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Connecting rod bearing clearence

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  • Connecting rod bearing clearence

    A friend of mine has a relatively new cheapo air compressor that had seized the connecting rod on the crank pin. Now normally I would have just told him to chuck this silly direct drive contraption into the garbage and forget it but the darn thing has less than an hour's running time on it and is fairly well built compared to most of these usually worthless compressors. It has an oiled pump, Aluminum piston with gasoline engine type piston rings and a cast iron cylinder -not a single piece of plastic in sight except for the cooling fan. Due to the fact it sat in his basement for a couple of years before seizing the compressor (that may have been the cause of the problem) it is out of warranty despite having barely been run at all.

    So much for the description of the thing now the question,

    Everything looks new except for the Aluminum connecting rod and there appears to be no damage to the crank pin except for a bit of discoloration but nothing I can feel, the rod however is slightly galled on the crank journal. Since this is a counter-weighted off-set cast iron crank pin mounted directly to the end of the motor armature and not a conventional crankshaft the rod is of one piece Aluminum construction that slips over the pin and does not have a removable cap like a normal connecting rod. The plan here is to bore out the large end to accept a pressed in Oil-Lite bushing that would then be bored to size to fit the crank pin, now, will that type of bushing work at 3600 RPM (splash type oiling)? But the most important question here would be how much bearing to crank pin clearance would I need for this considering a 7/8" diameter crank pin?

    Actually it's metric I'm sure it's just that I haven't measured it precisely yet but 7/8" is going to be extremely close so I used that as reference.

  • #2
    Radkins - it's a good question and all depends...

    personally I would not use oil-lite as it's sintered bronze and is more designed for use in applications to where its not submerged or surrounded by oil and is more so meant to hold it,
    so it's not a solid structure and does not handle spike loads/reciprocation loads no where near as well and in fact can get pounded out real quick,,,
    it is however very safe seizure wise and most all the time just "makes more room" as the highs have someplace to go due to it being partially porous...
    but the thing is - is why take a chance with it if you have splash type oil system in the first place, use something solid and compatible with the crank, what about just honing the original rod that was galled? is there enough good meat left to contact - and call the rest "oil retention"?

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    • #3
      Also consider this, if the rod can't be brought around and you do go the solid sleeve repair then you might want to build one with a flange - and make it not only be able to rotate freely on the crank but also the rod itself...

      this way if one surface gets in a little bind the other will take over and do the rotating - put the thin flange to the crank counterweight side and it will be self retaining,,,
      you will have over double the wear surface - and a slight more overall clearance with the two combined but should not bother anything...

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      • #4
        Any reason to not press/tig in an aluminum bushing? Or perhaps make an entirely new aluminum rod. High performance after market connecting rods for Briggs&Stratton engines are routinely made from 7075. I'm not sure of the type of alloy that Briggs uses as stock issue but irregardless they have no issue with 3600 rpm, splash lubrication, and high temps. If placing the rod try to keep small end/big end weight close to stock figures for balance purposes.
        Not sure if the stock air compressor rod comes equipped with an oil hole in the big end but it may be an idea to add one if it didn't.
        I don't have access to my manuals at the moment so I won't quote a figure but I would think one would not be too far off base by using the rod clearance figures for the common .875" rod from a B&S motor.
        Any indication as to what caused the failure of the original rod, contaminated lube, crank pin out of round or other dimensional issues? I realize this may be hard to nail down when the rod's big end is already toast but it does warrant looking into as much as possible in order to prevent a recurrence.
        Home, down in the valley behind the Red Angus
        Bad Decisions Make Good Stories​

        Location: British Columbia

        Comment


        • #5
          Originally posted by A.K. Boomer View Post
          Also consider this, if the rod can't be brought around and you do go the solid sleeve repair then you might want to build one with a flange - and make it not only be able to rotate freely on the crank but also the rod itself...

          this way if one surface gets in a little bind the other will take over and do the rotating - put the thin flange to the crank counterweight side and it will be self retaining,,,
          you will have over double the wear surface - and a slight more overall clearance with the two combined but should not bother anything...

          That's an idea I hadn't thought about and is certainly worth considering. I did wonder about the wisdom of using that Oil-Lite bushing and that's why I asked about that. Ok the Oil-Lite bushing idea is out, I have some brass pins I could make one from I suppose but yes the original rod journal is only slightly galled in one spot so maybe after checking it for roundness I may consider just using it as-is. Still for no more trouble than it would be I like the idea of having that double bearing surface so I might just do that instead, anyway how much clearance would be about right on a rod journal of this size?

