There is a limit to how much you can over bore a block before the cylinder walls become to thin. The common ones are .030, .040 and .060 bored over, obviously new pistons required. When a block has been bored .030 over it usually means it's been rebuilt once, but not always. It also adds cubic inches to the displacement, a 350 bored .030 over is 355 CI.

Scott


[Modified by Scott78, 10:43 AM 7/17/2002]

 

30 over on a stock bore 350 gives you 5 more cubic inches.
That is it.
Now you have a 355.

 

Years back you could get .010, .020 ...etc over pistons, and I think you still can. Probably special order? Rebuilders began to realize that boring .030" would clean up the majority of blocks. So, over the years the .030" bore became the "standard" for rebuilders.

 

what is the point in that, seems to me that 5 more ci really arent going to make much of a difference? or do they?

 

It's not done so much for the cubic inches but to refresh the bores. My 327 is now 331cid, hardly worth the cost of boring and new pistons (for a paltry 4 cid!), but it was recommended to "clean up" the bores. As I understand it, when you get to .060 range it's time to start thinking about a new block..... :cheers:

 

**********it adds a few HP due to a few more cubic inches,( somewhere between 5 and 10 hpdepending on the rest of the motor) and helps the rings seal better by having round cylinders. not a lot of HP, but add it all up from a bunch of small gains and they help quite a bit... better cam, intake, exhaust, etc.etc

 

Primarily done on high milage/damaged cylinder bores to clean up the bore and make rings seat like new again. Sometimes guys bore to add cube's and HP though... :cheers: :cheers:
Eddie

 

Ya, not of great significance unless your machinist says your block is already 40 and too thin to go 60 over...now what does 10 under mean ?? :seeya

 

sleaves

 

10 under means typically that the crankshaft wear areas were cut down .010 to clean them up and make the bearing surfaces smooth,and uniform across the length of the crankshaft. my L-48 had about 93k on it when rebuilt, and rec'd a typical rebuild job, .030 over on the block, .010 under on the crankshaft, the original rods were re-babbited,new rings and pistons.

 

Yes I do :cheers:

 

"bored 30 over" sounds cool when I say it. :cool:

Oh yeah, it was done to my block for all the reasons posted above.

 

Also, 355 looks :cool:er on paper, or on screen in this case, then 350.


[Modified by Fevre, 12:55 PM 7/18/2002]

 

Do you really need new pistons or can you get new rings to make up the difference?

:confused:

 

>>As I understand it, when you get to .060 range it's time to start thinking about a new block.....

Depends on the vintage. Early 283 blocks would take a .125 overbore (to 4"). That would create a 301. The 302 in the late-60's Z-28s was essentially the same thing except the crank was specially made because the 283 was a "small journal" crank and it hadn't been made in years anyway.


>> the original rods were re-babbited,

If they were, you are in SERIOUS trouble :D

"Babbit" is a lead-tin-bizmuth alloy that was poured into connecting rods & blocks and then machined into a bearing surface in the days before removeable bearings had been invented or were practical. The last Chevy to have them was the 216/235 straight-6 (I think 53 or 54 was the last year with poured rod bearings - the mains were removeable for several years previous)

When rebuilding a shortblock with removeable (insert) rod bearings, the rod caps are ground down a few thousands and then bolted to the rod. This makes the hole slightly egg-shaped. The assembled rods are then honed back out to the correct size. The process is called "resizing" and is unrelated to re-babbiting.

>>Do you really need new pistons or can you get new rings to make up the difference?

You don't EVEN want to hear the kind of racket a piston with 30 thousandths of skirt clearance would make. :D

To answer the question...YES, if there's enough wear to warrant boring the cylinders you MUST use new pistons because you don't just "bore a hole" to some standard size. Every piston is slightly different. You assign each new piston to a cylinder, number it, measure the skirt, and then bore the corresponding cylinder so that the correct amount of clearance is created. (typically about .0015" - note: that's one and one-half thousandths, not fifteen thousandths)

 

What kind of C/I and HP gain would you get if a 350 was bored .060? :confused:

 

Well, if a 350 bored .030 is a 355, then a 350 bored .060 must be 360cid. HP gain would depend on the engine.

 

I have a 69 block 3956618 Dated A139 which is bored .060 over and threw in a 400 crank to boot for a 388cu motor...looks so pretty I hate to put the body back on...not...
If you are going to rebuild a motor, might as well stroke it too. The torque you gain will be worth the effort.
Neil in Tenn


[Modified by Nomad78SA, 10:11 PM 7/19/2002]

 

Neil, I have got to admit, That ride IS sweet with or without the body! :cool: :cheers: Besides mine being a 72, our specs are almost identical. How many horses are you pushing? I've not yet had a dyno but was told by the previous owner I'm running around 430.

J Shipp