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Thread: Measuring Cylinder Bore for Forged Pistons

  1. #1
    Garrett booster
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    Measuring Cylinder Bore for Forged Pistons

    Hi everybody. I have a low-mile 2.5 TBI block that I'm planning to "build" for my turbo Spirit. Before taking it to the machine shop, I wanted to learn how to measure stuff myself with my new micrometer and bore gauge. If anyone could "check my work", I'd really appreciate it. Measuring thousandths-of-an-inch in new to me haha.

    First, did I measure out 3.445 on the micrometer correctly?


    And second, where can I find the tolerances for our engines?


    As I checked taper and out-of-round, everything was within .001 tolerance (most of the measurements were within .0005 except one). Hopefully I'm interpreting this correctly.

    Am I correct in saying that this engine is within spec? Also, what kind of tolerances are y'all running with forged pistons?


    Thanks

  2. #2
    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: Measuring Cylinder Bore for Forged Pistons

    It's more the feel of getting the measurement with the mic when using the telescoping bore gauges. I saw many of my students crush it down and make a smaller measurement as a result.

  3. #3
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    Re: Measuring Cylinder Bore for Forged Pistons

    Yes I heard that was an issue. So I set the measurement of the micrometer first (after I zero’d it gently with the little bar it came with). Then I placed the bore gauge in the micrometer and zero’d the bore gauge. Then I used that to measure the cylinders.

    Hopefully I didn’t crush anything. The trickiest part was figuring out the washer situation with the bore gauge.

  4. #4
    turbo addict
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    Re: Measuring Cylinder Bore for Forged Pistons

    As far as clearances, it depends on what forged pistons you were using. Venolia pistons called for crazy high clearance like .0075" That will make your 2.2/2.5 sound like a diesel until warmed up. I use .004-.0045 for Ross, Diamond, JE, Wiseco, etc. It also depends on how much boost, power adders you plan on running. The pistons you bought should have come with instructions to determine proper piston to wall.
    You really should be using a deck plate to measure, bore, hone, cylinders. Those cylinders will move around some with deck plate torqued up. I know some picky people even torque up a transmission case to the block. IIRC, Cindy rents deck plates out out. Someone in your area may even own one you could borrow/rent.
    Good luck,
    Todd

  5. #5
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    Re: Measuring Cylinder Bore for Forged Pistons

    Thanks, Todd. Ah so the bores will have to be machined for forged pistons even if they are close to stock spec. I didn’t realize the clearances were that “big”.

    I know someone up here in the northeast who might have a deck plate.

  6. #6
    turbo addict
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    Re: Measuring Cylinder Bore for Forged Pistons

    Most machine shops want the pistons in-hand so they can open up the bores to attain that #. You CAN order forged pistons based on what your actual finished bore size is. They will make the appropriate adjustments based on what you specify for piston to wall. That is the beauty I guess of ordering pistons. You can get them YOUR way, not what someone stocks. Yes forged pistons call for much looser piston to wall than cast pistons.

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