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Piston/Cylinder lapped fit--no rings

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  • Piston/Cylinder lapped fit--no rings

    Time to ask for some help now. I plan on building a model i.c. engine, 1" bore x 1" stroke. Cast iron piston and cast iron cylinder liner. I would like to do this without any piston rings. I have a lathe and a mill to work with. I have always used Viton o-ring on my previous engines, because they work so well and don't require a great knowledge of "ring making)--(at which I have proven to be a dismal failure). My new engine is going to be a two stroke with ports in the cylinder walls, and I know that a Viton ring would bulge out into the ports and die very quickly. On all of my previous engines, with either a cast iron cylinder or a cast iron liner, I have drilled and reamed the cylinder to 1" bore with a 1" reamer. then I apply a bit of light oil and run my 3 stone brake cylinder hone thru it a few times to knock down any high spots and to texture the wall a bit. Then I turn a round piece of aluminum to 0.002" smaller diameter than the bore size, coat it with 600 grit lapping compound in an oil base, and work the round aluminum thru the cylinder with a twisting motion until it gets free enough to chuck the aluminum in the lathe and hold the cylinder in my hand while running the lathe at a low rpm. I work the cylinder back and forth on the aluminum (which is longer than the cylinder barrel by about 1" plus chucking length of another inch). This is scary business, and you have to be prepared to immediately let go of the cylinder if it "grabs" and let it rotate with the lap until you can shut the lathe off and work it free by hand again. After about a minute of lapping, I'm done with the cylinder. I then turn the piston to about .002 less than the finished bore size, coat it with 600 grit compound, and repeat the process, sliding the piston (with a "handle" attached to the wrist pin) in and out of the cylinder over it's full length with the same twisting motion.--this knocks any high spots off the piston and gives a very nice sliding fit into the cylinder. HOWEVER---I know that this is not sufficient to give a fit that will create good compression without rings on the piston!!! I have read about internal expanding laps, external adjustable laps (which I have neither of) and 1000 grit lapping paste. I do know that Chuck Fellows, who is a remarkable machinist tried to make an engine with a ringless piston a year or two ago, and was unable to get the engine to run, because of compression issues.--So--I am asking for help, encouragement, knowledge about what I wish to do---and I don't want any suggestions that I give it up and learn to make proper rings please.---Brian
    Brian Rupnow
    Design engineer
    Barrie, Ontario, Canada

  • #2
    Mr. Rupnow
    The making of laps is not beyond your capabilities, either external or internal. Internal laps of that size are easily purchased.
    The larger problem is determining a running fit between what sounds like two dissimilar metals-perhaps I misunderstand?
    Perhaps there is a way to "glow" the fit prior to running, so that compression is not lost.
    I am not an IC mechanic, but AIUI, the piston is actually eccentric, or somewhat oval shaped, prior to operating temperature. This too can be accomplished on the lathe, but I can't give any data thereof.
    Encouragement, I offer, help if I can. Knowledge? ehh, sketchy.

    Comment


    • #3
      I've made ringless hot air stirling engines that work, but its a very fine line. You definitely need a very precise bore (which you can get from lapping). An IC engine, I'd be less inclined to go ringless ....things are faster and there are higher temps, I wouldn't want it to seize. then again weren't all little model airplane engines, the .049, ringless?

      I think you should figure out how to make rings. Like anything else, its just a bunch of little steps, each of which aren't too difficult. Besides, I never liked the idea of Viton rings in an IC engine...something goes wrong and you could get over 300C.....not me.

      as for lapping, you need an expanding lap to do what you want. as they are expanded, the clearance between it at the bore becomes almost nothing and that's when by feel you get things very very round and straight. I must have posted these a dozen times....but its exactly what you need for this job:

      http://bbs.homeshopmachinist.net/thr...ring-Cylinders

      post 19 shows the type of lap I make and used countless times. They're very simple as per pics but work very well.
      Last edited by Mcgyver; 02-16-2016, 05:23 PM.
      in Toronto Ontario - where are you?

      Comment


      • #4
        I believe that model ringless piston engines have slightly tapered pistons to allow for the fact that the top end of the piston gets much hotter and expands more than the surrounding cylinder. the bottom end of the piston can be sized to a close fit with the cylinder (this ensures compression) as long as they have the same coefficient of expansion. Possibly a couple of thou clearance would be enough at the top end of the piston. You may have to experiment and/or do some calcs based on gestimated temperatures.

