Old School Barstock 2 Stroke

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Brian Rupnow

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In my quest for a simple two stroke engine which I could make from barstock, I looked at many different engines. I didn't want a high rpm screamer with a propellor, nor did I really want to use a design that required me to make rings and figure out how to get them past the ports without breaking them. I also wanted to use spark ignition with Naptha (Coleman fuel) as the fuel, mixed with a bit of 2 stroke oil.--and the potential of maybe experimenting with a glow plug. The 1912 Hubbard Marine engine model seemed to offer up everything I was looking for. It has a 24 mm bore, and a 25 mm stroke, no rings, is water cooled, port induction. and a whopping great compression ratio of 4.45:1. The main body is an aluminum casting with a steel liner. There are videos of this engine running on the internet, and from my research, many have been built and ran successfully. After much messing about, I have come up with a design using barstock which copies all of the Hubbard engines critical dimensions and port dimensions. At first glance, it is uglier than Frankenstein's bulldog, but it does hold a lot of promise.

 
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I have been able to incorporate adjustable ignition timing, and my old standard Chrysler ignition points. The carb shown is a Traxxas Pro15. The piston and the liner will be machined from cast iron. Almost everything else except the flywheel and crankshaft and crank bearings will be from aluminum. The crankshaft will be "two piece"--that is to say, the main crank, the crank throw, and the rod journal will be 3 parts pressed, pinned, and Loctited together forming one "Piece", which will support the flywheel and be driven by the connecting rod. The other crank throw and shaft which runs out to drive the ignition cam will be pressed, pinned and Loctited together to form the second "piece". This second piece is actually a "follower crankshaft" which is driven by the rod journal fitting with a "slip fit" into the crank throw. This helps greatly with alignment issues between the crankshaft and bearings.


 
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This shows the overall dimensions of the engine. I will be posting the detail drawings as I make the individual parts. I will not post them before making the parts, because I have found that quite often the machinist at my house has to go back and remind the engineer at my house that "You can't make it this way!!!---Change the damned drawing please!!!
 
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This shows the overall dimensions of the engine
.

Brian those dimension give me .69 cu in not 1.2:wall:

and the location of the carb on your drawing need a rotary valve or
reed valve and their is no mention of this.??:confused:
 
Luc--I checked the original drawing file and they list the displacement as being 13 cubic centimeters, which converts to 0.8 cubic inches. My solid modeling program gives me 0.89 cubic inches, and I trust it more than the cubic inches given on the original drawings. The cylinder head is a pretty funky shape, and extends way down into the top of the cylinder, which makes the math somewhat difficult. You're right about the reed valve on the carburetor.--Actually, I had another look at the old drawings, and the carburetor has a 1/4" steel ball valve setting in a vertical tube against a seat. The vacuum in the crankcase lifts the ball and draws air/fuel mixture in thru the carburetor. As soon as the vacuum turns to pressure, the ball is dropped back into the seat by a combination of gravity and pressure from the crankcase side.
 
The original plans have an elliptical cam machined on one side of the flywheel, which operates a short stroke piston pump to circulate the water. I don't know right now if I will do the same or not. First thing will be to get the engine to run, with just a plain Jane disc of metal as a flywheel. Then, if I get it to run at all I will worry about refinements like a cam and waterpump. Willy--I'm as confused as you in regards to the glow plug. I have never used a glow plug, but like you I thought they would only work with a "specialty" fuel with a low combustion point. Also, by my understanding, a much higher compression ratio is required to run with a glow plug. The plan set I have says that this engine runs really well with a "glow fuel" and a glow plug, and is very easy to start by hand. It also says that the engine works just fine with a conventional spark ignition system and plain "gasoline" but that it is more difficult to start by hand. I never start my engines by hand anyways. God created variable speed drills to start engines with. As far as a degree plate on my variable timing---No, not likely. I find that the variable ignition timing is wonderful for first start ups. Then, after I find the "sweet spot" where the engine runs best, I lock up the variable timing and probably never use it again. On engines with a wide range of speeds, it is great to have timing that you can advance as you open the throttle. I have seen one of George Britnell's engines that does that, with the ignition advance mechanically linked to the throttle advance.---Sweet!!!. However, this particular engine has a moderate range of throttle induced speed, so I will find the optimum timing setting for a low idle and lock it up there.---Brian
 
This looks like the low RPM two cycle I was looking for. Please build this so I may watch.
 
