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boring rods - what are my chances?

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  • boring rods - what are my chances?

    Hi all, well I can't put it off any longer. I've got to stop thinking about this and stop futzing with the smithy and actually do someting. These rods are out of a honda gx670 and cost over $100 new (a piece). I'm not sure what happened but when I tore it down it was clear that there was some kind of oil problem as the bores were pretty chewed up. The rest of the engine is ready to go but I need to get the rods done and I'm not going to spend $225 for two rods!

    Now I'll be the first to tell everyone that I'm about as green as you get with this so please tell me if you see something wrong!

    I set the rod caps up in a vise (no comments about the "vise" - it's the first thing on my short list of things to replace) and indicated back and forth until it was parallel to the table. I used a 1/2" end mill and took off about .030" (more than I wanted to but I misread the dial - I thought each division was .010" but really it was .042").

  • #2
    I filed the locating shoulder on the rod until it was a couple of thou shorter than the cap. With the caps torqued on I can clearly see light in the gap and the caps snap on smartly so good to go. The surface finish was usable but I wasn't really happy with it.


    • #3
      I just finished up the back plate for the 4-jaw. It's usable now but I'd like to true it up a lot more. Guess I'll add that to the project list... So next the rod goes in the 4-jaw and I use a test indicator inside the bore to center it. The bore is oval now but since two of the jaws rest against the seams it was easier than I expected. I centered up the jaws on the seam first and then centered it up and down to take equal amounts off the rod and cap.

      Now that the rod is in rought alighnment I put a wrist pin in and ran the test indicator back and forth along two axis on it. I used a small deadblow hammer and a prybar to get it to the point that the test indicator says it's dead nuts on. Each division is .0005 and with the angle of the lever I actually have half the the indicated difference. I used something like RDM with the wrist pin (taking the slop out in each direction and averaging).

      What do you think? Am I ready to make the cut? I'm worried that I have the jaws too tight and it's deforming the rod but then I wonder if it isn't tight enough and will shift during machining! I think I'll use HSS to keep the RPMs down. It's a 1.5" bore so I'm thinking 800 RPM. I'll make a test bar for measurements. I'm also thinking I'll leave it a little small and will sneak up on the right size with some wet paper.

      So, do I have a chance in %$#@ of making this work?


      • #4
        I don't know what the professionals around here would say, but 800 RPM sounds way too fast for something that is that unbalanced.

        I'd do it real slow, and not worry about hurry.
        Put on the full face mask, and stand away from it.
        Hmm, are there such things as kevlar workshop aprons?



        • #5
          Whenever I have cut rods on a lathe, I made a mounting / locating boss for the end that I am not machining on a fixture that I bolt to my face plate and clamp the rod to this fixture with toe clamps to keep from distorting the rod. I would remove a minimal amount of material from the rod side to keep from lowering the compression. And I would probably run at about 200 rpm at .003 IPR


          • #6
            Originally posted by dockterj
            So, do I have a chance in %$#@ of making this work?
            100/1 total failure.

            Sad, you aren't willing to spend $200 for a new set of rods. I guess spending about $450 per head shouldn't bother you.

            You're missing a few critical alignment procedures in your setup and overlooked pertinent facts.

            I haven't any time to type at present. Perhaps this may invoke a little more thinking and a few more comments from those that have done such machining.

            Back later, best regards.



            • #7
              Originally posted by Patch
              100/1 total failure.

              Sad, you aren't willing to spend $200 for a new set of rods. I guess spending about $450 per head shouldn't bother you.

              You're missing a few critical alignment procedures in your setup and overlooked pertinent facts.

              I haven't any time to type at present. Perhaps this may invoke a little more thinking and a few more comments from those that have done such machining.

              Back later, best regards.

              Way to be a dick, Patch. The guy said he was new and he wanted advice, not for some pompous ass to come down and tell him he's all wrong and then leave. At least he's trying to learn and sometimes you have to spend a little extra to learn a good lesson. But then maybe you don't know anything about that, since I'm sure you've never done anything the least bit wrong. Everything by the books ...

              Unfortunately, internet etiquette and tact are things that machinists (actually most people) are not good at. Don't take it personally, dockterj.

              Seems like you put a good bit of thought into the operation. I don't know much about motorcylce engines, but what you've done would work for a small engine. 'Course they are practically indestructable. I'll be curious to see what some of the more expierenced have to say about the operation.

              Don't worry about the balance thing. Ryobiguy - these are pretty lightweight pieces and, so long as it is securely held, there's no need to worry. I've run much larger and scarier items at higher speeds with no problem. (Recently I did a tractor hitch that was going from "wallored out" to 3.2" diameter. If you've any idea of what a tractor hitch looks like, that put about 40 lbs off center and spinning pretty quick. Plus it was forged steel and was a PITA to machine.


              • #8
                Patch certainly appeared smug and superior and not very helpful. Perhaps that was not his intent, and he really did have to leave.

                However, that said, I would ditch the 4 jaw.

                if you have it tight enough to hold, the bore is almost certainly going to turn out with 4 lobes due to the chuck jaws.

                if it is loose enough for that not to be an issue, it will shift and ruin the job.

                Not good choices.

                I would clamp it flat on a faceplate, and then bore it. The alinement with a pin in the other end is a good plan, and may help to hold it.

                Use of a mill and boring head would also be a good plan, if that is an option. It avoids the spinning part, and the need for a large swing.
                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.