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          • #6
            Could you possibly use the small end bushing from an automotive connecting rod? I know that the small end piston pin on my Jaguar is 7/8" and the specified clearance is .0002". The bearing is steel backed - phosphor bronze.

            There is a listing for these on eBay, individually for these bushings: http://www.ebay.com/itm/Engine-Pisto...c4d7be&vxp=mtr



            Richard

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            • #7
              Originally posted by radkins View Post
              That's an idea I hadn't thought about and is certainly worth considering. I did wonder about the wisdom of using that Oil-Lite bushing and that's why I asked about that. Ok the Oil-Lite bushing idea is out, I have some brass pins I could make one from I suppose but yes the original rod journal is only slightly galled in one spot so maybe after checking it for roundness I may consider just using it as-is. Still for no more trouble than it would be I like the idea of having that double bearing surface so I might just do that instead, anyway how much clearance would be about right on a rod journal of this size?
              I would stay away from brass...

              yes if the galling is not that severe and has not disrupted but a fraction of overall surface area could be the way to go...

              Willy's advise about checking things that may had caused the failure is wise, check the crank pins tolerance in all directions esp. load related...

              the double bearing surface can get you out of a jamb but only if the big end of the rod has enough beef for it, you will be weakening the structure,,, it will also be tedious as you will have to hold two tolerances very close,,, to find out the right tolerance I would just mimic close to that of an engine with the same rod end diameter or try to find untouched place on the rod and crank to find out what they were using...

              Comment


              • #8
                Originally posted by Willy View Post
                Any reason to not press/tig in an aluminum bushing? Or perhaps make an entirely new aluminum rod. High performance after market connecting rods for Briggs&Stratton engines are routinely made from 7075. I'm not sure of the type of alloy that Briggs uses as stock issue but irregardless they have no issue with 3600 rpm, splash lubrication, and high temps. If placing the rod try to keep small end/big end weight close to stock figures for balance purposes.
                Not sure if the stock air compressor rod comes equipped with an oil hole in the big end but it may be an idea to add one if it didn't.
                I don't have access to my manuals at the moment so I won't quote a figure but I would think one would not be too far off base by using the rod clearance figures for the common .875" rod from a B&S motor.
                Any indication as to what caused the failure of the original rod, contaminated lube, crank pin out of round or other dimensional issues? I realize this may be hard to nail down when the rod's big end is already toast but it does warrant looking into as much as possible in order to prevent a recurrence.
                Apparently we were typing at the same time and I completely missed your reply until now,

                I don't have access to a tig welder at the moment but I think the bushing just pressed in should be ok, as was suggested earlier I could make it a flange bushing with the flanges on the crank pin side so it would be sort of self locking. After thinking this over I probably will just bushing the rod with that pressed in flanged bushing since that would be the simplest thing to do, as long as I don't make really thick sidewalls it shouldn't weaken the rod significantly because for a small rod like that it seems fairly heavy on the big so strength shouldn't be an issue.

                That seems like a great idea about using the piston pin bushing from a gas engine (except for the lack of the flange but I could probably come up with something there), I hadn't thought of that but if I can find the right ID/OD or just something close enough to hone the ID then that should be the perfect material to use. Except for some Oil-Lite bushings and brass stock I don't have anything else on hand and apparently neither of those materials are suitable but I suppose if I can't find a piston pin bushing of a suitable size surely there will be some kind of bushing available.

                This is a cheap compressor, although better quality than most of this type, but since this project is working with what is essentially new parts and I see this thing as being a project that could result in a small compressor that could last a considerable time with some minor improvements I am willing to spend the time and a few bucks to do it right. Projects like this are just plain fun and I get a tremendous amount of enjoyment out of seeing success, especially if it means turning a relatively cheap piece of equipment into something reasonably reliable.
                Last edited by radkins; 05-03-2015, 06:24 PM.

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                • #9
                  Projects like this are just plain fun and I get a tremendous amount of enjoyment out of seeing success, especially if it means turning a relatively cheap piece of equipment into something reasonably reliable.
                  Exactly my way of thinking as well. Sometimes it's just the challenge of turning a sow's ear into a silk purse.
                  Or at least have some fun trying. Nothing ventured, nothing gained.
                  Home, down in the valley behind the Red Angus
                  Bad Decisions Make Good Stories​

                  Location: British Columbia

                  Comment


                  • #10
                    It may surprise some but all scraping is not just for flat surfaces.

                    Try "round/hole" scraping with a "three-point" and a "half-round" scraper.

                    You may be surprised at just how well it works.

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