Mike Brichta Posted March 12, 2021 Share Posted March 12, 2021 I'm looking at a 1970 402 engine that has been bored .070 over. Should I be concerned? I remember hearing stories about over-boring the 400 engines and causing overheating. Thanks!! Mike Quote Link to comment Share on other sites More sharing options...
714024SPEED Posted March 12, 2021 Share Posted March 12, 2021 I would be a little concerned if your building a street engine .0 70 over the 402 because the 402 blocks actually started life as a 396 so that would be 100over a stock block. But that's just my opinion. 1 Quote Link to comment Share on other sites More sharing options...
Dtret Posted March 12, 2021 Share Posted March 12, 2021 1 hour ago, Mike Brichta said: I'm looking at a 1970 402 engine that has been bored .070 over. Should I be concerned? I remember hearing stories about over-boring the 400 engines and causing overheating. Thanks!! Mike Is it possible that the 400 you may be thinking of is the small block with Siemese cylinder hole and the steam hole drilled in the block and heads? 1 Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted March 12, 2021 Share Posted March 12, 2021 I agree with Kevin and Dtret. Both statements are true. Personally, I would look elsewhere. 396/402 engines seem to be around for sale. Some nice ones also. And since they are considered "undesirable" by the bigger cube people, they can be had relatively cheap compared to a 454. 1 Quote Link to comment Share on other sites More sharing options...
Mike Brichta Posted March 12, 2021 Author Share Posted March 12, 2021 The engine is the numbers matching engine to the car that's why it was saved and rebuilt that way. It may be fine at .070 over. I just never heard of that. I'm not an engine expert. Thanks!! Mike Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted March 12, 2021 Share Posted March 12, 2021 Understand the numbers matching aspect and the rebuild. How bad was it that it had to go .070 overbore? What did the engine rebuilder at the time say about that? Quote Link to comment Share on other sites More sharing options...
Mike Brichta Posted March 12, 2021 Author Share Posted March 12, 2021 The story was that the engine was rebuilt once before at .060 over as part of a full restoration and the project was stopped and stored for several years. Maybe they lost interest or the funds ran out. The engine sat and water got into the engine and the piston walls had pits in them. So when the engine was gone over again, the builder had to go to .070. I believe that the story is true. I'm just a little nervous that the engine may overheat sometime down the road. I read on some Chevelle forums that these engines can handle up to .090. I just thought I would get some other opinions here. Thanks!! Mike Quote Link to comment Share on other sites More sharing options...
jft69z Posted March 12, 2021 Share Posted March 12, 2021 If nothing else, maybe get the numbers matching engine, just because.... Two options after that I suppose, run it as is & see what happens, or take it apart and have a reputable shop sonic check the cylinder wall thickness and determine if it's safe. They could also check for cracks, etc at the same time. Quick search shows reasonably priced sonic testers available, even on Amazon. You could probably just pop the heads and measure the walls by rotating the crankshaft to get the various holes. Still, a good shop may have more insight and experience with what may be acceptable limits. Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted March 12, 2021 Share Posted March 12, 2021 The story makes sense Mike. I would install it and see what happens. If it's not crazy on the compression it most likely would run good. Big radiator and good water pump. Quote Link to comment Share on other sites More sharing options...
Dtret Posted March 13, 2021 Share Posted March 13, 2021 If it’s numbers matching and Mike wants to keep it that way can the block have new cylinder sleeves put in? Quote Link to comment Share on other sites More sharing options...
wallaby Posted March 13, 2021 Share Posted March 13, 2021 That's odd.I have never heard of a .070 over piston. Usually, .060 is considered as big as you want to go, and the next step is .120. I have my original engine and it's at .060 now, but it needs to be rebored. Some castings will let you go to .120 without issue, and others have the cylinder bores not exactly centered, which causes a thin wall on one side when over-bored. That's what sonic checking does: it verifies that there is sufficient wall thickness. Cylinder sleeves are the only repair for thin walls, and one or more may be needed. The good news is that sleeves are a good repair; I think every aluminum block engine comes from the factory with them. Quote Link to comment Share on other sites More sharing options...
rotinrob Posted March 13, 2021 Share Posted March 13, 2021 Depends on the block and what you intend to do with it. Some latter blocks are also 454 casting and can take a large overbore without problems. Get it sonic checked or buy one they are much less expensive than they were years ago. Just remember that most blocks have thinner walls on the non-thrust side to save iron. The big problem is the cost of pistons as you will not be using an off the shelf piston and 396/402 blocks are plentiful and inexpensive if true numbers matching is not needed, plus starting with a standard block gives options down the road. rotin Quote Link to comment Share on other sites More sharing options...
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