                • #9

                  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 cut it off twice; it's still too short
                  Oregon, USA


                  • #10
                    Yes it's about as aligned as you'll get it but the 4 jaw will distort the bore. It would be better to make a fixture as described above. What you don't want to do is take much off the piston side of the rod. If you do you will lower the comp. ratio. I think 300-400 rpm at .002" feed would be ok with a slight radius on the cutter.

                    Does it use rod bearing inserts? If so how will you do the tang area? If not then you will need an extremely fine finish on the rod to run on the crank.

                    What material is the rod made of?
                    It's only ink and paper


                    • #11
                      First of all, this is an industrial engine, not a motorcycle; the bore and
                      stroke on this engine are 77 x72 mm (3.0 x 2.8 in), w/ 40.9 in^3.


                      The compression ratio is 8.3 : 1.... so .030" should change the compression

                      First, compute clearance volume Cv.

                      Cv/ (20.45 + Cv) = 1/8.3

                      Cv = 2.46 + Cv/8.3

                      .87 Cv = 2.46

                      Cv = 2.8 in^3.

                      Now, pulling .030 off the rod means:

                      (3.0/2)^2 * 3.14 * .030 = .212

                      So that will change your compression ratio by

                      .212/2.8 = 7.5%

                      A bit more than half a point.... meh, don't sweat it.

                      Chase down the oiling problem before restarting, unless you _KNOW_
                      the engine ran low on oil, it will happen again.

                      - Bart
                      Bart Smaalders


                      • #12
                        Thanks! I appreciate *all* the comments. I should have described the rods a little better. They are cast aluminum and have no bearing inserts. I don't think honda considers this engine rebuildable as there are no undersize rods available, there are no replaceable (or undersize) crank bearings, etc. You can get oversized pistons but I didn't even price them out...

                        Patch - your estimate of 100:1 is much better than mine (no chance at all) I probably already ruined these rods by taking too much off the caps but all I'm out is my time at this point and I need to invest time to learn anyway. Even after I bore these out it doesn't mean I'm committed to using them. I have a line on some "clone" replacements at a much better price but they are backordered with no ETA. If those come through I would probably go that route. The engine, with it's unknown lineage, isn't worth putting any more money into and if it goes boom later, well that's just my education, isn't it? I'm curious about what alignment issues I'm overlooking - I thought that I had accounted for everything even though my methods and execution may not be sound.

                        Ryobiguy - I haven't turned the power on yet but was planning on starting slow to see how unbalanced it felt. It's an aluminum rod so probably will be ok.

                        Knedvecki - if I understand correctly you would create a "top hat" shaped piece with the bore closely sized to the wrist bore on the rod and use that for alignement. Let me think about that. I don't have a face plate and this chuck isn't quite big enough for that to be easily done.

                        Fasttrack - thanks for the encouragement.

                        J Tiers - I had originally dismissed using a boring head on the mill as I thought this would be easier to align but perhaps I'll rethink that idea. If I made the alignment pin and clamped on each side (over the bolts that hold the cap on) it might be better. It doesn't appear that either side of the rod is flat or square to the bores so I'd have to shim everything but that would be too bad.

                        Tim - thanks for your comments. You're completely correct - the rods were trash to begin with and even with the cost of new at $100 I doubt I could get a machine shop to redo them at a comparable price.

                        Carl - aluminum rods, no bearing inserts. Apart from the alignment issues the finish is the thing I am most concerned with.

                        Barts - thanks for the computation. Also, I split the difference so it's would only be shortening the rod by .015 AND the cap is set at an angle so it's slightly less. I should probably take more off the cap and less off the rod but you're correct it probably won't be a big difference.

                        I really do appreciate your feedback!


                        • #13
                          if it was me (and i'm barely above the newbie stage), i'd do exactly what you're doing. if you ruin it, it was trash anyway, if it works, you saved a few hundred bucks. either way you learn something and can make use of the knowledge later. would i do this on Ferrari rods, no.

                          andy b.
                          The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining


                          • #14
                            Gee you're baulking the modern idiom of "Run and Throw" as far as REPAIRING is concerned Dockterj, good on yer.

                            Nother one to think about, as you have acess to a Miller, instead of wafting a lump of metal around and getting the lathe to try and walk around the shop, why not arrange a vertical (Clock it ) peg, the same dia as the gudgeon (Wrist) pin on a backing plate clamped to the mill table. By packing under the rod and clamping down, you should be able to clamp away from the big end bore you're trying to true up, and with carefull measurement and a boring head, rescue the rod. That'll annoy Mr. Honda.

                            Regards Ian
                            You might not like what I say,but that doesn't mean I'm wrong.


                            • #15
                              My first thought would be to do it on the mill with a boring head. Make a shaft to fit the wrist pin bore snug and use it to align the rod on the mill table. The setup will take a lot of time. Leave some material to hone them. Before you do the job call an auto machine shop and ask if they will hone them for you. You may have to rig up to hone it yourself.

                              When you bore it take off as little as you can from the top side of the rod towards the piston. That will keep the piston higher in the bore more to stock specs.

                              They looked like aluminum and I didn't see any notches in the bore, that's why I asked. You may have been able to put inserts in them but not now.

                              I would go ahead and try it, as you said, there's nothing to loose. I would get the new after market rods and use them after trying the reconed rods to see if they work. I don't think the cap would hold up well so I would not do the test run very long or high rpm.
                              It's only ink and paper