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Have you ever looked at an engine that someone has for sale on Craigslist or another “For Sale” site? Then after looking at it you make the purchase, cheap and go directly to your favorite machine shop. Upon arriving they look at the back of the engine and then, on the spot, they tell you “it’s junk” and won’t touch it? The reason they give is just two words, “Core Shift”. You may not know what this is but, you should have learned what it is much earlier in your high performance career.
If you want to tell your friends and/or competitors what to look for you just need to show them these 2 pictures. Both are SBCs; the one above has as close to “zero” core shift as I have ever seen, while the one below is a 4-bolt main engine with a lot of core shift. (This can happen in any engine so it is a common problem in all brands of ‘50s through mid-‘90s cars.)