Myth or fact

[Terry Wilkerson]

Does boring a motor .30 over cause it to overheat. Seem like I had on that was ok until the air temp neared 100 in the summer. I'm considering a .30 rebuild built by a local guy. Not sure of the year but it's a roller lifter and rocker now. Has vortec heads. Asking 3300. What do you guys think

 

[Blue Ox]

.030" over is not causing the overheat.

 

[TotalyHucked]

Nope, 30 over really isn't much of an overbore anyway. 60 over is starting to get into "the cause of overheating" territory. Anything that's 30 over and overheating is not caused by the overbore

 

[bucket]

Nope, 30 over really isn't much of an overbore anyway. 60 over is starting to get into "the cause of overheating" territory. Anything that's 30 over and overheating is not caused by the overbore

I won't even say that going .060 over is a cause of running hotter. It's just that once you get to a certain point, the cylinder wall is too thin and other issues may pop up related to the thinner cylinder wall, from a structural standpoint. No matter how thin the cylinder wall winds up being, if it holds up then it holds up and temps should be similar to a stock bore block.

 

[TotalyHucked]

I won't even say that going .060 over is a cause of running hotter. It's just that once you get to a certain point, the cylinder wall is too thin and other issues may pop up related to the thinner cylinder wall, from a structural standpoint. No matter how thin the cylinder wall winds up being, if it holds up then it holds up and temps should be similar to a stock bore block.

Exactly. I just meant that .060 is just starting to get into the possible realm of issues being rooted in the overbore. I know of plenty of engines that are happy at .060 but I also know of a few that constantly had issues after a .060 overbore/rebuild. Some engines had more core shift than others and have thin spots in the walls that others may not have. I do believe once the cylinder walls get to a certain point of thin-ness, there's the possibility for more heat transfer which would make the cooling system run hotter. But to get to that point and hope the engine lives, you'd have to ultrasonic the block to see how far you can go. So it's not realistic to experiment with.

 

[GTX63]

Exactly. I just meant that .060 is just starting to get into the possible realm of issues being rooted in the overbore. I know of plenty of engines that are happy at .060 but I also know of a few that constantly had issues after a .060 overbore/rebuild.

The 400 small block would be a good example.

 

[Bennyt]

SBC's are becoming quite rare. I would consider boring the block 0.020 that way it can be bored 2 more times at 0.040 and 0.060 instead of just once more.

My opinion based on experience...Unless there was some core shift issues which does definitely happen, a 400 won't run any hotter at 0.060 than 0.030 and only run 2-3 degrees warmer than a 350 if measurable at all. In my youth I worked at a shop that did R&D work and we did so much weird stuff I've literally tested the same block at various bores and they were all the same temp. Best to always sonic test a 400 as they can get thin at the siamese. We even did blocks with different bores side to side and in center. We would cut cams with different profile at different sections, half the block sleeved at one bore and the other at a different bore. It allows you to test valvetrain without building 5-10 different engines.

The consensus was that a 400 that ran hot at 0.060, ran hot at 0.030 as well after back to back testing and what was hot, was still within operating temp range. Again, my opinion based on work done 25 years ago. You don't have to agree with me but if anyone is throwing away any 0.060 400 cores, I'll take them.

 

[TotalyHucked]

SBC's are becoming quite rare. I would consider boring the block 0.020 that way it can be bored 2 more times at 0.040 and 0.060 instead of just once more.

My opinion based on experience...Unless there was some core shift issues which does definitely happen, a 400 won't run any hotter at 0.060 than 0.030 and only run 2-3 degrees warmer than a 350 if measurable at all. In my youth I worked at a shop that did R&D work and we did so much weird stuff I've literally tested the same block at various bores and they were all the same temp. Best to always sonic test a 400 as they can get thin at the siamese. We even did blocks with different bores side to side and in center. We would cut cams with different profile at different sections, half the block sleeved at one bore and the other at a different bore. It allows you to test valvetrain without building 5-10 different engines.

The consensus was that a 400 that ran hot at 0.060, ran hot at 0.030 as well after back to back testing and what was hot, was still within operating temp range. Again, my opinion based on work done 25 years ago. You don't have to agree with me but if anyone is throwing away any 0.060 400 cores, I'll take them.

Now that's good info! Not many people can say they've tested stuff back to back like that. Wish I would've tried working in a machine shop when I was younger, we've got several good ones around here

 

[Terry Wilkerson]

This is all good to know. I will be pulling my engine this spring. Not sure if I will rebuild it or not. If it's already .030 I think I'll look for another to rebuild.