There's definitely some BS out there about blocks, but it's not all false information.
There's an article on 440Source's website about big blocks, and there actually are a lot of differences over the years. All Chrysler ever really said was not to take the later big blocks more than .020" over, the printed reason was "thinner walls". The truth of the matter is that the late B/RB blocks often have slightly thicker cylinder walls. The concern was actually metallurgical, as the '76-'78 blocks are of a slightly softer (lower nickel/tin) alloy. It was just easier to explain it to Joe Public in simple terms. That information was actually released when the blocks were new, though, and time has proven it to be a non-concern, especially as the blocks have actually gotten a stronger with use. Heat cycling is a beautiful thing. Thank the B/RB engines' ridiculously long connecting rods and excellent rod:stroke ratio. There's so little side load on the pistons that accelerated cylinder wear or failure did not become the issue they'd expected. The same lesser alloy was used in the small blocks of the era, but I have absolutely no concerns over the strength of the '78 360 block sitting in my garage.
The one that's complete trash, and I don't know where it started, is the "early 360" myth. This one centers around the water jackets. According to the legend the '71-'73 360 blocks have thicker cylinder liners because they were made with 340 water-jacket cores. The casting process isn't that simple, especially with a somewhat complex casting like an engine block. With the minor revisions made in the bottom end for the larger crank bearings, the jacket core mold has to be specific. Then there are the obvious questions: if the preceding scenario was the case, why would Chrysler change to a specific 360 water-jacket core after three years of success using the 340 core? If the 340 water jacket and crankcase were sufficient to house the 360 crank, why didn't they just start boring 340 blocks to only 4.00" and cutting larger mains, and simply use a new casting number on existing tooling, if at all? I mean, think about it: the Direct Connection race 340 block (the "X" block) used a 318 casting number. No matter; no reputable engine builder with whom I'm familiar has ever actually ever found such a block, and those who claim they exist cannot provide any evidence to back up their claims. Yet, guys will fall all over themselves and pay big extra dollars for the "good" '71-'73 block. If you're one of those guys, please let me know. I know the location of no less than six early 360s, all sleeping quietly in junkyards. Before you ask, I already took all the J heads. :eusa_dance:
I think my favorite one, though, has got to be the "dash number" myth. This is the last number on the block casting ID, such as 2780930-340-6. I've had it told to me over and again that this is the number of cylinder-liner cores that were replaced during the making of the original mold. I have no idea where this one started, but it was the subject of a magazine article many years back. While replacing a broken core is possible, it's not what you'd call "advised." I'd love to meet the adventurous soul that decided the -9 block in my '85 AHB was a go. :icon_biggrin: Though able to withstand 3,000° molten steel, sand molds of any kind are fragile things. Anyone who's ever worked in the production area of a foundry could tell you that anything more than one trashed core would probably result in the mold being scrapped, and it's likely that even one would fail the overall mold. However, that's not even the truly laughable part. The casting number is part of the tooling used to make the mold (the "pattern"). You can't change the casting number of a mold. Period. It is simply not possible. You can change the number on the tooling, yes, but once a mold is made from it, you are stuck with whatever identifying marks were on the pattern that made it. I've also been "told" that the -6 340s were the T/A blocks, the number denoting to their carburetion. Uh ,no. If you have a T/A block, trust me, you'll know it by the big "TA" in the casting number and the thicker main webs.
Mixed truth/myth: the .040" overbore rule. This is the largest overbore Chrysler recommends on any LA/B/RB block (all of which are thinwall castings), and they're right. Yes, there are thousands of .060" engines out there running happily every day with no problems... that's where the myth part comes into play. No, going .060" over will not cause you to blow out a cylinder wall; that was never the concern. The concern is that above ~.045" past original, the cylinder walls will actually flex in hard usage, compromising ring seal and causing a loss of power in comparison to what you'd make at .040" or less. You've got to remember, these recommendations were made by Chrysler powerplant engineers of the '60s and '70s. Tom Hoover & his crew were hardcore racers; the Ramchargers were all guys that worked in the Chrysler engine labs. They designed the stuff in the first place and those recommendations were for similar-minded guys. Will you notice an issue in your daily-driven Fifth Avenue? No. It'll go tens of thousands of miles without so much as a hiccup. I know of a +.090" 318 with about ten years' use on it, in fact (I would not recommend attempting this, in fact I attribute it's longevity to light use and dumb luck). Would you see it on a dyno pull? Yes, assuming you had a smaller-bore engine of exact spec otherwise to use as a baseline, like your dyno sheet prior to the larger overbore assuming nothing else changed. If you're going racing, stick with the smaller overbore and get every last HP out of your engine. Blocks are cheap. If you're building a weekend cruiser/daily driver, don't worry about it. Yes, I'm aware of the effects of "positive" or "good" core shift in the major thrust direction for larger overbores, but it's not worth exploring in this thread.
