283C.I.D. Overbore

[John M.]

Hi guys,

Just got a call from my machine shop telling me that .060 overbore will not clean up my original #s block ,says it will clean at .080 over is this too much for a 1960 283 or not? I have heard conflicting advice some say don't go over .060 and others say that 283's can usally go to 4.0" with no problems. If I should sleeve the block I will but not looking to spend money needlessly.

thanks,John Mc Graw

[John H.]

Re: 283C.I.D. Overbore

What year (casting date) is your block? Early 283's are a little shaky if you go over .060". Later ones had thicker cylinder walls and the casting cores were more stable (less core shift), although I don't know exactly when the change was made. A good machine shop should have reference material on this, and should be able to do an ultrasonic check in each bore to determine cylinder wall thickness rather than "guessing".

[John H.]

Re: 283C.I.D. Overbore

What year (casting date) is your block? Early 283's are a little shaky if you go over .060". Later ones had thicker cylinder walls and the casting cores were more stable (less core shift), although I don't know exactly when the change was made. A good machine shop should have reference material on this, and should be able to do an ultrasonic check in each bore to determine cylinder wall thickness rather than "guessing".

[Dwight Farmer]

Re: 283C.I.D. Overbore

John, you should have no problem on the '60. Mine is a May 60 car with the original bored 0.080 in. and its done fine for the last 12 years. No overheat etc. I understand back in the 60s, many of the "thick wall" Corvette 283s were bored 0.125 inches just to get the displacement up for drag racing.

[Dwight Farmer]

Re: 283C.I.D. Overbore

John, you should have no problem on the '60. Mine is a May 60 car with the original bored 0.080 in. and its done fine for the last 12 years. No overheat etc. I understand back in the 60s, many of the "thick wall" Corvette 283s were bored 0.125 inches just to get the displacement up for drag racing.

[Joe L.]

Re: 283C.I.D. Overbore

John----

As earlier responder John mentioned, sonic testing is the way to go. Even if the engine can be bored to .080" without "getting wet", you want to KNOW if there is sufficent wall thickness remaining to insure a durable and reliable engine.

Also, as earlier mentioned, core-shift may be the primary "unpredictable" that can affect whether your engine can stand the overbore. You can get a ROUGH idea of core shift by examining the camshaft bore from the front of the block. The camshaft bore is within a round boss on the front of the block. If the camshaft bore is well-centered in this boss, your engine probably doesn't suffer from significant core-shift. However, if cam shaft bore is significantly "off-set" and off-center, you have significant coreshift which does not bode well for large overbore potential.

[Joe L.]

Re: 283C.I.D. Overbore

John----

As earlier responder John mentioned, sonic testing is the way to go. Even if the engine can be bored to .080" without "getting wet", you want to KNOW if there is sufficent wall thickness remaining to insure a durable and reliable engine.

Also, as earlier mentioned, core-shift may be the primary "unpredictable" that can affect whether your engine can stand the overbore. You can get a ROUGH idea of core shift by examining the camshaft bore from the front of the block. The camshaft bore is within a round boss on the front of the block. If the camshaft bore is well-centered in this boss, your engine probably doesn't suffer from significant core-shift. However, if cam shaft bore is significantly "off-set" and off-center, you have significant coreshift which does not bode well for large overbore potential.

[Jeff]

No sweat for street use...

.080 over for a street motor, especially one slated for a collector car which will be pampered, is pretty much a no-brainer. Back when they were contemporary race engines, even early 283 castings were routinely punched .060 and then .080 and more for freshening after much use/abuse.

I wouldn't spend any time or money on testing for wall thinkness or shift. One thing that will help you is that your block is 'aged' by virtue of having gone through countless heating and cooling cycles and whatever 'moving' it's going to do is long since done. We never built a new race motor from a 'green' casting. We used to pull the 'aged' 4 bolt block from the transporter and put the new block in the truck to begin the 'aging' process for our next block.

By the time most blocks would have needed to be .125 they had been blown up. While .125 would machine up just fine in a static situation, in early 283s the thin walls between adjacent cylinders would overheat and the cylinder holes wouldn't stay round during a dynamic event. The rings wouldn't seal, resulting in tremendous blow-by and a loss of power from several aspects related to the blow-by. It was fairly common that a 292 (060 over) would outpower a 301 (125 over) and that remained the case until the 327, 4" bore block became avilable for use with a 283 crank, which combo became the 67 Z motor, and changed the whole 292 vs 301 power picture.

JP

[Jeff]

No sweat for street use...

.080 over for a street motor, especially one slated for a collector car which will be pampered, is pretty much a no-brainer. Back when they were contemporary race engines, even early 283 castings were routinely punched .060 and then .080 and more for freshening after much use/abuse.

I wouldn't spend any time or money on testing for wall thinkness or shift. One thing that will help you is that your block is 'aged' by virtue of having gone through countless heating and cooling cycles and whatever 'moving' it's going to do is long since done. We never built a new race motor from a 'green' casting. We used to pull the 'aged' 4 bolt block from the transporter and put the new block in the truck to begin the 'aging' process for our next block.

By the time most blocks would have needed to be .125 they had been blown up. While .125 would machine up just fine in a static situation, in early 283s the thin walls between adjacent cylinders would overheat and the cylinder holes wouldn't stay round during a dynamic event. The rings wouldn't seal, resulting in tremendous blow-by and a loss of power from several aspects related to the blow-by. It was fairly common that a 292 (060 over) would outpower a 301 (125 over) and that remained the case until the 327, 4" bore block became avilable for use with a 283 crank, which combo became the 67 Z motor, and changed the whole 292 vs 301 power picture.

JP