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Bore measurment

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  • Bore measurment

    I posted this on my other home on the web...and didn't really get substantive I thought I'd try here!

    I was taught to use a dial bore guage zeroed on the same outside micrometer used to measure the pistons...then insert the bore gauge in the cylinder and note the deviation as your piston to cylinder clearance...and do the math if you wanted to place a numeric value on bore size.

    I'm considering a PAW assemble yourself "block kit" for my impending 351W rebuild which may require that I juggle pistons for optimum fit.

    My question is the relative benefits of the dial bore gauge method. I was taught that telescoping gauges were not precise enough for engine work because they cannot be read in situ and error is introduced during removal...but these seem pretty good:

    What about using an inside micrometer for bore measurment? I see these on the surplus market relatively frequently.

    Are the benefits of the 3 point dial bore guages worth the cash? Can you still zero them with an outside mike or do you have to use a set ring type standard?

    Thanks for your insight and experience.

  • #2
    I expect others with more experience will give you better answers, but I'll throw in my ideas for what they're worth.

    A lot depends on the skill of the operator, as they say. I think, in the hands of a skilled toolmaker, telescoping gages can give you an accuracy of less than a thousandth of an inch. That being said, some of the other techniques you mention (an inside micrometer, for instance) may less demanding of skill and "feel" to get a reliable reading. You also need good-quality gages. I think Starrett telescoping gages are better for "feel" than the cheap imports, for instance.

    Another parameter comes into play, too: what is the quality of the bore you are measuring? Is it truly round? What kind of surface finish does it have? Those factors will influence the reliability of the readings you get.

    I assume, for a 3-point gage, you'd need a ring standard. I don't see how you could reliably set it off a micrometer. I've never used one, however, so I may be missing something there. A 3-point gage will probably give you the most accurate reading...assuming a round hole. If the hole is oval, you could easily fool yourself. That's one advantage of two-point measurement: you can measure several diameters to see if they are all the same.

    Just a few thoughts. Dunno if it's any help....

    [This message has been edited by SGW (edited 04-26-2002).]
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    • #3
      My thoughts:

      The idea is to be able to repeat your measurement with accuracy much better than your tolerance requirements so you know where you are at. If you need to hold the bore to .0005" then you should be able to measure to .0001".

      I use the 2-pt dial bore guages nearly daily for bearing fits into rollers. I "master" the gage to the bearing....not to a pre-specified dimension. Then machine the bore for the desired fit. I simply snug a set of micrometers on the OD of the bearing, then set/zero the bore gage to the micrometers. Hence, no need for expensive standards or any backwards math to figure the differences. This way, if I need a .0002" interference, I bore .0002" undersize. This is generally for bore diameters under 2".

      On bigger bores, say 4" and larger. I use inside mics. Here's the tip.....make sure that you can repeat the measurement over and over....since there is a "touch" involved to make sure that the gage is square and centered. You'll need to access your method and determine if it is good enough for the required outcome. I can't measure a 4" bore to the .0001" with my inside mics. I'm comfortable to the .001" and maybe with some special attention I might fool myslef into thinking I can measure it to .0005". I don't do enough work in this size range to justify the purchase of a more precise bore gage.

      As far as the telescoping spring loaded types. For me, I only use them when I need accuracy to about .002" or so. I find the most variation in this method....but they canb be very convienient as they don't need to be mastered. You simply slip it in, pull it out and measure. Repeat several times to make sure you get the same measurement.

      Back to your question,
      Yes you can master the 3 pt gages with mics, since there is only 1 "live point" and 2 followers to aid in centering.

