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  • This is shaping up to being a "no machining day". However, I have modelled a starter hub to one side of the engine and a power take-off pulley to the other side. I might get a chance to machine them today, but I have a lot of "Honey Do" things to look after.

    Brian Rupnow
    Design engineer
    Barrie, Ontario, Canada

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    • No jig, bob. The fan and the hub both had a 3/16' hole on center, so I just ran a # 10 bolt thru them and cinched it up tight with a #10 nut. After soldering I just unscrewed the nut, removed the bolt, and I was finished.
      Brian Rupnow
      Design engineer
      Barrie, Ontario, Canada

      Comment


      • Today was not one of my stellar days. I machined the drive hub and the power take off pulley, and they turned out well. They are held onto the flywheels by four #4-40 socket head capscrews. Drilling the four holes in the starter side flywheel went well. Tapping the first flywheel hole went well. Tapping the second hole didn't. I broke the tap off flush with the surface of the flywheel. After repeated tries to get the broken tap out, I went across town to my tool shop and bought two new taps, two new drills and two 1/16" diameter center cutting carbide endmills. On arriving home, I discovered that my newest Diana Gabbeldon book had arrived, so I made an executive decision to read for the rest of the day. Something of interest--that #4 hand tap and it's drill are about 13 years old, and gets used fairly often. I measured the drill, and it measures at 0.088" diameter. the man who sold me the new taps and drills said that the standard tap drill for a #4-40 bolt is 0.094" diameter.---Who knew--
        Brian Rupnow
        Design engineer
        Barrie, Ontario, Canada

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        • Who knew???
          Ummmm, every chart, Machinery hand book, internet……..

          Comment


          • Originally posted by sid pileski View Post
            Who knew???
            Ummmm, every chart, Machinery hand book, internet……..
            +1 and it is not uncommon to use a slightly larger drill than the "standard" size. It results in a slightly lower percentage thread but the strength reduction is very slight AND it makes tapping much easier with far less chance of tap breakage. In reality, there is no standard size drill for a given tap size, all depends on the desired thread percentage.

            Didn't they cover this in engineering school?

            Here is a sample chart reflecting the above : http://theoreticalmachinist.com/PopU...SizePopUp.aspx

            Last edited by Sparky_NY; 11-24-2021, 10:11 PM.

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            • Originally posted by Sparky_NY View Post
              AND it makes tapping much easier with far less chance of tap breakage.]
              I don’t think using a 13 year old tap that gets used fairly often had anything to do with tap breakage.

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              • I've just looked at all the tap drill charts I have, and a half-dozen online, and they all say #43 (0.0890") is the one to use for #4-40 for aluminum, brass, plastic, which should give around 75% engagement.

                For steel, stainless steel and iron thread engagement is recommended to be about 50%, and in this case the #4-40 tap drill is #41 (0.0960").

                For #4-40 threads (major diameter 0.112", minor diameter 0.0813")
                #43 (0.0890") = 74.9% thread engagement
                #42 (0.0935") = 60.3% thread engagement
                #41 (0.0960") = 52.1% thread engagement

                So the 0.094" tap drill (#42?) recommended by Brian's supplier will work fine as an all-purpose compromise.
                SE MI, USA

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                • Once again the old and arthritic have defeated the powers of darkness. It cost me a 13 year old #4-40 tap and a new 1/16" carbide endmill, but in the end the broken tap was removed and the hole was rethreaded at the correct size. (on #4 taps, that trick actually works about one time out of three for me.) So, the engine now has a starter hub on one side and a power take-off pulley on the other.

                  Brian Rupnow
                  Design engineer
                  Barrie, Ontario, Canada

                  Comment


                  • The engine is almost fully assembled. I had trouble with the first cylinder head, so had to machine a second one. The engine is pretty well finished except for "tidy up" and cosmetic stuff. I'm not sure you can see it, but the camshaft gear ended up hanging below the baseplate by about 0.020", so I've had to epoxy a pair of filler plates on the bottom of the base to raise everything so the gear doesn't rub on the table top. This was an easy change to the engine plans, I just made the base out of 5/8" stock rather than 3/8" thick. I didn't want to remake the baseplate, so the filler plates will do for me---I have to tidy them up to match the contours of the engine base. The only remaining things to be made are the cast iron piston rings.
                    Brian Rupnow
                    Design engineer
                    Barrie, Ontario, Canada

                    Comment


                    • Does anyone have a source for 1/4"-32 Helicoils? I've managed to bugger up the threads in my sparkplug hole in the aluminum cylinder head. There is a considerable amount of time and work involved in making these heads, and I don't want to scrap this one. My options are #1--Find someone who manufactures a sparkplug with a 5/16" thread---(That would be my absolute best option)---Or find a source for 1/4"-32 Helicoils,----Or ---Make my own sparkplug with a 5/16" thread. I have made my own sparkplug before, but I don't really want to. Thank You----Brian
                      Brian Rupnow
                      Design engineer
                      Barrie, Ontario, Canada

                      Comment


                      • Something more--There are manufacturers of M8 metric sparkplugs, which are very close to 5/16". M8 is 0.315" diameter. 5/16" is 0.312" diameter. Perhaps my easiest fix here is to buy an M8 sparkplug and an M8 tap. They seem to be readily available on the internet.
                        Brian Rupnow
                        Design engineer
                        Barrie, Ontario, Canada

                        Comment


                        • Why not plug it and retap to the 1/4-32?

                          Sid

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                          • I don't trust my aluminum welding capabilities that well. Granted, I can stick two bits of aluminum together, but I really would like to find an answer that doesn't include the possibility of ending up with a ball of slag.
                            Brian Rupnow
                            Design engineer
                            Barrie, Ontario, Canada

                            Comment


                            • I thought the spark plugs you always use were 10mm? If you have enough room you could make your own threaded insert by drilling out a bolt or threaded rod and tap it to your spark plug thread size, then drill and tap the head and loctite the insert in.
                              Larry - west coast of Canada

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                              • I use two sizes of sparkplugs. The largest 1" bore engines get a 10mm sparkplug, because I have room for it and because they are a lot cheaper, (I can buy them at an automotive supply store). The smaller engines (this one is 7/8" bore) get a 1/4"-32 sparkplug because there just isn't enough room for two valves and a large sparkplug.
                                Brian Rupnow
                                Design engineer
                                Barrie, Ontario, Canada

                                Comment

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