Understanding Core Shift in the 455 block- why sonic checking is so important.

Discussion in 'Street/strip 400/430/455' started by Jim Weise, Feb 14, 2009.

  1. PunchT37

    PunchT37 BOP Sympathizer

    Because the walls would get too thin. A block is cast with a certain amount of wall thickness.
    Example; ALL Pontiac blocks are the same external dimensions. They started out in 1955 with a 287 cubic inch and kept that block all the way through to 455 CID. The deck height at 10.24 and the bore space at 4.62 didn`t change. Nor did the head bolt pattern. There are no big and small block Pontiacs. BUT, that doesn`t mean one can bore out a 287 to a 060 over 455. One would have to have inch thick walls to do that.:shock: The block would be very heavy and would over heat with too thick of a wall.

    Course I left out the 265/301 blocks out. They were late, low deck bastard blocks that most don`t use.
  2. MN GS455

    MN GS455 Well-Known Member

    Nice piece on sonic testing. I am a firm believer in sonic testing as well.

    There is another important factor to consider after sonic testing, and that is final bore position. When I bore a block (after line honing and truing the decks with a BHJ Block Tru) I will have my sonic test data sheet right there to reference.
    After centering the boring head on the BHJ Bore Tru fixture, I sweep the head down in the cylinder bore with the dial indicator and find out where it will be removing the most material to correcty true and index the bore.
    If it looks like it will be cutting heavy on the thin part of the cylinder, you have to decide if correcting the bore position is worth sacraficing bore integrity...sometimes you can split the difference, sometimes you are just left to bore off the original position. in extreme cases, you may need to move the bore over to maintain thickness. This is a "last resort" option and I would never use such a block for a race engine where the bores were that far out of index.

    That said, Block filler goes a LONG way to strengthen the cylinder walls, especially in a thin or shifted block. It will make the engine run cooler, and the rings last longer. If anyone is interested, I could elaborate on that subject a bit more.
    There is almost no engine block that won't benefit from some amount of block filler.

    Ron @ the machine shop.
    Julian likes this.
  3. Slugger

    Slugger Slugger


    I was just asking why moving the cylinder wouldn't work to center the bores in the sweet spot when I read your post. I was wondering if it would wise to sleeve standard bore block back to a standard or .010 bore that wasn't overbored from previous overhauls?

  4. staged70

    staged70 RIP

    Around here a sleeve is 120.00 per cyl thats 960.00 for the whole block. If you just do a couple cylinders I guess it might work. Can you build a race block with sleeves and block fill then bore the snot out of it?
  5. MN GS455

    MN GS455 Well-Known Member

    Slugger and Staged 70,

    Most blocks will take a .020" or 030" overbore. I don't know of any shelf stock pistons that are .010" , Sleeving the whole block is kind of impractical from a cost standpoint. I do it now and then with some NHRA Stock/Super Stock engines for 2 reasons though. First, to get a block back to a legal bore size after it is max bored and worn. Second, I can use a thin wall (3/32) sleeve made of ductile material which is 100,000psi tensil strength VS stock which is 30,000. These sleeves offer much better ring seal, bore and ring life. Again, very expensive, but it allows me to index the cylinder from the start as well and have uniform sleeve thickness.

    A block with ductile sleeves requires less block filler to do the same job, and you can't beat the RZ, RPK and RA wall finish capability for the rings. Ductile sleeves love the new Advanced Profiling steel rings from Total Seal too.

  6. gmcgruther

    gmcgruther Well-Known Member

    Here is my question for you engine machinist, If the care shift is out of wack in the lifter bore area, that means the cam bores were out to, yes that is corrected by buick when bored but as a engine machinist I have seen several Chevy , Ford , and Pontiac's that were not cam bored properly ! Has there been anything like that in the Buicks ? I know there is about 10 to 20 hp worth of power loss if not centered correctly.. As a late engine machinist , I never once worked on any Buicks. I only changed the oil and filter or your typical tune up, but never machined nor rebuilt a Buick. My time has come for me to get schooled in Buicks and know whats out there. I personally thank you guy's for the information you are providing , and that's why I have asked some questions people are not to awear of. Has any one deburred there engine block ? I have done this on all engine builds. David Vizard explain why you should deburr you engine . He say's it does'nt give the engine any sharp edges to possible start a crack and destory your engine and saves your skin the best....
  7. Mark Richards

    Mark Richards Active Member

    I had tossed around the idea of going all out and sleeving an entire block so that one, I could reuse my forged .030 over pistons and two, add durability to the block. I don't know about you're neck of the woods but here in North Texas, it's getting difficult to find these old gems hanging around your local salvage yard. It seems that just about all you can do these days is search out and buy parts from your local "Amazon Parts Store":Dou:.

