There is no such thing as thinwall blocks. They DO NOT exist.
We have sonic checked over 50 blocks, and have found absolutely no evidence that later model blocks have cylinder walls that are any thinner then earlier blocks. In fact, we have found later blocks to have less core shift, meaning the cylinder walls are of a more uniform thickness all the way around. This makes perfect sense, considering that B engines were in production for over 2 decades. During this time, it would be expected that there would be small improvements in the technology of casting processes, quality control, etc, which would help create a better product.
There was also an article from Mopar Action magazine by Andy Finkbeiner of AR Engineering in which they sonic check 20 blocks and come up with the same findings we did. They also explain how statistically 20 blocks is enough of a sample to estimate the rest of the 440 blocks out there. We've tested over 50 with the same results. This article also has some great info regarding hardness of the iron used, in which they find that the earlier blocks do have slightly harder cast iron, by about 10%.
Another issue is block weight, or the quantity of cast iron which is actually used in the block. Like the Mopar Action article, we have weighed many blocks and have found later blocks to weigh more, or have more cast iron in them. Since the reasoning behind thin wall blocks is that the factory used this practice in the late 70's to save money on iron (which is a known fact with SB Chevy engines), the fact that later blocks weigh more shows that Chrysler had no intention of using less iron to save money. Which is great news for us racers and performance enthusiasts.
So, the bottom line? Save and use those late model blocks. And go .060" over with 'em all day long. Also on that note, we know of several machine shops that have been going .060" with late model blocks for over 20 years with no problems.
