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Thumper---A new 1 3/8" bore i.c. engine

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  • #76
    Today yielded a new top plate which bolts to the top of my cam box, and a 1 3/8" diameter piston. The piston has had everything done on it that can be done on my lathe. Tomorrow it will spend a bit of time with my mill and rotary table to get finished up.

    Brian Rupnow
    Design engineer
    Barrie, Ontario, Canada

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    • #77
      Today was cylinder day. The cylinder is made from cast iron. The piston does fit the bore---with a little persuasion. That is exactly what I wanted. Tomorrow I will run my brake cylinder hone thru the bore of the cylinder, and if I need to, will coat the piston with a bit of aluminum oxide #600 paste and lap the piston into the cylinder. I have ordered two compression rings from Debolt in USA.
      Brian Rupnow
      Design engineer
      Barrie, Ontario, Canada

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      • #78
        Brian, your workmanship is so cool,

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        • #79
          Thank you Ringo. I really do try. I will never be as good as some of the machinists on this forum, but I know that I have interesting ideas.---Brian
          Brian Rupnow
          Design engineer
          Barrie, Ontario, Canada

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          • #80
            The fins are thicker than the grooves (it's usually the other way around). If indeed this matters, it is simple to fix if necessary.
            Last edited by aostling; 03-14-2020, 08:28 PM.
            Allan Ostling

            Phoenix, Arizona

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            • #81
              Brian, what kind of piston/bore clearance are you thinking about?
              I cut it off twice; it's still too short
              Oregon, USA

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              • #82
                There is a bit of parallax going on in that picture. The fins and the grooves are both 0.093". Piston to cylinder clearance---That's a moving target. My worst fear is that after a lot of machining the piston falls thru the cylinder ---not good. I deliberately aim for a slight interference, then after honing the cylinder if I still have any interference I will lap a bit with 600 grit aluminum oxide paste. So---In theory, I suppose there really is no gap. Of course it only stays like that for the first 100 revolutions. Better minds than mine say you should have a diametral clearance of 0.002". Trouble is, if you aim for that, it quickly changes to .005" to .007"---again, not good. I am running cast iron rings on the aluminum piston. Again, better minds than my own say that because of differential expansion properties between aluminum piston and cast iron cylinder the aluminum piston will seize in cast iron piston. I haven't had that happen.
                Brian Rupnow
                Design engineer
                Barrie, Ontario, Canada

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                • #83
                  Today we have a cylinder head.--And trust me, lads, there are some strange and wonderful set-ups involved with making this part. Everything came out like the drawing asked for, but there was a bit of tongue biting and butt clenching involved!!!
                  Brian Rupnow
                  Design engineer
                  Barrie, Ontario, Canada

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                  • #84
                    --Today I drilled and tapped both ends of the cylinder for #8-32 bolts, and happily everything bolts together properly. I honed the cylinder with my 3 stone brake hone, a total of 50 slow strokes end to end. I hone it dry with no lubricant. The piston was quite happy to start into the cylinder for about half it's length, then started to tighten up. I coated the piston with #600 grit aluminum oxide paste, and mounted the cylinder in the three jaw chuck on my lathe. I have a temporary handle which fits into the piston and is locked there by the wrist pin. With the lathe on it's lowest speed I began slowly working the piston into the cylinder about 1/8" at a time, ready to let go immediately if it "grabbed". When it started getting "grabby" I squirted a little #30 oil onto the piston and kept working it back and forth until it started to come out the other end of the cylinder. At that point I stopped, cleaned piston and cylinder with laquer thinners, then gave both a scrub with an old toothbrush and some dish detergent. When dry, the piston will fall thru the cylinder. If I put my hand over the end to seal it, the piston stops. This did NOT give a tight enough seal for the engine to run without rings. To run without rings, as my marine engine does, takes a much tighter fit and a much longer time lapping. I'm happy with the days work. I've had to give up taking my "Fat man's walk" in the local mall because of the virus. Yesterday I returned to my solitary walk in the woods for exercise. It is beautiful there, but we still have about a foot of snow in the bush, ---I'll be glad to see spring come.---Brian

                    Brian Rupnow
                    Design engineer
                    Barrie, Ontario, Canada

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                    • #85
                      Brian, I have to ask, about the piston clearance with aluminum piston in a steel cylinder.
                      Does the alum piston swell up when it gets hot?

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                      • #86
                        Ringo--I will let you know. I've heard that can happen, but I have never actually seen it. I have 16 or 17 running i.c. engines, but most of them use viton rings. I've never had any of them give any indication of "seizing" from piston expansion. This will be only the second engine I have made using purchased cast iron rings. My first was the vertical hit and miss, and the piston was fitted to the cylinder in exactly the same way.
                        Brian Rupnow
                        Design engineer
                        Barrie, Ontario, Canada

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                        • #87
                          I thought the prior engines you used cast iron piston, no?

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                          • #88
                            The coefficient of thermal expansion (CTE) of aluminum is 0.000013 inches per inch per °F. If the 1.38" diameter piston rises 300F° its diameter will increase by 0.005 inches.

                            The cast iron cylinder has a CTE about half the value for aluminum. So if it too rises 300F° its diameter will increase by 0.0025".

                            The net effect would be that the clearance between piston and cylinder wall would decrease by about 0.002" to 0.003" for an assumed 300F° temperature rise.
                            Allan Ostling

                            Phoenix, Arizona

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                            • #89
                              Originally posted by aostling View Post
                              The coefficient of thermal expansion (CTE) of aluminum is 0.000013 inches per inch per °F. If the 1.38" diameter piston rises 300F° its diameter will increase by 0.005 inches.

                              The cast iron cylinder has a CTE about half the value for aluminum. So if it too rises 300F° its diameter will increase by 0.0025".

                              The net effect would be that the clearance between piston and cylinder wall would decrease by about 0.002" to 0.003" for an assumed 300F° temperature rise.
                              I agree with the numbers but its the diameter difference between the cyl and piston values that would decrease by .002 to .003. The clearance is equal all the way around , or put another way is the change in radius values. so the (wall) clearance would decrease by .001 to .0015 More clearance is needed the bigger the bore because of the expansion properties. Going by Brians description of the fit (no measurements), it sounds like he will be VERY close to rubbing when hot.

                              Maybe SId or gbritnell will chime in with a good clearance value for a bore this size.
                              Last edited by Sparky_NY; 03-16-2020, 08:36 PM.

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                              • #90
                                Some of the prior engines had aluminum pistons, some had iron pistons. I would have to go back and look to see what material I had used with what specific engine. All of the cylinders were cast iron. Theory is that a low rpm engine can handle an iron piston with no problems, but a high rpm engine uses aluminum pistons because of the reduced mass.
                                Brian Rupnow
                                Design engineer
                                Barrie, Ontario, Canada

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