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I've heard that people often bore out their cylinders .30 over when they re-build the engine. Does that mean the cylinder is .30 inch wider? How much of a horsepower increase would I get if I did that on an 1984 GT that has an all original, Holly 4 barrell, 175h.p. 5.0?
 

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what it means it that they are increasing the diameter of the cylenders by .030 or 30 thousandths in machinest talk. i would honestly say that just the new rings and everything will make the motor feel healthier. you might realistically see 15 to 25 hp over stock, but to me that's optomistic
 

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Yeah, boring it .030 over makes a 302, what, a 305? 307? You're not gaining a heck of a lot of cubes when you do that, but you will gain from the fresh rebuild. Like Geno said, a healthy engine will be your biggest gain.
 

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It makes it a 306cid
 

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The gain is negligible. It is done primarily for one reason, and that is to make the cylinder walls straight and uniform from top-to-bottom. In other words, it is to remove any "ring ridges" at the top of any cylinders, or to make any cylinders that are out-of-round round once again, or to remove deep scratches in any cylinders that cannot be removed by honing, etc.

Another reason to overbore a block is to unshroud large intake and exhaust valves, but I dare say hardly anybody here would have such a need to do so.
 

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Fox hit the target. That's the reason why you bore the cylinders, to correct cylinder wall imperfections. Just a .030 overbore wouldn't add much power at all by itself.
 

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Alot of people think it gives more power but all it does is take out the imperfections and gives you a fresh bore and tightens everything up like new
 

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I think even a 347 stroker adds about 40-50 hp but thats .60" over and a longer stroker
 

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WOW!! i didnt know that, I guess you learn something new everyday :bigthumbsup good to know.
 

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the gain from the overbore is negligable because the extra 4 cubic inches are offset by the extra friction from the increased cylinder circumference. the restored compression may help you though if thats a problem now.
 

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I think even a 347 stroker adds about 40-50 hp but thats .60" over and a longer stroker
A 347 is a 3.4" stroke and a 4.030" bore (.030" overbore). A .060" overbore - which is NOT recommended for a "factory" 302 block - would be a 352.
 

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hahaha 307 haha yeah it makes it 306 i would say like a 15hp gain and feels a lot stronger
 

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A 347 is a 3.4" stroke and a 4.030" bore (.030" overbore). A .060" overbore - which is NOT recommended for a "factory" 302 block - would be a 352.

Yeah you are correct it's .30" over I dont know why I said 60 And I am also not running a Factory block
 

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hahaha 307 haha yeah it makes it 306 i would say like a 15hp gain and feels a lot stronger
Yes maybe 15hp and stronger, but only because your old 302 was aging and losing hp from blow by. :bigthumbsup Old motor = loss of hp over time...:gringreen
 

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my 302 on the dyno made 6 HP more than my friends 306 on the dyno.
 

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You will feel a little bit of a difference in the fresh rings and bearings in the bottom end, maybe 15 or so horsepower with a full rebuild. It will be just a little better than stock, but barely noticeable. Plus if it's done right, you wont get that tick at idle if you had one before, and hopefully your getting your deck and heads milled while your at it. You may get about .10 or so compression gain too when milling the heads.
 
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