Thread: To Bore..Or not to Bore...

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Recently I have been talking about building a sbc motor (400) and it was commented that boring out the cylinders just refreshes the cylinders and doesn't add any HP. I may add the argument made was very good.

I am very curious on what you all think about this subject, I see guys rebuilding thier engines of all kinds and alot (most) mention .030 over or more of boring.

My question was if your increasing your cylinder volume, arn't you gaining hp?

So let me know what you all think, inquiring minds want to know?

brickman The reason I bored my 305 .060 over was when I pulled it from a car it had over 150,000 miles on it and the cylinders were worn. To make the cylinder walls smooth I had to take it out that far and thats about as far as you want to go with a 305 since the walls were not very thick. As far as making more horespower, its what you put into the engine afterwards. If I had just stuck some new pistons in without doing anything else to the engine I probably wouldn't have seen any gain, but I put a new cam in, ported the heads, new rods,ground the crank , new intake manifold and carb and got about 40 more horespower.

Boring a small block adds around 5-6 cubes per .030 over. So, if you are making 1 hp per cube (which is a little higher than most street motors), that will be 5-6 hp per overbore. That may be a lot to a ricer considering most of their performance parts under $300 only add up in incriments of less than 5hp, but in the muscle car world, it's nothing to be concerned about, especially, if you're bracket racing.

Also, with every bore, you run the risk of going too thin. Heat dissapation is done more efficently through thicker metal. As you bore out, you take reliability down.

So when I start rebuilding my engine, I will be just as well to do as little needed to refresh the cylinders. If i can get by with just honing and maybe .010 over just to have a good tight fit would be best. Closest to stock, the better it will run and last.

I'm all for that, it is important to me to get the best combination of endurance and performance I can get.

Usually a .030 bore is standard you can bore .040-.060 depending how brave you are . Some people roll the dice for that extra bit of power . I personally go .030 every time ,especially when it comes to a 400 chevy, there isn't much room between those cylinders and water jackets.

Great advice tech, I will do that. Streets, now I understand why everybody has .030 bored motors, it's just the first step you go to to get an even bore job, very good thanks.

And thanks to everyone who answered, you have solved another puzzle for me.

What do you know, senior statous! About time I get acknowledged for my age !!!

How do you judge when it's time to bore? I have an a 350 apart now. I wasn't going to dig past the heads but a couple of the pistons rocked slightly in the cylinder. I figured OK rings so I pulled em. There is no decernable ridge or cylinder wear except in one cylinder. It looks like a tiny bit of something (maybe a sand grain) got between the wall and piston. If you run your fingernail across the scratches you can just barely feel it. I'm going to replace the pistons and rings but I'm not sure if these scratches warrent getting the cylinders bored. Thanks for your help. ---Rich

That is what a flex hone is for, Jersey. It roughs up the walls without changing the dimensions much.

Originally posted by brickman
Great advice tech, I will do that. Streets, now I understand why everybody has .030 bored motors, it's just the first step you go to to get an even bore job, very good thanks.

And thanks to everyone who answered, you have solved another puzzle for me.

What do you know, senior statous! About time I get acknowledged for my age !!!

Heck, everyone answered my questions also. now I don't have to bother everyne again. Thanks

There are a few things I know Jerseyrodder, one is that the cylinder walls are measured with a micrometer for a certain size. Let's just say for example your stock bore in your engine came out at 3" exactly. Now the engine has 100,000 miles on it and it has loosened up the piston in the cylinder. There is an amount of wear that can be tolerated and honing will fix, say .04000 of an inch. They will measure the cylinder all the way to the bottom of the stroke because too much tapper is bad too. After that certian amount, you have to bore it out. I'm sure there are fellows here that can give you exact dimensions.
Also scaring such as lines in the cylinder walls can mean metal has been scratching there and you will loose compression down through those scratches.
I am no expert but there is very little tolerance that a ring will absorb before you suffer power loss and eventually start using oil.

Thanks for the info guys. This is why I love this site. No lame explanations. Hard facts. Now I can take part in a conversation with my machine shop guy and know what the hell he is talking about. Hopefully it just needs to be honed. ---Rich

Ok Streets ol' friend, I have a question for you. What is "Line Boring" and what advantage is it.

Does it have to do with blue printing an engine?

I have done several stock rebuilds in my day but always picked up the short blk's already assembled to get the warrenty's. This time I am planning and building this one myself and I am just trying to learn so I hope you all don't mind all the dumb questions.
The more I listen to you all, the more I am getting convinced that 400+ HP out of my 400sbc is no problem.

You would line bore for 5 reasons: Spun main bearing, insufficient crush gap, out of alignment main bearnigs, just an out of round main bearing hole, or if you bought new caps or somebody pulled the old ones and forgot where they went. The process involves milling the main bearing cap and block slightly. Then, the mains are installed and bored to their proper dimension. It's expensive, so I wouldn't buy a block that needs it. A line hone doesn't require the cap be milled, and cleans up minor damage or misalignment.

Where I get my machine work done, a line boring costs very little. Like 76 said, they cut the bottom of the cap slightly to make the *bore* smaller and then they run a true straight bore through them to make them round again. This ensures that the bearings will seat correctly and that the caps are not out of round and is a pretty good way to make sure that you get the same amount of clearance between all caps on a good crank. So it doesn't change bearing or main od size requirements which is good. The hone is just minor stuff though. Also, very much like Streets said, a *seasoned* block is often better than a new block since the core has already shifted and been put through its break in where it changes quite a bit during the heat cycling. I would recommend boring to straighten up the cylinders on all projects, and a minor true decking is often good too, this ensures that each cylinder is the same length and that you get a good seal on cylinder head gasket. This makes sure that the deck is exactly the same height from the crank centerline on both sides...something that stock cores often don't have. Just don't deck it too much since this *pulls* the heads in closer together and your manifold will not fit the way it would have before (also misaligning the intake ports, which can be cured by milling some material off of the manifold) since the heads are closer together and the endgap on the block to the manifold will be larger. Oh, and not to mention it brings up the compression. IMHO I would recommend that a core be tanked or baked, magged, given a boring, torque plate honing, slight cylinder decking if it needs it, and a line bore or hone, depending on which it needs. If the stock crank is being used then turn it and get undersized bearings. I feel that an engine with the right machine work done to it will be more dependable, less hassle down the road, less likely to give you surprises during assembly or later and will give you more power in some cases. If you are on a budget though, invest in a good cleaning, boring and hone, and just get the rest of the stuff checked if you are unsure. It's just money right???