        Phil

        Comment


        • #5
          Originally posted by phil burman View Post
          I believe that model ringless piston engines have slightly tapered pistons to allow for the fact that the top end of the piston gets much hotter and expands more than the surrounding cylinder. the bottom end of the piston can be sized to a close fit with the cylinder (this ensures compression) as long as they have the same coefficient of expansion. Possibly a couple of thou clearance would be enough at the top end of the piston. You may have to experiment and/or do some calcs based on gestimated temperatures.

          Phil

          This as well as, or instead of, tapered cylinders, grooved pistons for oil retention and a rich oil mixture to aid sealing.

          http://www.mecoa.com/faq/abc/abc.htm

          Also check out the FAQ page as there may be some useful tidbits there as well.

          http://www.mecoa.com/faq/index.htm
          Home, down in the valley behind the Red Angus
          Bad Decisions Make Good Stories​

          Location: British Columbia

          Comment


          • #6
            Perhaps this may be of some help. http://modelenginenews.org/techniques/materials1.html

            There is also a section on making piston rings.

            Geoff

            Comment


            • #7
              Automotive industry has for many decades used Aluminum alloy pistons in both Iron and Alloy blocks. Since the pistons invariably get hotter than the surrounding cylinder walls, they must have adequate clearance to prevent seizing at high temperatures, but must also be adequately "tight" to prevent "flopping" around while running still cold; such wiggling of pistons creates what the engine folks call "piston slap", an undesirable and disconcerting sound to even the inexperienced.

              The problem is compounded when using alloy pistons in iron blocks. So, the pistons are "cam-ground", (whatever that means!), and are egg-shaped a little bit, just as was mentioned. At right angles to the wrist pins (and thus the piston skirt), a steel insert is cast into the slug. This hunk causes expansion to occur in two directions unequally, allowing the "Fit", or clearance to be fairly close while cold,. to prevent "slap".

              Sorry, my blurb had nothing to do with rings. Don't know much about them, 'ceptin' some designs use a lot of them, 6 rings per piston on some diesels. One other piston imponderable, though; piston wrist pins are located off-center, in almost all IC engines. On a 4-inch bore, as much as 1/8-inch. That's why the piston must be oriented properly in the bore, usually by a mark or indent which faces "forward". imp
              IF IT'S ELECTRICITY, IT'S ME.

              Comment


              • #8
                I was skeptical the first time I made piston rings, but was pleasantly surprised at w well or went. I purchased a length of cast iron from McMaster Carr, turned the OD, drilled and bored the ID. And parted off 4 rings, although I only needed 2. I was sure I would break some fitting them to the piston. I clamped the rings , one at a time in the mill vise and cut them with a slitting saw. Open the gap and put a spacer in and heat the area across from the gap to dull red and allow to cool. Much easier than I ever thought. Bob.

                Comment


                • #9
                  Model Engine News covers the lapping topic well for smaller bore engines. (Ron incorrectly calls it honing.) http://modelenginenews.org/techniques/cylhone.html

                  I think you are going to have difficulties with a 1" bore holding good compression, due to differential thermal expansion of the piston and cylinder. If you want to try, you might have the best luck with a cast iron piston in a cast iron cylinder.

                  You really should try to get the hang of the George Trimble cast iron ring method. It is not that hard, and you can make 10 rings as easy as you can make one, so you can have piles of extras.

                  Is there a supplier of pre-made CI rings? I think Stuart used to sell them from the UK.

