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Tech Tips 4

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Big Block Chevrolet beginners guide Jan 26/10

I have been playing with big block Chevy engines most of my life and I am often asked questions regarding them by guys starting out. I have rebuilt them, raced them and broken them over the years. My knowledge is based on experience with the Mark IV design used on engines made from 1965 to 1990 as well as the aftermarket blocks. I hope that the information below will help some of you avoid the mistakes I made on my early engines.

I can not answer your questions on newer big blocks as I have no experience with them but I will be happy to post on the site any information I am sent.


BBC cylinder heads came with two combustion chamber shapes. The smaller bathtub shaped chamber is known as “Closed chamber” and was used primarily on earlier engines and on some later large trucks. If domed pistons are used in the engine the cylinder head must match the combustion chamber the pistons were designed for. Flat top or dished pistons can use either cylinder head design. Domed pistons used with aftermarket cylinder heads can also be an issue. Some manufacturers rotate the combustion chamber slightly which can in some cases cause piston to head interference. My unscientific method to check for this is to install the cylinder head on the bock with no head gasket. I rotate the motor by hand after installing a couple of head bolts finger tight. Twice now I have found pistons that had worked with the cast iron factory heads hit the new aluminum cylinder heads I have just purchased. In both cases the interference was a very small corner of the combustion chamber . I rounded the small area with a Dremel tool and retested it.

The big block Chevrolet cylinder heads were produced with three shapes of intake ports. Rectangular , Oval and Round. Oval port heads work very well on street engines producing more low end power than the rectangular port heads.  Stroker motors and race only applications should be equipped with the rectangular port heads to achieve the full performance potential of the engine. The round port heads also known as peanut port heads are a better choice for your tow vehicle than in your performance car.  Can a rectangular port intake manifold be used on Oval port heads? Yes when used with rectangular port intake gaskets. Some mixture turbulence I am sure will occur but the loss of power on a street driven engine is not at all noticeable.

Keep in mind that mixing a newer block with an older set of cylinder heads or vise versa can present problems. The water passages between the block and heads were reshaped when the engine was redesigned as Gen V for the 1991 model year. This will result in water leaks if the engine family and head family do not match.   Engines with a two piece rear main crankshaft oil seal were produced from 1965-1990 know as Mark IV (Mk IV)  engines. 1991 and later Gen.V, VI and VII production BBC motors have one-piece rear main crank seals. Most aftermarket performance parts (including blocks) were designed for use with the 1965-1990 Mk IV engines.

The blocks were produced with two different deck heights. “Car” blocks which also were used in pickup trucks had a deck height of 9.8 inches. “Truck” blocks used in dump trucks and other larger trucks used a 10.2 inch deck height. The quick way to identify what you have is to look at the space between the water pump and the block’s “deck” surface where the cylinder head attaches. The water pump almost touches the head on the  “car” 9.8” blocks while there is a space of about 3/8” above the water pump on the “Truck” blocks. Truck blocks are great to build a stroker motor but will require a special intake manifold or spacers to use a “car” intake. Special push rods, Pistons and distributor will also be required. Cylinder heads from the “car” and “truck” engines are interchangeable within their families and combustion chamber design.


4 bolt main blocks were used on some high performance cars and many of the truck blocks. My personal opinion is that you do not need a 4 bolt main block for a big block engine for all street and most performance applications. I have never had any problems with the 2 bolt main blocks I raced shifting at 7000 rpm. I can not say the same for connecting rods….


Connecting rods come with either 3/8” or 7/16” rod bolts. It is my opinion that the 3/8” connecting rods are the weak link in an otherwise excellent design. If you must use the smaller rods for any sort of performance application buy a good set of aftermarket rod bolts. It will be money well spent. Ohio Cranksaft ( ) offers 7/16” rods with the good aftermarket bolts for little more than the cost of new rod bolts and having your 3/8” rods resized. For the price of the 7/16 rods I use them even in mild build ups and upgrade to H beam rods in full race applications.

I have seen three diameters of push-rods used in the engines. There is very little cost difference between the sizes but you will need to change the guide plates on the heads if you decide to upgrade yours.


Engine mounts…. Big blocks will bolt to the same frame mounts as a small block engine provided you use the small block engine half of the mount on the big block engine. Yes some cars did have special big block frame mounts but it was not that the engine would not bolt to the SBC  frame mounts it was that the factory wanted the BBC to be mounted higher / lower / or further to one side than the SBC so that all the accessories would clear. Before you spend the time and money hunting down a set of factory BBC frame mounts try the SBC mounts. I personally have tried this several times and have not had a problem with it yet.

Flywheels and Harmonic balancer…. All Mk IV big blocks except the 454 can use the same flywheels and harmonic balancers. The 454 is an externally balanced engine and requires the flywheel and Harmonic balancer to be equipped with an eccentric balance weight.

Crankshafts ………Both steel and cast crankshafts were installed by the factory. 396 and 427 engines used the same cranks. Steel crankshafts are very common in the 396, 427 truck engines. An inexpensive 427 can be had by installing the one of these crankshafts in a 454 car block.

Small block parts… 396 and 427 engines can use a flywheel or flex plate from a small block engine. Small block starters can also be used but some of the small blocks used smaller windings than the big block and could be a little short on power for bigger engines or those with higher compression ratios. Look for starters with a 3/8” long spacer between the solenoid and the copper tab that comes out the starter body near the front. Starters without the spacer are the low torque design.  Standard flat tappet ( not roller ) valve lifters (solid or hydraulic)  from a small block engine can be used in the Mk IV big blocks. To the best of my knowledge these are the only small block parts that can be interchanged with the big block engine.

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