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Technical Pistons down in the hole variance

Discussion in 'The Hokey Ass Message Board' started by 30tudor, May 23, 2020.

  1. I put new flat top pistons in the 4 corners of my '64 327 block today, only had enough wrist pin keepers on hand to do 4 so I did the corners. This block has been tanked, bored 030 over and decked. It is very clean and is going together on an engine stand. The crank is a '67 with standard uncut journals. Rods are full floating Scat, also new.

    1 piston measures .007 and the others are .016 and 2 at .017 down. The two at 007 and 016 are on the same journal and the 2 at 017 are on the same journal at the opposite end of the block.

    I used a dial indicator on each piston to arrive at their tdc and a straight edge with feeler gauge to get my readings.

    How much variance should I expect from one to the other? What's acceptable? School me on how I should be measuring this please if my method is unsound.
  2. blowby
    Joined: Dec 27, 2012
    Posts: 6,592

    from Nicasio Ca

    Sounds like your deck job got a slanted cut but I would swap the two pistons/rods and recheck. Also make sure you are using the center of the piston to eliminate piston rock variances.
  3. Ericnova72
    Joined: May 1, 2007
    Posts: 369


    Did you check center-to-center on the connecting rods?? How about compression height on each piston??
    Crank checked for equal stroke at each journal??

    Very common for resized rods to vary quite a bit, since they just cut the cap, bolt it back together and hone it round....if it don't clean up good they cut the cap some more and hone some more. Since hone cuts from both cap and rod side of the hole, rod gets shorter every time the cap is cut.
    If the guy cutting the caps isn't worried about cutting each cap the same and just throws it on the machine an takes a "tiny" cut its possible for a rod to lose quite a bit.
    If the rods weren't all equalized in the rebuild process, you would have to go through and measure each rod, then height measure each piston and juggle short pistons to longer rods to even it out some.
    You've got individual stroke length at each journal too Front journal may have different stroke than rear journal. Factory stuff is tolerance all over the place..
    Don't forget we've also likely got two different heights on each deck in play.

    The two on the same journal variance, that can be in the rod, the piston, and the fact the two opposite deck heights on the block probably aren't the same.

    Plus you may have side-to-side and end-to-end variance on the block in play too. They are not very square at all from the factory, generally taller at one end due to how both decks are cut at the same time by pushing it through a very large broach. The whole thing springs a little coming in and going out the cutter.
    With new aftermarket rods and pistons, the block is likely your issue.

    Probably getting an idea on "how come race blueprinting costs so much" about now right?!!!
    Last edited: May 24, 2020
    Lepus, Deuces, ottoman and 8 others like this.
  4. sunbeam
    Joined: Oct 22, 2010
    Posts: 4,848


    I have set up a block in my mill with the crank rods and pistons and zero decked it. Turning every piston to TDC as the the cutter was at that hole. Milled until the lowest piston was skimmed and run a .040 head gasket. Number pistons, rods and send to have it ballanced.
    Lepus and 30tudor like this.
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  5. Mike VV
    Joined: Sep 28, 2010
    Posts: 1,917

    Mike VV
    from SoCal

    Where are you touching the piston for your depth measurement ?
    Pistons WILL rock in the wrist pin, giving widely varied "in the hole" numbers.
    You need to add some tape to the skirt, top and bottom to keep the piston from rocking in the bore. Even if you have only .003" or .004" piston to wall gap, it will still show up when measuring the depth.

    Desoto291Hemi, 30tudor and big duece like this.
  6. irishsteve
    Joined: Jan 10, 2017
    Posts: 727


    While it would be nice to have a engine that had the same compression across the board it wont matter how it runs.Most of us have done compression checks ,and had readings that varied 5-10 lbs. in several cylinders. Ive seen low buck do it yourself guys replace a piston in one hole with a 9:1 while the rest were 10:1 simply because they had one,or a friend gave it to them. Engine sounds fine when done.
    ted kovacs and 30tudor like this.
  7. squirrel
    Joined: Sep 23, 2004
    Posts: 46,099


    Do you know if the machine shop used a jig to square the block, when they decked it? Chevy blocks are notoriously unsquare from the factory. Do you have a way to measure the deck height at each corner, from the main bearing bore?

    But as mentioned, tolerances add up. Measure each rod, measure each piston, or use one rod/piston to check the 4 corners as suggested.
    Deuces, ottoman, VANDENPLAS and 3 others like this.
  8. This is an important thing to note when building a performance motor.

