Could someone explain what the terms "bored 30 over or bored 40 means?
I know it helps to increase horsepower, but exactly what is done?
Can this hurt the engine? How much horsepower can be gained?

What is the cost for a mechanic to charge for this?

Thanks as always

 

It's actually what happens during a rebuild if the cylinder walls are scratched or pitted.

30 over makes the cylindar 0.030 inches larger in diameter. It really doesn't add a terrible amount to displacement or horsepower. It just kind of happens when it happens.

 

overboring .030" is pretty much standard on a full engine rebuild. the machine work is relatively inexpensive compared to cost of a new set of pistons and labor cost of the rebuild.


[Modified by Turbo-Jet, 9:14 PM 12/8/2001]

 

Many times when an engine has high mileage or seen hard usage, the cylinder walls will be scored or scraped by the piston rings. As a result of this, the engine loses compression as gases leak past the piston rings (condition known as blow by). In a high mileage engine, the cylinder walls will even develop a ridge where the piston rings reach the top of their cycle as friction eats away the metal.

To eliminate this ridge and any scrapes, the machinist uses a boring machine to scrape and hone the cylinder walls. No-one rebores an engine unless it is required. As material is removed, the diameter is larger and thus it is no longer possible to use the old pistons; they're too small. To make it easier to get replacement pistons, since the diameter of the cylinders when the motor was new is known, the amount of material removed is standardized- 30 thousandths of an inch over the 'as new' diameter; hence the expression "30 over." Although this sounds like a small amount, in the close tolerances of an engine, this is huge.

There is a possibility of doing damage if the cylinder walls are thin or poorly made as this can lead to warpage and other problems but vast majority of the time, this is not a problem. In fact, most motors can be rebuilt/ rebored a couple of times before there's a concern. The machinist will know, based upon the measurements he makes, whether it's prudent to rebore or not.

The bizarre part is once they've made the cylinder walls perfectly round and smooth, they take a hone and lightly score the cylinder walls again. The pattern of this honing is referred to as hatching and is necessary for the new pistons and piston rings seat properly. Seating is the process referred to as 'breaking in' the engine. If the hatching isn't there, the piston rings will make too perfect a seal and the motor won't turn over.

If an engine was in such condition as to need to be rebuilt, it will make a marked difference in horsepower and performance when it is rebored although the displacement isn't normally changed all that much. What usually makes the biggest difference, however, is what pistons, cam, crank and other hardware are installed during reassembly. Since you have the motor apart, most people take the opportunity to upgrade and that's where the real horsepower gains are made. That's why when someone says they've bored 30 over, they normally follow up with what kind of pistons they've used and/or what fancy-schmancy cam they've installed.

I hope this answers your questions. If not, repost and we'll try again!!


[Modified by Mac, 8:01 AM 12/9/2001]

 

I like to elect my engine to 2, ten year terms. And there isn't necessarily term limits. If they want to go longer than that, great. ;) :lol:

Oh, not that kind of terms. Silly me.

 

As Mac says very well, you don't increase the cubic inch displacement too much by boring. If I remember my math, the volume of a cylinder is (PI x Radius squared x Height). A normal/standard 327CI Chevy has a 4 inch bore from the factory. A .030 over engine would have a bore diameter of 4.030. I don't remember the "stroke" off the top of my head, but if you do the math, a 327 bored .030 over works out to about 331 cubic inches as I recall. You figure the volume of one cylinder, then take it x8 for a V8 motor.

"Stroking" and engine means that the length of the standard piston stroke travel has been increased. You read/hear of members with "stroker" motors. That's what this refers to. If you look at my "formula" above, stroking increases the "height" of the cylinder, which also increases the total displacement. Hope this helps. Chuck

 

If memory serves, the bore of a 350 is 3.48" and the stroke is 4.00" and a 30 over 350 cid becomes a 355 cid but that's off the top of my head; don't quote me.

One of these years, I'm gonna get around to building myself a 'stroker' engine. From what I understand, the crankshaft from a small block 400 cid will fit a 350. The resultant engine displaces 383 cid. The main advantage of 'stroker' engines is increased torque but balancing becomes an issue. Imagine how much fun it would be to build a stroked 350, roller cam & roller rockers. Hmmmm, feel the power, the ultimate power!!!! Bwaahaaa Bwaaaahaahhaaahaa! :crazy: :crazy:

Sorry! I'm okay now. The pills are working. :crazy:

 

I think you got it backwards, it has a 4.00 inch bore. 327, 302, and 350 have the same bore, I think. I believe you have to have at least a 4.00 inch bore to use 2.02 valves. Don't forget sleeves. I have a 427 that is already 060 over. I think it will be less expensive to sleeve everything and start with new pistons than to buy a new 4 bolt block. It has a steel crank so it would be cheaper again to keep the crank and rods than to buy a new setup for a 454. But then again, I could be wrong.

 

RE: the crosshatch-
Crosshatching is done to provide a "fluffy" surface (like sandpaper) to allow the rings and the bore to mate to each other. In the first 500 miles (or so) the crosshatch and the rings "rub each other the wrong way" and the two surfaces eventually establish a pattern that is acceptable to both parties. The degree of Crosshatching (angle & grit) depends on the rings being used. For example, chrome faced rings break in differently than castiron unfaced rings. The machinist must make allowances and judgement calls for this based upon the ring manufacturers specs and his experience.

Edit- forgot to mention- final honing is required- without it, the surface is too rough and will cause premature ring failure. With too smooth a finish, the rings will not seat, and you will not have good compression. This same principle applies to wheel cylinders, calipers, and master cylinders.

RE: Overbore... pistons are not all created equal .... there is a lot of variation between manufacturers. A .030 piston from one manufacturer can be different than anothers. The machinist should have the intended set of pistons in his hands before he bores a block to make sure he leaves enough metal for the final hone step where the crosshatch is applied.

Pistons can be made in any oversize.... even .0015 if you want. It will just cost you an arm and a leg to have them custom manufactured.

If an engine has very little wear, and no taper, I have even bored engines .020 with no problems. .030 is a "usual" first bore because it eliminates any guesswork on the part of the boring machine operator. It takes a lot of time to continually re-mic the bores (using older equipment), and .030 usually will clean up most worn, standard bore blocks.



[Modified by Tom454, 8:55 AM 12/10/2001]

 

Mac:
You should have been an Auto Shop teacher. :yesnod: Extremely well defined. Congrats! :cheers: