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boring rods - what are my chances?

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  • #16
    The angled caps avoid most of the change in compression.

    If it were a 2 stroke, which it presumably is NOT, a change in rod length from milling the would also change the exhaust timing a bit. No idea how significant that would be , but it might be larger than expected in degrees.

    The other question would be that since you COULD re-center slightly by boring the big end off-center and back on the correct length, whether you would still have enough meat in the cap if you did that.

    it would be easier to bore to correct centers on the mill, where you could use the table movement to set the correction as an offset from the wrist pin.
    CNC machines only go through the motions.

    Ideas expressed may be mine, or from anyone else in the universe.
    Not responsible for clerical errors. Or those made by lay people either.
    Number formats and units may be chosen at random depending on what day it is.
    I reserve the right to use a number system with any integer base without prior notice.
    Generalizations are understood to be "often" true, but not true in every case.


    • #17

      1) We make split cams at work and place shims in the gaps when we run the finish bore. It has 2 benefits: Lets you crank the 2 halfs together good, helps the tool by not having to jump the gap at the split. But they are steel and you can crank on them pretty good.

      2) Your surface finish is goign to have to be pretty much "ground" You might want to sneak up on your bore with a flex hone.

      3) before testing, consider if you run the risk of turning a $225 bill into a much larger one by damaging something else if there is a bad finish or too much slop.

      Best of luck


      • #18
        1. I don't understand the alignment method ( thats OK, I don't have to) but somehow you need to re-assemble the rods on the crank EXACTLY like they were when you honed them.

        2. I would expect a tolerence of something like 1.4997" to 1.5002" or some such numbers and you don't get that with typical HMS stuff. Watch a machinist do a rod on a Sunnen machine and you'll see whats involved.

        3. I started on a Briggs 5 KW generator rod just like you but found I was over my head and bought an expensive rod. It croaked from a stuck governor and the crank journal was coated in melted aluminum. I etched it off with hot lye and found a layer of burnt oil and a perfect crank underneath. Hope you are as lucky.

        4. Sometimes I have to re-think the cost "Now I have a pile of junk, if I spend $225 more on it I'll have a good Honda gx670, does that make sense?" You'll have plenty of oppurtunities to make chips with the Smithy when you run and fight another day. Good Luck.


        • #19
          Done them on vintage engines in the past but always done them between centres.
          Myfords did a jig just for this than ensured you kept alignment with little trouble.

          Gave it to my mate when I got rid of the Myford and stopped racing.


          Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


          • #20
            Gee, I did this about 6 months ago, when my snowblower locked up, because the oil slinger broke
            and the crank starved for oil..during a 11 inch snow fall. never forget that !

            Good idea using the wrist pin for allignment.
            While I rebored mine on a bridgeport, and used a prebored hole in the vice jaws to jold my wrist pin,
            which has to be dead nuts on .
            your setup should work.

            Here is a hint for you and others on a method I use for delicate parts that I do not want to shift or move.
            Get some Hot Melt Glue and glue the rod to the chuck .
            Don't worry, it will peel off easily when done.
            It stabilises to whole assembly and also acts as a vibration dampener.
            It is great stuff. don't be afraid to use it, takes a bit to cool, so you may want to layer it.
            Put it on the jaws where clamping as well.
            The nice part, is it will hold shims or other blocks which may be needed, without worry !
            Good luck
            Last edited by Rich Carlstedt; 06-10-2009, 07:56 PM.
            Green Bay, WI


            • #21
              I think your chances of success are pretty good... if you find out what the original cause of the failure was and fix it before you reassemble the engine. You should also make sure that the chamfer on the face of the rod still clears the radius on the edge of the rod journal after you have machined .030" away from 1 side. You might want to take a light test cut and then remove it from the chuck to to verify how much out of roundness you are getting from the chucking forces. It shouldn't need to be chucked so tight it distorts it much. Use some cardboard or leather pads on your last pass to spread the contact forces a little. A small radius on a sharp tool with a fine feed should give a good enough finish for the bearing surface. Overhead cam cylinder heads get their cam bearing bores machined with a single point tool all the time in line boring operations and it works fine. Try Lemon Pledge furniture polish in the trigger spray bottle as a cutting oil. It has beeswax which gives a nice finish on aluminum (and it smells lemony fresh!)

              "Nothing is impossible for the man who doesn't have to do it himself"


              • #22
                Originally posted by Tim Clarke

                I've never tried rods in the lathe. Sized a truckload or two back in the dark ages, when I worked in a auto machine shop. We never took much off the cap, a couple thou. then honed the big ends with a Sunnen hone, and then the small ends were cut on a machine that held the rod in alignment on the finished big end. [with new pin bushings, which were a little undersize] Also, the center to center distance was set to spec's. That's where I think you might find trouble, keeping the center distance close to spec, and as importantly, the same.

                I truely hope you haven't made scrap of your rods. If the centerlines of the rod and pin bores aren't parallel, you're in for big trouble, concentrates the stress on the edges of the inserts, and will tend to bust the rod just below the pin bore.

                Hopefully, there is a way out of all this, and you'll be fine. I just can't think of a way, based on what I see and read in your posts.

                Of course, if your rods needed work, you're really out nothing, you'd have to buy new, or get a reman set anyway.

                Best of luck, TC
                I agree, a Sunnen Hone is the way to go. I'm concerned, however, that the rods have been shortened some .015 (half of .030) now since that changes the geometry of the engine. Furthermore, this will affect engine balance and performance somewhat due to the lightening and the fact that there are now two short connecting rods. This will lower the comression ratio in the two affected cylinders. .015 difference in head space is a great amount in an engine that was so closely detailed as the 750s.

                I presume that there are no inserts in the connecting rod bearings and that they ride directly on the crank journal. That's a perfect setting for a Sunnen honing Job. I've worked on many Harley-Davidson engines and whenever I had something like this come up, I would find a shop that could do the honing for me since a Sunnen Hone is not even on my short list along with food, clothing and shelter. LOL.

                Good luck on the project but I do agree with Tim Clarke here.


                • #23
                  Gnm109, I would not think the balance would be affected to badly if he gets them the same, it is just a 2 cylinder engine. I kind of got the impression from your post that you might think it is a 750 motorcycle engine.


                  • #24
                    Large Diesel rods are commonly reconditioned on specialized boring machines. You don't normally see those in ordinary auto machine shops.

                    They locate the rod using the wristpin, with a clamp over the beam and a pair of jackscrews at the big end.

                    In these machines the rod is fixed and the boring head turns. It's still a boring operation.

                    The trick here is, rather than trying to clamp everything near the big end, they use the little end to oppose the boring torque, the jackscrews keep the big end from moving around, and the strap across the beam just keeps the rod from falling off.