    SBCs are notoriously crooked. That is part of why they get decked. If you look at a properly built performance mill you will notice that the pistons are numbered this is for two reasons one is because there will always be a slight variance in weight and you need to know that for balancing purposes. The other reason is that pistons and rods often get juggled to get as close as possible to distance down the hole. The is because of variance in the pin height and and rod length.

    Most builders are not just balancing the engine but they are also shooting for a specific quench length (distance) that is measured from the top of the piston.
    Deuces, 30tudor and dana barlow like this.
  9. big duece
    Joined: Jul 28, 2008
    Posts: 5,913

    big duece
    from kansas

    I would wager to bet there is more human error involved in the decking of the block, opposed to finding different lengths of Scat aftermarket rods. I was taught once you find true top dead center, use a dial indicator straddled over the center of the piston for measurement. Zero out the indicator on the block, and then take a reading in the center of the piston to eliminate as much piston rock as possible as mentioned earlier. images (2).jpg One other thing to try is take ONE rod/piston combination, and install it in each corner of the block so you know for sure your using the same length combo for measuring. I have a tool similar to this one...
    Deuces, Budget36 and 30tudor like this.
  10. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,098

    from Brooks Ky

    Thinking about what you described...........You are basically saying you have 3 assemblies that are .016/.017 which is essentially saying they are all the same even though they are at different ends of the block.
    I would take the two pistons that you installed on the same journal at the end where you get the .007 reading and switch them. Then measure again.........
    Deuces and 30tudor like this.
  11. Driver50x
    Joined: May 5, 2014
    Posts: 71


    How are you going to use the engine? Is it a high end performance/racing engine? Or is it more of a mild performance, lower budget street engine? Answering these questions will help determine how close to perfect it needs to be.
  12. dana barlow
    Joined: May 30, 2006
    Posts: 3,887

    dana barlow
    from Miami Fla.
    1. Y-blocks

    .010 of each other is not bad,fine for running a round the street. If your building a all out race car,getting closer is a good thing. Even for racing with in .003 is thought of as real good.
    Driver50x, Deuces and 30tudor like this.
  13. The block in my 56 got decked all caddy wompiss. Not to say what others have mentioned all ready can be the issues as well.
  14. saltracer219
    Joined: Sep 23, 2006
    Posts: 749


    Use the same piston/rod assy. to check all 4 corners. that will tell you if your deck is off. This assuming the crank stroke is accurate and the main line is straight.
    Deuces, Desoto291Hemi and XXL__ like this.
  15. Thank you all. Here is what I'm working with so I am not getting the reading from exactly the center of the piston but I am getting it from above the wrist pin and not an outside edge. I don't have a straddle type holder. This is a street motor and in fact the pistons all have their rings on while I'm doing this. Measuring the depth was mostly an afterthought or curiosity after putting them in. I will take the .007 assembly out today and move it to at least one other hole for comparison.

    DSCN2615.JPG DSCN2617.JPG
    Deuces likes this.
  16. 2X, I use to use a slip pin and use the same piston in all four corners. My machinist thought I was nuts when i wanted to straighten out a .005 difference. This was on a 15 to 1 menthol motor.
    Deuces likes this.
  17. PackardV8
    Joined: Jun 7, 2007
    Posts: 888


    Agree the SBC blocks are likely to be out of plane in several directions. Unless the block has been square-decked with a BHJ fixture, don't even bother checking the deck heights. You'll just get frustrated.

    Agree reconditioned rods are likely to vary in center-to-center length, so unless they've been reconned by boring on a Tobin-Arp machine, don't bother checking deck height.

    However, the OP is using new SCAT rods. Every set of those we've checked was dead-nuts on C/C length.

    The OP is using a STD crank, but whether from GM or when reground, the stroke length can vary a few thou.

    jack vines
    Deuces likes this.
  18. I moved the assembly of .007" to another hole and found it to be .015". At what point in difference should I become concerned? I'v seen as much as .010" so far, if the others aren't any worse should I run it, am I being too anal for a street motor? What difference would you guys be comfortable with?
  19. ^^^^^ Ask yourself if all chamber in heads are of equal cc's? You can chase this like a dog does it's tail or just settle
    Kevin Ardinger and Driver50x like this.
  20. big duece
    Joined: Jul 28, 2008
    Posts: 5,913

    big duece
    from kansas

    Was the block line bored? If I was paying good $ to have the deck milled down to what would have been said parallel to the crank centerline, then that's what I'd expect to have done. If your engine guy sold you blueprinting services, I would question it. I would check all work performed, if you have a bore gauge... For performance, you may never see the difference.
  21. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,098

