I have got numerous opinions on the subject. Some say yes do it to 6 over. Others say no it eventully messes up the block. Now I dont want to do something really stupid to my car. So could someone please explain the whole process to boreing(sp?) out an engine and what it means to go like 6 over or 4 over. Also please tell me if it really does ruin the block after so long. Thanks
Boring is the process of cutting away metal from the cylinder walls, in order to 'true' the surface. This is typically done when the engine is rebuilt. Since metal is removed, the cylinder bore is now a larger diameter than it was. It is 'oversized'. This oversize is described in thousandths of an inch (.001" is one one thousandth of an inch). Typically, the cylinders are bored out only as much as they need to be to be uniform. The many miles put on an engine causes wear in the cylinder walls, as the pistons have moved up and down untold millions of times. Over time, there will be a lip at the top of the bore, and the bores can be a little out of round. Again, we are talking very small amounts here. I just had my 5.0's engine rebuilt, and the block's cylinder bores was taken .030" larger than the original bore. So my engine is "thirty over", in gearhead speak. .060" would be 'sixty over". I will assume this is what you are hearing people talk about.
Boring an engine out, at least in the Mustangs, will gain a little bit of displacement, but not much. It is far too expensive and invasive to do just to get a few more cubic inches. The entire engine has to come out, apart, machined, and then new pistons and rings installed, etc. A total enigne rebuild. Now, back in the 1950's, some of those old cars had engines that could be taken .060" or even .080" over bore. These old engines had thicker iron cylinder walls than the more modern Mustangs of the sixties.
Once a typical Ford small block is take .030" over, I'm pretty sure that is all the further it can be done. If my engine blows up, I might need a new engine block! There just isn't as much metal. It can be taken .060" over (I think), but then you will run into constant overheating issues. Most machine shops probably wouldn't do it even if you begged them to.
As for big blocks, well, I don't want to comment on that because I haven't owned a big block since my old Thunderbird and I'm not too familiar with them.
If your engine needs a rebuild, any competent machine shop will already plan to bore out your block. If you really want to add some displacement, that would be the time to get a stroker kit. Suddenly your 302 inch motor is a 347 inch motor! Like magic. The kits can be had in the $1,000 or so range for the parts.
If you have a nice healthy engine, and I hope you do, there are plenty of ways to add some power without pulling the motor apart. Hotter cams, an aluminium intake, headers, etc will make a big difference, and you won't need to waste money on an unnecessary rebuild.
Check your local library and see if they have books on engine rebuilding. You'll learn a lot just by reading through them
Michael
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1966 289-2V C-4 Convertible. All original drivetrain. Factory center console and power top. Dual res. MC and power brake upgrade. Rust bucket brought back to life, I am the 2nd owner of "Lucy".
1990 GT 25th Anniversary 5.0 AOD Convertible. Dynomax cat-back, K&N, the usual crap. Welded in subframes, welded in reinforcement plates (Ford riveted them in. Go Ford! )New engine and transmission. Date coded radiator hoses. Second owner of "Katie".
Small block Fords can be bored .040 over but not further, if you go more than that you risk hitting a water jacket or developing overheating problems.
A .040 overbore will net you about 5 extra cubic inches not enough to make a differance in performance. So the only real reason to bore a block is to true up the cylinders.
During rebuild you should be checking for cylinder wear this will show up as bore taper or a differance in cylider bore diameter from top to bottom with more wear at the top of the cylinder than at the bottom.
If there is a differance of more than .010 you need to have the machine shop bore out the block but go the minimum necessary. If your block wear measures less that .010 you can have the block honed and run .010 over rings.
The guys that are telling you to go .060 over are probably Chevy guys 'cause they obviously don't have a clue about a Ford.
Cubic inches in SB Fords are gained with stroking the motor not an overbore, there are a ton of stroker kits avaliable, deal with a reputable company and you should be fine.
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1966 Fastback
1997 V6 Mustang
1998 GT
"Government is not the answer to our problems, government is the problem."
This is about the most helpful imformation I have gotten in a very long time. Thank you to you both.
Another question tho, I have a 351 cleveland engine. Its the original engine from the car. I believe it would be a good thing to rebuild considering it has never had one. Now is this something I could do myslef after a lot of studying and trial and error on other engines. Or does it have to be done by a machine shop incase it needs to be bored?
