Sonic Checking the Mopar Big Block

If you ever want to start an argument at a Mopar gathering you can always ask someone which model year had the best engine block casting. There are more rumors and half truths floating around on this issue than Elvis sightings.One big issue that must be dealt with is the information that the factory published in its Mopar Performance publications. Over and over again, the factory has said that blocks cast after 1976 are “thin wall” castings, and should not be bored more than 0.020. It turns out that this statement isn’t completely true and that is the source of much confusion. Hopefully we’ll be able to clear this topic up some by pulling out some modern technology, namely, a sonic checker.Sonic checking was once an exotic, space age technology. These days, companies such as Dakota Instruments are selling hand held checkers for less than $1000. This might not be in the budget for the average Mopar fan, but it is well within reason for a serious engine builder. A sonic checker uses high frequency sound energy to measure material thickness in much the same way as a police radar measures speed. The sonic checker sends out a signal and then measures the time required to get the echo back. The longer it takes for the signal to return, the thicker the material. Sound travels at different speeds in different materials so the sonic checker needs to know what material is being tested, but that is an easy front panel input. The PR-8 model that Dakota Instrument sells is designed for measuring cylinder wall thickness as well as cylinder heads and other items which racers are typically interested in. We’ll use the PR-8 unit in this article to test a number of big block castings in order to see if there is any difference between early and late model castings.The PR-8 Sonic Tester used in this article was obtained from Dakota Instruments.

THICKNESS We were able to gather up 20 bare blocks of various year, CID and casting numbers. While 20 blocks might seem to be too few to accurately tell what the factory did, it turns out that it is plenty.

This is what a ton of Mopar big blocks looks like!


We are actually measuring individual cylinders for wall thickness not blocks, so 20 blocks means 160 cylinders to measure. A good rule of thumb for statisticians is that if the sample size is greater than 30, then you can predict the total population with a fair amount of precision. Since the blocks we gathered up were a random sample of blocks built over a long period of time, our sample should be very representative of what you might find at a swap meet or wrecking yard.

One of the first things you learn when using a sonic checker on a block is that the cylinder wall castings are not of uniform thickness. The cylinder walls are thicker on the sides facing the inner and outer parts of the block than on the sides between cylinders. (PICTURE OF CYLINDER WALL WITH LABELS) The thick sides are the thrust surfaces of the cylinder wall, named that since they take the thrust load from the piston. In a motor that rotates clockwise, the major thrust surface will be on the passenger side of the cylinder, the driver’s side of the cylinder wall is called the minor thrust surface. When looking for an ideal block, we want one with thick, uniform thrust surfaces. More material on the major thrust surface is a nice touch as long as it doesn’t come at the expense of the other surfaces.

Pre ’76 Post ‘76

Average thrust surface thickness 0.204 0.220

Standard deviation of measurement 0.041 0.038

Minimum wall thickness 0.065 0.108

Maximum wall thickness measured 0.300 0.300

This 440 block cast in 1977 has wall thickness of 0.276. So much for that thin wall theory!

So what do these numbers mean? From a first glance it appears that blocks cast after 1976 are actually thicker on average than blocks cast before 1976. A closer look at the numbers however tells us that since the standard deviation is so high, we can’t really say that these two numbers are very different from each other. For those who might have skipped statistics class in high school, the standard deviation is a measurement of distribution. In this case, it tells us how likely it is for one cylinder to vary in thickness from another cylinder.Given these numbers we can predict what percentage of cylinder blocks will be thick enough for high performance use. According to Bill Jenkins book, The Chevrolet Racing Engine, he prefers a minimum wall thickness for a racing engine to be 0.215 thick. He goes on to say that the Chevy specification is .223 average on the thrust sides but he doesn’t say what the tolerance allowed is. Given that the average thickness is less than this 0.215 figure, what chance is there that a Mopar big block would have all of its thrust surfaces greater than 0.215? Actually, this isn’t too hard of a problem to solve. Since the odds of any thrust surface being greater than average thickness is 0.500, the probability that all 16 surfaces are greater than average is 0.50 times itself 16 times. The same exact odds as flipping a coin 16 times and having each flip be heads, or about 1 in 65,000.If we lower our standards a bit it might be easier to find a block which will work. For instance, if we will accept 0.160 as the minimum wall thickness on the thrust side, what percentage of blocks will be acceptable? Since 84 percent of the cylinder walls are going to be thicker than 0.160, 0.840 times itself 16 times will tell us the number which are going to be acceptable. That is, if you have a 84% yield for each thrust surface, and you need 16 good ones, what are the odds of getting 16 in a row good? Turns out to be 6%, or 6 blocks out of 100 are good enough for racing if you use 0.160 as the minimum thrust thickness.