    from Brooks Ky

    Ok, when you moved the .007 to another suggestion was to swap it for the one that was on the same journal and known to be .016/.017. If you swapped them rather than just moving to a different hole, it would give you an indication of whether the deck was square. In other words, if the .016 piston was put in the .007 hole........and now it reads .007 ( or close), then the deck is most likely cut on an angle. Conversely, the original .007 piston should now read .016 (or thereabout).
    No you are not being anal by being concerned about what you are doing. Any time you try to "do something well" and learn something in the process.......that's being smart, not anal.
    If it turns out that the deck is cut wrong, tell the machinist to fix it. He should want to correct it if he is conscientious. Have him take both decks to .007. He may have to go to .005 to get it right.
    Do you understand what I'm trying to explain, that by switching pistons you can tell if it's the deck that's the problem. Good luck!;)
  22. indyjps
    Joined: Feb 21, 2007
    Posts: 3,826


    Ha, Ive done that. Talked a machinist into only boring one hole, he made me swear I wouldnt put his name with that job. Threw a .040 forged 2 relief, in with 7 cast .030 4 relief that came out of the engine.

    The engine owner was a friend of friend, had no cash, and just ran the hell out of this car. I was curious to see what would happen. Nothing....... ran as hard as it would go everyday for over a year, then the car was sold.
  23. Deuces
    Joined: Nov 3, 2009
    Posts: 18,539

    from Michigan

    I also have one of those with a digital 0-1" travel indicator....
  24. Joe H
    Joined: Feb 10, 2008
    Posts: 901

    Joe H

    On a recent 455 Pontiac, I measured each cylinder for piston depth using the same rod and piston. By using the same piston/rod, I was able to see the slant in the factory machining front to back, but also left to right ( heads were not setting 90 degrees to bores).
    My machinist used a heavy steel fixture to deck the block using the crank bore to locate off of. He also used the front two holes as starting point since they had the least amount of depth. I wanted them even or below, nothing stick above the deck.
    When I assembled the engine with eight rods and pistons, I found all eight holes were different by .010" to .012". Only the #1 hole was right. I assumed each new rod would be the same and each new piston would be the same, they were not! I swapped the worst ones to get them closer, but no way was all eight going to be even.

    So ask your self these questions,
    Do the cylinder heads have identical cc's in each chamber?
    Are the gasket going to crush to the same thickness?
    Were the bores machined to the exact bore diameter?
    Was the crankshaft polished or ground to the exact same diameter and indexed 90* from each other?
    Are the bearings the same thickness?
    How about spark plugs, are they going to change the cc's per hole?
    Do the heads flow the same CFM in each port?

    Lots of variables to deal with just for a street engine to bomb around with.
    30tudor likes this.
  25. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,098

    from Brooks Ky

    Those are all good points Joe, but thats why you check as many things as you don't want the variations to be any worse than necessary. I've always thought the old saying "integrity is doing the right thing even when no one is watching" basically applies to everything someone does in life. If you always try to do small things the best you can, then it usually carries over to the more important things . Sometimes finances dictate choices, and sometimes other factors come into play that force a choice to be made. My feeling is that building a hot rod and its attendant engine is a chance show yourself what you are capable of.
    Last edited: May 25, 2020
    30tudor likes this.
  26. sunbeam
    Joined: Oct 22, 2010
    Posts: 4,848


    If you move the indicator to a point farthest away from the piston pin and rock the piston you will barely see the dial move.
    30tudor likes this.
  27. Thanks all. I'm still learning. To be sure the same rod and piston assembly should be used in at least the 4 corners to get started with. Lesson learned. At this point I have to think one side of the block was machined true to parallel to the crank the other side not so close, looks like there is a slope. I will drop by a performance engine shop tomorrow and discuss this with them. I will also drop by the machine shop that did the work on the block tomorrow and have a conversation with them. What they consider allowable may shock me but I'll ask.

    The remaining circlips will be here in another day or two and I expect to either take the crank out and send the block out again or put the remaining pistons in, measure them also, and if they're all with in .010" screw the heads on and go.

    I'm still wondering from my original post what amount of difference does anyone see in their own builds and where do you draw the line.
  28. Joe H
    Joined: Feb 10, 2008
    Posts: 901

    Joe H

    I very seldom check them unless the customer asks. The factory rebuilds I do wouldn't notice the differences.
  29. patsurf
    Joined: Jan 18, 2018
    Posts: 365


    most common sense answer so far!!
  30. dreracecar
    Joined: Aug 27, 2009
    Posts: 3,005

    from so-cal

    Compression hight is changed all the time in race engines to even out the cylinders, as long as you have enough piston to valve clearence , you will be fine and .010 difference isnt that much
    30tudor and saltracer219 like this.

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