                  Comment


                  • #10
                    You want one of Dan Gelberts lathes!
                    http://youtu.be/sFrVdoOhu1Q
                    His piston and bore fit is amazing.
                    Think your stuck with making a bore, plug lapping it then slowly lap reducing a piston till it fits, not impossible, look at the fit on a gas srynge, and that's glass
                    Mark

                    Comment


                    • #11
                      Bobslad--He could probably trade that lathe for a small island in the Caribbean!!---That does bring up a very good point though. Many of the posts I have read show that lapping and/or honing are only done following grinding operations. I don't have any grinding capability here. Just a lathe and a mill. Now I kind of question if the necessity for grinding completely completely stops me in my tracks from attaining a piston to cylinder fit like I would probably require to make an engine with no piston rings??
                      Brian Rupnow
                      Design engineer
                      Barrie, Ontario, Canada

                      Comment


                      • #12
                        Originally posted by Bob Fisher View Post
                        I was skeptical the first time I made piston rings, but was pleasantly surprised at w well or went. I purchased a length of cast iron from McMaster Carr, turned the OD, drilled and bored the ID. And parted off 4 rings, although I only needed 2. I was sure I would break some fitting them to the piston. I clamped the rings , one at a time in the mill vise and cut them with a slitting saw. Open the gap and put a spacer in and heat the area across from the gap to dull red and allow to cool. Much easier than I ever thought. Bob.
                        Im not sure why you would need to do this? (Heating after opening the gap)

                        AFAIK, the process of fitting rings is that you generally start with a ring that is way over your bore size, but still not so big it would fall out of the ring grooves on the piston.

                        You put the ring into the bore and make sure its straight by using the piston as a handy dandy bore sized ring straightening device.

                        You then measure the gap at the end of the ring with a feeler gauge (remove the piston) to check the end gap. I forget the specs but there is an end gap per inch of bore spec for various engine types (IIRC, higher power engines usually have more gap per inch of bore (To allow for more expansion?))

                        Of course, if your ring does not yet fit in the bore, you gotta stick a file into your end gap till you can get it to fit.

                        I don't think the actual size of the ring is very important, but I do think you want to start from oversized and compress it down, Not heat/bend an undersized ring out because then its not circular anymore.

                        Ie: you reduce the OD of the oversized ring by increasing the gap. you don't increase the OD of an on sized ring by heat bending it

                        IMO, compressing a circular ring should end up with a circular ring.. compressing a non circular ring is going to end up with a non circular ring.

                        Also, usually the bore is left with significant 45 degree, cross hatched tooling marks from a 220~ grit hone. These need to be redone when you change rings, as its these tooling marks that grind the rings to match the bore during the break in period.

                        PS: you don't need 2 rings per piston, tons of 2 stroke racing motors get away with 1 paper thin ring, they just don't last as long and have a little more blowby.
                        Last edited by Black_Moons; 02-16-2016, 09:42 PM.
                        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

                        Comment


                        • #13
                          Black_Moons, you need to read George Trimble's explanations as to why rings are held open and annealed. The main point is that the ring is machined to match the cylinder ID, then spread open and annealed. The idea is that, when the cylinder forces the ring back to circular, it so happens that the force between the cylinder and ring is uniform around the circle. The ring is gapped after the annealing operation so that the gap doesn't disappear when the engine is at operating temperature.

                          Comment


                          • #14
                            Originally posted by brian Rupnow View Post
                            Bobslad--He could probably trade that lathe for a small island in the Caribbean!!---That does bring up a very good point though. Many of the posts I have read show that lapping and/or honing are only done following grinding operations. I don't have any grinding capability here. Just a lathe and a mill. Now I kind of question if the necessity for grinding completely completely stops me in my tracks from attaining a piston to cylinder fit like I would probably require to make an engine with no piston rings??
                            There's no problem lapping or honing turned pieces; no grinder is required. The job takes longer the worse the starting finish and geometry, of course.

                            Comment


                            • #15
                              Brian, when I built my Webster, I bought pre-made cast iron rings from Otto Engine Works. IIR they had rings in stock down to .625 inch (16 mm-ish). Goggle Otto Engine ,Blue Balls rd.
                              .....talk about an address that's easy to remember. LOL.

                              BTW , rings were 5 dollars . What's some time saved worth?
                              Like has already been said, you will only need 1 ring; however, what has not been mentioned, you will need to put a tiny pin in the ring groove gap adjacent the port. This will keep ring from slowly rotating on the piston.
                              The idea being keeping the ring "ends" away from the port. Don't work about ring snagging port,it won't happen if you simply break edges. No need for a honking ass bevel either, just clean off the sharp edge after you bore the cylinder. Motorcycle air-cooled, single-ring 2 stokes routinely see 8-9k rpms in this configuration.
                              Bricolage anyone?....one of lifes fun games.

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