    I am the proud owner of two blocks .030 over and the other .060 over. The .060 was pulled from a '72 225, had been rebuilt and looked like a great contender to replace my dying .030. A couple of days following the transplant, the thing started to rattle. I said bearings, Pops said he didn't hear anything, pulled the pan and viola, the rod and main bearings were toast. We sent it to the machine shop for a check up and he said the block was twisted. I was not as informed as I am now with all the ways that a block can be "twisted" but something was wrong with it. It sits in the shed at my dad's house and, it being the only available contender, may find itself back from the grave with 8 new transplants. 8-900 dollars for a stronger block with fresh tubes is suitable compared to purchasing a short block from parts-train.com just to use the block. Springing for the TA455 would be like buying a diamond for your wife but having to save up for the ring...if the military paid better, maybe, but this guy is trying to stay in the game on a more reasonable budget.

    Would custom pistons at .050 with a half-filled block in my .030 motor, sonic check permitting, be better than going for the full sleeves in the .060?


    Mark Richards
  8. Jim Weise

    Jim Weise 1000+HP

    You should have no problem finding a suitable rebuild candidate in TX. You can probably by a complete engine as a core for half the price your talking about here. Some locals on the east and west coast they are getting harder to find, but should be plenty in TX.

    I would not sleeve 8 cylinders. While stronger in the cylinder walls, overall block durability would be compromised.

    Not to mention that it doesn't make financial sense at this point.

  9. supremeefi

    supremeefi supremeefi

    However don't take your sonic check numbers to the bank. Let me explain why.
    I recently had an Olds 455 block sonic checked. It showed clear "core shift" in one direction, on one side, or did it.
    I use a BHJ BorTru plate when machining all my builds.

    After installing the plate on that same bank it then became obvious that the block was improperly machined from the factory. The cylinder location was off both at a 12-6 o'clock location and at the 9-3 'clock location as well as at an incorrect angle. Once it was bored and re-checked it brought it in to a much more uniform dimension. However without the plate it would have been just a guess as to which way to compensate.
    300sbb_overkill likes this.
  10. Brian Albrecht

    Brian Albrecht Gold Level Monkey

    Great thread, thanks to all that contributed!
  11. matt68gs400

    matt68gs400 Well-Known Member

    IMG_9195.JPG IMG_9192.JPG For the lifter bores, it seems like looking at the machined top of the lifter bores can be deceiving. Best to measure thickness on both sides with a micrometer?

    If there is a fair amount of core shift, are these still acceptable blocks for under 500 hp?
    Gary Farmer likes this.
  12. Gary Farmer

    Gary Farmer "The Paradigm Shifter"

    I'll answer this question if no one else will.

    Stock-moderate builds will be fine. They survived all this time lugging around huge land yachts for many decades, and I seriously doubt they were babied the entire time.

    Nothing is 'perfect', it's just that the thin walled 455 needs all the 'perfection' it can get in order to hold together in high powered builds, especially when people have their names and reputations at stake.

    It's simply good business sense, nothing wrong with it.

    As far as throwing away all the 'junk' blocks that aren't 'just so', it's ridiculous. If we were to take every engine ever made and sonic test them, we'd be shocked at just how off spec they all are, with relatively few being spot-on or even close to spot-on.

    It would make those blocks incredibly valuable, however, at least to hardcore racing enthusiasts.

    Makes sense to me anyway.
    rmstg2 and matt68gs400 like this.
  13. Mart

    Mart Gold level member

    Quick question, on these sonic checkers, is the probe radiused for the most accurate thickness measurements?
    Have a flat surface on probe end of the one I'm using to check my spare block. Flat surface checking it is very accurate, seems I can get readings that have a variance checking bores.

    Attached Files:

    Last edited: Jan 20, 2018
  14. Jim Weise

    Jim Weise 1000+HP


    On they 455 bore size the issue is not too bad, but if your checking a 350, it can be an issue. My probe was having issues, so I bought a replacement one that was about half that diameter.

    I have heard of guys rounding two sides off that diameter probe to help that issue.

  15. Mart

    Mart Gold level member

    Ok, thanks Jim.
    I figured there might be a more accurate solution.:)

    This is Derek's sonic tester and I didn't want to modify his probe until he see's that it is necessary.:)
    Last edited: Jan 20, 2018
  16. 8ad-f85

    8ad-f85 Well-Known Member

    There's some good sonic tester info on Speedtalk, esp. about altering the probe and dealing with calibration.
    Olive oil works good as a fluid, or spit.
    There's nothing entirely accurate about it anyways regarding cast iron.
    There's nothing like finding a thin spot after $2k worth of port work, despite the tester's reading.
  17. Mart

    Mart Gold level member

    The vasiline seems to give very accurate readings on flat surfaces. Mic'd China rail @ .297 & sonic tester is within .005!
    I'll check out the probe mods.....
  18. 8ad-f85

    8ad-f85 Well-Known Member

    It's when you have irregular surfaces on the backside that it gets tricky.
    Water jackets can be really ugly.
  19. Mart

    Mart Gold level member

    Yup, I understand the rugged surface inside casting. Just have to find a nominal overall size. :) Im pretty sure I can mod and get this probe to provide fairly accurate readings. We shall see....
    8ad-f85 likes this.
  20. Mart

    Mart Gold level member

    Well, my SB 1970 350 block checked out real well. The modded probe works good. Thanks Jim for the info provided! 1516805664466-1128811810.jpg
    300sbb_overkill likes this.

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