      Good luck,


      • #4
        3 point dial bore gages need to be set with
        a master setting or a given standard (known
        diameter that will not change. A dial bore
        gage is used to measure or investigate
        out of round, bell mouth or taper in a hole.
        I would use a dial bore to check an engine
        if I had a dial bore gage with setting rings
        Good quality telescoping gages (Starret,
        Mititoyo) can get you to +/- .001 if you are
        careful. Telescoping gages are usually very
        accurate to 4.00 inches past that it can
        become iffy. To measure to 6.00 inches I
        would use an inside mic: only to check dia-
        meter. A dial bore gage is your best bet
        when measuring below .001


        • #5

          Your bore gage - mic method is fine.
          Telescoping gauges tend to read a couple tenths to a thou. big, depending on bore size, and feel.

          I. d. mics are good too.
          In all cases a light feel is the way for accurate measurement. You put a couple tenths "pressure" on your i.d. mic and check it with your o.d. mic with a couple more tenths pressure, and you're off a thou..

          With an i.d. mic, I hold one end stationary in the bore and swing the other adjusting until it will swing thru center with very light feel, not drag.

          I start a little small and advance the mic a couple tenths at a time like a go, no-go gage, to get the feel I like.

          Telescoping gages are set a little big, the nut snugged, one end held stationary, the other end swung thru the bore.

          The gage will find center and collapse to swing thru.

          Those Chinese import gages are useless. You can buy Starrett by the size as you need them.

          I bored printing press frames for ten years like this, using telescoping gages from 0 to 1 1/2 ", and mics to twelve inches
          (tolerance .001 on location and dia).

          Never used bore gages much, too much time setting for many sizes per frame.



          • #6
            My experience is for engine work ID mikes are quick, easy and gets the job done. Bore gage is frustrating if finish is south. Use outside mikes to check ID. Advice in previous postings is great!


            • #7
              Sort of off-topic, but I'm curious as to what you're building the 351W for, and to what specs?

              One of my current projects is a 1969 Mustang, and a 351 will soon replace its 302. The stroker crank from Summit is looking very nice: big-block cubes, small-block weight--best of both worlds.


              • #8
                If you want a good bore gauge get Starrett's #452B or 452M (metric) .oo1" - these are the best automotive bore gauges.

                If better accuracy is required then the top of the line Starrett #84 series 3 point dial bore gauges are the way to go. They can detect out of round conditions easily. Accuracy to what you ask for - they can cost more than the motor is worth.

                If you want to use internal mics the tubular micrometers work best but careful measuring is required to insure accurate results - the mic can be skewed off the diameter as well as not being perpendicular to the bore.


                • #9
                  You also get smallblock block flex and smallblock bearings.
                  Jim H.


                  • #10
                    Remember, pistons are tapered,(smaller at the top for thermal expansion) and bore wear is not uniform,(less wear at the bottom,more at the top. Measure the piston 90 deg to the wrist pin. measure bore at the top, middle and bottom and at angles for a total six readings. and use the mean average, also bore at least.005 under size to allow for honing.
                    Non, je ne regrette rien.


                    • #11
                      CompositeEngr: Don't even think of going to the 347 302 if you have a 351W to put in it's place. Any 351 is a stronger block than almost any 302 except the Boss 302 and the Ford Racing 4 bolt blocks. An aluminium head 351 will weigh in very close to a stock all iron 302. If you wish, email me off list to get a discussion going about Ford small blocks. WALT WARREN


                      • #12
                        I'll talk to you off list...

                        I'm building for a '69 Mustang as well...

                        I'm assuming your talking about the 351 Stroker crank not going to a 347 type stroker.

                        As far as the "small block bearings, small block flex" argument...the Windsor's generous 3" bearing is essentially a big block bearing. If you run a '69 to I believe '72 block the extra 10 lbs in the main area should keep things together fine.

                        <font face="Verdana, Arial" size="2">Originally posted by CompositeEngr:
                        Sort of off-topic, but I'm curious as to what you're building the 351W for, and to what specs?

                        One of my current projects is a 1969 Mustang, and a 351 will soon replace its 302. The stroker crank from Summit is looking very nice: big-block cubes, small-block weight--best of both worlds.


                        • #13
                          abn: IF you wish to include me in the Ford 351W discussion, feel free to email me. WALT WARREN