.030 over bore question

Complete waste of time. The purpose of boring is to make your cylinders "fresh". Only do it if the walls have wear.

strtrcr50 said:

Some people do it to help unshroud the valves.

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The BEST byproduct of .030" over or more is in fact... allows one to unshroud the valves on 2.02" and larger.

There are a plethora of .030" over pistons as well as .155" over pistons. This includes numerous compression heights, ring types etc.

Yes, in most cases it's due to the cylinder requiring the overbore to correct issue such as elongated wear.

dmc919 said:

ok thats what i thought, it just seems like when people are talking about the performance mods, the first thing they say is bored .030 over, like its some big performance adder.

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As mentioned... it IS a huge performance adder WHEN you have aftermarket cylinder heads.
If a person spends 1700.00 for nice heads with larger valves, why put them on a 4.00" bore or smaller?
Do some research on "flowed" cylinder heads. The company doing the benchwork almost never use a 4.00" bore. In most cases it's @ 4.060" or more.

To unshroud your intake air charge is a tremendous performance gain.

dmc919 said:

ok thats what i thought, it just seems like when people are talking about the performance mods, the first thing they say is bored .030 over, like its some big performance adder.

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People will also mention synthetic oils or advanced timing like its some sort of exotic upgrade.

I dunno, if I had a 306, I'd mention it in my list of mods, too.

347HO said:

As mentioned... it IS a huge performance adder WHEN you have aftermarket cylinder heads.
If a person spends 1700.00 for nice heads with larger valves, why put them on a 4.00" bore or smaller?
Do some research on "flowed" cylinder heads. The company doing the benchwork almost never use a 4.00" bore. In most cases it's @ 4.060" or more.

To unshroud your intake air charge is a tremendous performance gain.

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We're talking .015" of gained intake valve clearance... Is it really that much of a gain?

It was always my understanding that .030 overbore was simply to remove any cylinder wall defects on a damaged or high mileage motor. Now, to step up to a 4.125+ bore, I can see where the unshrouding effect comes into play, as well as the performance gain from more piston surface area (and increased displacement).

I guess I'm curious, too... I'd like to see flow numbers in a bore size comparison.

Gearbanger 101 said:

That's fine if you're running a deep breathing top end with a set of high dollar aluminum heads and are trying to eek out ever little pony there is, but for your average joe, its kind of a waste of time and money unless you're already in need of a rebuild, wouldn't you say?

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Actually, I do agree with you. I was just pointing out there are other reasons besides the added displacement. It's something I'm considering. I don't know about high dollar, but I do have twisted wedge heads and a pretty well matched induction setup.