guess. Not like it isn't worth it 😎.

1968 Impala SS
496 Stroker
T56 Conversion
3.73 Posi-Trac
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post #6 of 7 (permalink) Old 10-24-2019, 08:05 AM
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For reference attached is a picture of the Mark II 427 with dual 4 barrels next to a Mark IV 427 with single 4 barrel. I find it interesting that both of these engines were in development at the same time in the same building. The mark II engine was designated as the Z-11 in 1963. The Z-11 had only 57 cars made and as of today only 7 survivors are known about. These cars also came with an Aluminum front end. (e.g. fenders, hood, bumpers, radiator support...) Also none of the cars came with stereos and most were ordered with heater delete. In all the cars were around 300 lbs lighter than the most stripped down 409 car. The funny thing is most of the cars were sold right off the dealer lot and not directly to racers.
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post #7 of 7 (permalink) Old 10-31-2019, 01:14 PM
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The best valve cover gaskets I had luck with are the Victor ones with metal between two cork gaskets. I've been out of the game forever though.

'07.5 Chevrolet 2500HD-6.6 D-Max
'67 Hell Camino-BB 427/TH400/4.10 His
'67 Biscayne-BB 427/TH400/??? Hers
'66 Chevelle 300 Deluxe- 230/PG

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