Author Topic: block boring  (Read 5427 times)

dab67

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block boring
« on: January 20, 2006, 02:04:16 PM »
 ??? Anyone know how far you  can bore the 305, 307 and 327 blocks of the late 60's and early 70"s? Is there a spec book somewhere for each casting number that would give you this info? I tend to see when looking to purchase a block that some are bored +.30. But what is considered a save bore size if that is the correct way to say it for each block.

Thanks CRG!

Buddy

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Re: block boring
« Reply #1 on: January 20, 2006, 03:33:58 PM »
They can handle .060 but I wouldn't go any more. You can also re-sleeve them to original bore.

Buddy

dab67

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Re: block boring
« Reply #2 on: January 20, 2006, 04:31:28 PM »
Thanks Buudy!

I've heard talk of going .120 over!!!!!!!! To mean that seems awfully extreme if not impossible!!!!!!!!! Is the +.30 consider just a clean up of the cylinders?

dab67

asm69

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Re: block boring
« Reply #3 on: January 26, 2006, 03:58:36 AM »
Is it considered a safe practice to sleve cylinders. My 302 block may have a sleve in one of the cylinders. I'm
not sure. But, I will be finding out shortly.

dab67

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Re: block boring
« Reply #4 on: January 26, 2006, 12:41:41 PM »
asm69:

From what I have been told, it is better to re-sleeve than continue trying to bore. I haven't done either as of yet, boring continues to reduce the thickness of the walls decreasing the strength, I have been told it can cause, if over done, piston slap. Like I stated this is what I have been told. I'm new at this also and have gotten a ton of great information from the people here at CRG!

dab67

CNorton

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Re: block boring
« Reply #5 on: January 26, 2006, 04:58:25 PM »
Blocks cast after the mid-sixties shouldn't be bored more than .060.  Any more than that and you're playing with the distinct possibility of a split cylinder wall.  The most serious problem will not come from the engineered dimensions but from the fact that the casting process allowed for a range of "core shift" that can result in the cylinders, lifters, etc. to be moved slightly to one side or the other.  In stock form they usually don't have a problem but when the boring process opens up the cylinder .060 or more there MAY be instances of cylinder walls thickness in the range of .100.  This is not conducive to good ring seal or reliability. 

Sleeves in small blocks are also problematic.  You may  get away with it on street motors but the relatively close bore spacing can cause adjacent cylinders to distort from the pressure required to push the new sleeve into place.  If you're trying to salvage a numbers matching block it might be worth the risk but, in general, when things are that far gone it's more cost effective to start over.

c

dutch

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Re: block boring
« Reply #6 on: January 26, 2006, 05:36:54 PM »
asm69:

From what I have been told, it is better to re-sleeve than continue trying to bore. I haven't done either as of yet, boring continues to reduce the thickness of the walls decreasing the strength, I have been told it can cause, if over done, piston slap. Like I stated this is what I have been told. I'm new at this also and have gotten a ton of great information from the people here at CRG!

dab67

    Unless it is a block that has big value - boring past a certain point isn't worth the risk of hitting water or in the case of a daily driver, the over heating / ring seal problems that will likely come with it...
    Piston slap isn't created by the boring process - its just a function usually, of poorly sizing the newly bored cylinders to the new pistons that would (should) be used in the process.
    I have no experience with sleeves - but would expect that unless its a big dollar situation (read non-daily driver vehicle) that it would too cost prohibitive for most owners to do, let alone the majority of most machine / engine shops to accomplish properly and well.
    For the availability of most any small (or big - for that matter) blocks, especially for situations where the vehicle isn't a collector piece requiring matching numbers - anything over a .030" bore would probably prompt me personally to look for another subject block to spend my $$'s on - especially if the goal was to use it for a long time or under severe conditions such as racing... IMHO  --Randy

dab67

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Re: block boring
« Reply #7 on: January 26, 2006, 08:10:25 PM »
And this is why I ask questions!!!!!!!!!!!!!! Look at all the information that has been presented just on this subject!!!!!!!!!!

Excellent!!!!!!!!!!!!!!!!11

Miclabel

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Re: block boring
« Reply #8 on: January 26, 2006, 11:52:51 PM »
My 67 283 is .120 over on the bore without sleeving it. Its called a four inch over bore. from what i understand very few castings allow this to take place. Most engines will support .90 but it's not recomended you go over .60. due to cylinder thickness.

69 4 Speed

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Re: block boring
« Reply #9 on: January 29, 2006, 05:49:29 AM »
I've gotta agree with dutch...If you've got to go over .30  on a block to clean it up...then try looking for something else.   You'll be money ahead, especially if you looking for something that is going to be used long term.     J.R.

sam

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Re: block boring
« Reply #10 on: February 03, 2006, 01:32:22 AM »
You should be safe at 30-40 over on a SB. In the old days they used to bore 283's to 120 over to make them 301's. Thats where the 302 Z/28's came from in 67. Sam