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1970 Mustang Grande with the original 351 cleveland.
Hello. The machine work would need to be done by a machine shop. It's not that the work involves knowledge of some dark art that you are unfamiliar with or unwilling to participate in, it just requires the use of some pretty expensive machinery. At least, expensive from the hobbiest's perspective. Most people aren't really set up to bore blocks and plane heads in their garage. If you are going to rebuild it, it will need to be bored out a bit, because, as was mentioned earlier, there will be a step at the top of the cylinder at the end of the piston ring's upward travel. The problem with that is that the rings wear down slightly with use also, so, when you replace the old rings, the upper, outer edge of the new ring goes up slightly farther than the old one did and hits that step pretty hard, damaging the the new piston or ring, or both. You would want to bore it out the minimum needed to clean up that's also a size that you can use over the counter pistons for, like .010 over. Hope that helps.
So I could do the engine rebuild myself, but at some point have to go to a machine shop. Either way im going to have to replace the pistons and rods correct? Now I took note of what MizzouMike76 said at the end of his reply about doing other things for extra power. How big of a difference would it make by going from a 2 barrel to a 4?, and what all would I have to change out other then the carborator and intake?
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1970 Mustang Grande with the original 351 cleveland.
Hi again. To change over to a 4 bbl all you need to do is replace the intake. That, in and of itself, would make the car a bit quicker, but to get the most out of the 4 bbl you would also want to lose the original exhaust manifolds and go with some freer breathing headers. Between the two, you should pick up about 35 hp. The stroker kit that he mentioned would help a bunch. What that does is increase the distance from top to bottom of the piston's travel. The farther down it goes, the more air/fuel mixture it sucks into the combustion chamber. The greater volume of air/fuel mixture causes the compression to be higher at the top of the piston's travel, both of which are good things. More fuel burning under higher pressure means more push downward. The only downside to that is that the longer stroke slightly lowers the maximum rpms the motor can turn without flying apart. Not by much, though. If your car will do 135 mph right now, stroking it would drop that to maybe 132mph. You give up a tiny amount on the backside to pick up a whole bunch up front. For a street car, or a drag car, for that matter, it's well worth it. If you plan on running NASCAR or something, you would need to go ahead and get a big block.
As far as doing the work yourself and having the machine shop do just the machine work, that wouldn't really save you much. Nothing if you figure your time as being worth anything. If they do it start to finish, you normally come away with a one year/12,000 mile warranty, which is a good thing. If you do it, you get zilch for a warranty. Something to think about.
You know honestly all I want is to have full part in everything that happens to my car. Or well as much as possible. The warranty I could careless about. If I mess it up its on me and not anyone else. I dont really trust the mechanics here in my town to much. So atleast if I do it myself I will know its either done right or I will need to buy another engine haha. I have asked a lot of people about this type of stuff and they all tell me they have nothing as far as advice goes to help me out. So thank you to you all for all the advice and information.
Cobra6 The book you reccomended is a good book, only thing is I can order one from a catalog that I got that deal with my engine specificaly. But I will take a look at that book also. I sorta have family who own other ford engines so knowing as much as possiable is a very helpful thing. Thank you to you all again.
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1970 Mustang Grande with the original 351 cleveland.
Boring Engine Block. Is it Good or Bad? 
)New engine and transmission. Date coded radiator hoses. Second owner of "Katie".
The machine work would need to be done by a machine shop.
It's not that the work involves knowledge of some dark art that you are unfamiliar with or unwilling to participate in,
it just requires the use of some pretty expensive machinery. At least, expensive from the hobbiest's perspective. Most people aren't really set up to bore blocks and plane heads in their garage. If you are going to rebuild it, it will need to be bored out a bit, because, as was mentioned earlier, there will be a step at the top of the cylinder at the end of the piston ring's upward travel. The problem with that is that the rings wear down slightly with use also, so, when you replace the old rings, the upper, outer edge of the new ring goes up slightly farther than the old one did and hits that step pretty hard, damaging the the new piston or ring, or both. You would want to bore it out the minimum needed to clean up that's also a size that you can use over the counter pistons for, like .010 over. Hope that helps.
