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Piston clearance

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  • Piston clearance

    First, thanks to all who answered concerning my carb problem. I've got some good ideas to go on.

    Now I have a question on piston clearance. What should the clearance be between piston and cylinder wall on a new(rebuilt) V8? Just bought a short block for a truck project and am wondering if the 6 thousandths I'm finding is excessive. Thanks

  • #2
    Believe it or not, Studebaker uses a "select fit procedure" to check piston clearance. I'm sure that this wasn't the answer you were looking for but, you'll have to get into the manual for the specific procedure, as it's rather protracted. Depending upon how you're measuring, (and that DO make a difference), yes, that is too much clearance. The book calls for .002, using the procedure.

    Sonny
    http://RacingStudebakers.com
    Sonny
    http://RacingStudebakers.com

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    • #3
      Sonny- We re-checked the pistion clearance and it is 4-6 thousandths, depending on the cylinder. I'm in a bit of a bind since the pistons were already 80 over. The easiest solution is take another block I have, boil it out and bore it to fit the pistions. The machinist I'm working with now has really reasonable rates so that won't be too expensive. However, the machinist also offered the solution of knurling the pistions. Do you think that would work? thanks- Leon

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      • #4
        Leon, is this a 'rebuilt' shortblock that has already been bored? If so, who made the pistons?

        The reason I ask.......if it is a rebuilt shortblock, I am in the same situation with a 259 block I have. It was a rebuilt shortblock with Jahns cast pistons. I pulled it apart to check the dimensions and assembly since I didn't know who built it. The bores are right on at .040" over, but the Jahns pistons have .006 - .007" clearance (!!). This is way to much, even more clearance than a forged piston would need.

        So, I am also trying to decide what to do. Current plan is to get new pistons, but the vendor is checking with the manufacturer to get the actual finished piston diameters, to ensure the clearances will be reasonable.

        Thanks, Paul
        Paul
        Winston-Salem, NC
        Visit The Studebaker Skytop Registry website at: www.studebakerskytop.com

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        • #5
          Paul- This block was rebored. And I'm sure the pistons are forged, but I don't know who made them. I'm going to repost my question on knurling- I'm hoping that is a solution. The problem is(as it always is) the mechanics and machinists are always using their Ford/Chevy experience and refuse to learn anything new. Even if I hand them a complete Studebaker shop manual before they start the job they don't bother to read it. Remember, most mechanics are guys who couldn't make it through high school. Almost always better to do it ourselves.

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          • #6
            Paul,
            I used Jahns, cast, solid skirt, racing pistons in a street engine I built in the 60's. I think clearance was .010"! I'm sure it was much greater than what was specified for a forged piston. You should identify the piston manufacturer and check with them.
            That type of piston, with it's loose fit, is a little noisey. It rocks some in the bore; and so the rings may wear on a radius. They are also very heavy. They are not the best choice for daily driver; but I don't think they are unuseable on the street.
            You may find you have no pin offset. Which also adds to the noise some, and makes a little more power.
            Mike

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            • #7
              Leon,

              The only experience I have had on knurling pistons is a good friend who, against several peoples adivice, had the pistons knurled in his 396 Chevy engine instead of boring/replacing the pistons. At least he did have the bores honed<G>.

              The engine sounded great at first, but after not too many miles (several thousand?), that 'piston slap/knock' was back! The second time, he bored the engine, and had not more problems.

              My take on piston knurling, from my limited experience with it, is that it is good for a car you want to get back running and make a quick buck selling, but not good for a long term keeper.

              For what it's worth, my two cents worth.

              Paul
              Paul
              Winston-Salem, NC
              Visit The Studebaker Skytop Registry website at: www.studebakerskytop.com

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              • #8
                I agree..
                JEFF[8D]

                quote:Originally posted by r1lark

                Leon,

                The only experience I have had on knurling pistons is a good friend who, against several peoples adivice, had the pistons knurled in his 396 Chevy engine instead of boring/replacing the pistons. At least he did have the bores honed<G>.

                The engine sounded great at first, but after not too many miles (several thousand?), that 'piston slap/knock' was back! The second time, he bored the engine, and had not more problems.

                My take on piston knurling, from my limited experience with it, is that it is good for a car you want to get back running and make a quick buck selling, but not good for a long term keeper.

                For what it's worth, my two cents worth.

                Paul
                DEEPNHOCK@worldnet.att.net
                '61 Hawk
                '37 Coupe Express
                http://community.webshots.com/user/deepnhock
                HTIH (Hope The Info Helps)

                Jeff


                Get your facts first, and then you can distort them as much as you please. Mark Twain



                Note: SDC# 070190 (and earlier...)

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                • #9
                  Hi, I'm new to this forum. I saw this topic and thought I might add this response. Look forward to reading more!

                  Forged pistons require more clearance that cast pistons because they expand more when at operating temperature and as a result are noisier until the engine is up to temp. I'm not sure, but the stock pistons were probably cast and could be run with tighter clearances. In my opinion, you should always ask the piston manufacturer what they recommend for piston clearance. Otherwise it's a shot in the dark unless you are using a cast replacement. As far as boring goes. In my opinion, you should have your pistons in the hand when you get the block bored at least for the final hone. The machinest can measure the pistons add the recommended clearance and you get a good sealing and performing engine. But that's just my way and I know what they say about opinions! My dad get's his blocks bored, orders pistons and rings and slides them in the hole and goes by feel a lot more than feeler gages. We don't agree, but his engines always run well and he's built a lot more than I have!

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