LOW DECK VS. RB MOTOR Another variable that should be touched on briefly is the length of the cylinder wall. Any mechanical part is going to be weaker as it gets longer. A simple test with a yard stick versus a ruler will show you the dramatic difference in deflection as the unsupported length increases. By sliding a rule down into the water jacket of a block, you’ll see that the unsupported section of a B block’s cylinder wall is about 5.25 long, while a RB cylinder is about 6.0 inches long. This means that the unsupported length for a RB motor is about 14% longer than on a B motor. A 14% increase in unsupported length doesn’t sound like much, but when you crunch the numbers, it turns out to result in a xx% increase in deflection for the same load. Fortunately, a racer can solve this problem with the use of Hard Block or other water jacket filler. By filling the bottom of the water jacket, the length of the cylinder wall which is unsupported becomes shorter and therefore stiffer.HARDNESS One item that people rarely discuss is the hardness of the cast iron itself. But the hardness of a cylinder block is actually quite important since a harder block will be stronger, and more resistant to wear. Since the thickness of the Mopar blocks seems to be the same over the years, we were curious to see if the hardness of the blocks had changed during that time. Folklore among machinists is that the blocks cast in the 60’s and early 70’s are much harder than the later blocks.

A small sample was taken from each block in order to test the hardness. It is fairly easy to remove one of the transfer lugs from the side of the block with a hacksaw and then this material can be tested in a bench mounted hardness tester. If your engine builder has a hardness tester like this, pat yourself on the back!

The hardness tester uses a calibrated load to press a small ball into the material. The softer the material, the further the ball is pressed into it. The hardness value is then read directly from the scale. The numbers for the big blocks was interesting. All of the material samples tested at 200 or more on the Brinell hardness scale until we got to the 1976 and later blocks, then the values dropped more than 10%. This would indicate that the material strength was reduced in 1976 by about 10%.



Hardness samples from six of the blocks.

Some Typical Hardness Values

1964 413 HB 200

1969 383 HB 205

1971 400 HB 205

1974 440 HB 205

1976 400 HB 180

1977 440 HB 181

As a reference point, according to the 2000 GM Performance Parts catalog, the GM big block race engine has a hardness specification of 210 to 230 on the Brinell scale. In Bill Jenkins book , “The Chevrolet Racing Engine”, there is a picture of a block being hardness tested. Evidently, sharp racers such as Grumpy were using this technology many years ago to select the best blocks.

WEIGHT The weight of the various years should also give us a clue as to the thickness of the cylinder walls. For some reason the MP book doesn’t provide us with accurate weights for the various years so we arranged for some quick testing on a large scale.

The 1977 440 block with the thick cylinder walls weighs in at a porky 234 pounds!


The following results are interesting.

Bare blocks with main caps

1964 413 block 220 lbs

1966 426W block 227 lbs

1977 440 block 234 lbs

1969 383 block 220 lbs

1972 400 block 218 lbs

It is interesting to see that in the case of the 383 motor, it weighed the same as the 413 block. According to the MP engine book the B motor should weigh 225 pounds. No data is provided by MP for the RB motor but it appears that the RB is maybe 0 to 20 pounds heavier, depending on the individual casting.

The 1977 440 block that weighed in at 234 pounds was the heaviest block the author has seen. When sonic checked, this block proved to have very thick cylinder walls with all thrust surfaces well over 0.200 thick. But the hardness for this block was only 180 on the Brinell scale which showed it to be made of the late model cast iron. Probably still a very good candidate for a healthy output motor.

As a point of reference, the big block Chevrolet Bow Tie racing blocks all weigh right at 270 pounds bare. Of course, that is for a block with cylinder walls which are 0.400 thick, so the comparison isn’t exactly fair.

Shortblock weights – these are bare blocks with crank, rods, pistons but no camshaft or lifters.

1970 383 361 lbs

1978 400 351 lbs

1974 440 376 lbs

1978 440 376 lbs.


CONCLUSION Well it appears that the factory information on big blocks isn’t the whole story. While the blocks cast after 1976 do not appear to be “thin wall” castings, they do appear to be cast from a material which is inferior to the earlier castings. It could have been this knowledge that led the MP engineers to caution racers against using the later blocks. If you don’t have access to a sonic checker you might just rely on block weight to help you decide. While a heavy block isn’t guaranteed to have the material in the correct place, at least it is a start in the right direction. For a high horsepower application, a heads up engine builder should consider having the blocks hardness tested as well as sonic tested.

It isn’t readily clear as to the value of the later blocks versus the earlier blocks. While the later blocks appear to be just as thick or thicker, they are also constructed of a material which is softer. I suppose it comes down to individual testing of various blocks for all of these properties and then picking the one that is the best available choice. Obviously, no one should pass on a 440 or 400 motor just because it has a casting date that is 1976 or later. And if you can get one cheap because the seller doesn’t read Mopar Action, then all the better!

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