This is a totally new one on me!!! I have never seen this type of throttle control and one way valve on a carburetor before. There is a needle valve and metering jet directly below the brown 1/4" ball bearing, which "floats" in the vertical passage. When the piston travels upward, it creates a vacuum in the crankcase, which allows atmospheric pressure to lift the ball of it's "seat" against the tube immediately below it and be sucked into the crankcase, picking up a charge of fuel on it's way past the metering jet immediately below the ball. Once the piston has reached the top of it's movement and starts back down the cylinder, pressure builds in the cranckase, and that pressure, helped by gravity push the ball back onto it's "seat" and prevents backflow of pressure thru the carburetor.----And the really neat thing is that the red screw on top of the carburetor is threaded, and screwing it in or out determines how far the ball can lift off the seat, thus giving throttle control!!! this is very old school, and very clever.---I love it!!! Apparently this system works best with the fuel tank ABOVE the carburetor, depending on a gravity feed to the carburetor, and you must have a fuel shut-off valve on the tank for when you are not running the engine.

 
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Some advice please.--Before I go any farther with this design, I decided it would be smart to see if the main body was going to fit into my 12 x 28" metal lathe and still spin for boring the main bearing hosing. It does---but it's darned close!! In the first picture, you see the body at 1:1 scale mounted and centered in the 4 jaw chuck. The top of the most extended jaw clears the nearest obstruction to the center by a good 1/2". In the second picture, I have rotated the four jaw to the point where you can see how far that same jaw actually sticks out past the chuck. ---Scary stuff indeed!!! That jaw is held by two complete turns of the chuck key so, two turns of thread engagement between "not connected" and "up tight against the part". Balance will be totally out the window--I'm aware of that, but won't be turning at a high speed so it's not that big a deal. The third picture shows the full scale cut-out of the main body laying on the faceplate. It looks like some form of attachment to the faceplate would probably be safer and less apt to fly out and damage the lathe or me. The only thing I can think of right now is that profile of the part is the 'finished" profile. If I started the operation with the part 2" wider, I could put a bolt hole on each side of the part in the "extra" material and bolt it to the faceplate (tap the faceplate if I have to) and finish all the boring, and then after removing the part from the faceplate cut away the extra material and finish the profile. I'm open for reasonable suggestions here.---Brian


 
Not sure how big your chuck is in relation to mine, which might make a difference. Last week I had a similar situation in a 5" 4 jaw, although my part was a little wider in comparison with it's length (more square I guess, but not a square). Anyhow, I spun it at around 600 RPM and balance wasn't an issue at all. It was a piece of ali about 3" thick and I couldn't detect any unusual vibration. I didn't feel overly comfortable with the jaw swinging out there like that but it all went well and was a non-event. I'd recommend spinning it up and see how it goes.

Edit to add: It was a through boring operation I was doing as well - step drilled 3 sizes to 20mm then boring bar out to about 7/8", so reasonably heavy machining, not just skim cuts.
 
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You may find balance to be a bigger deal that you suspect. I would use the face plate and bolt on a couple lumps of steel to balance it.
 
The total length of that part shown is 5.009", with 4.019" from the center up to the top of the cylinder. If I start with a piece 8 1/8" long and center it on the faceplate, that will do away with any balance issues. I have to think on the quickest, cheapest, securest way of attaching it to the faceplate yet. The finished width of that part is only 2.27", so if I buy a piece of 3" stock, that will leave lots of room on each side of the cylinder portion for bolts, and maybe tap the other end of the part on center and bolt thru from the back side of the faceplate thru one of the slots.
 
Luc--I'm not sure how that set-up would look. I think it would mean setting the part up on my rotary table and turning the rotary table by hand as I moved the table left or right into the turning cutter. Very clever.--Had never thought about that method. At any rate, my 3 jaw chuck is attached to my rotary table, and I don't want to disturb it. I like the faceplate option.
 
The total length of that part shown is 5.009", with 4.019" from the center up to the top of the cylinder. If I start with a piece 8 1/8" long and center it on the faceplate, that will do away with any balance issues. I have to think on the quickest, cheapest, securest way of attaching it to the faceplate yet. The finished width of that part is only 2.27", so if I buy a piece of 3" stock, that will leave lots of room on each side of the cylinder portion for bolts, and maybe tap the other end of the part on center and bolt thru from the back side of the faceplate thru one of the slots.
Just hold it down with a strap, using the two slots near to the head end. Just a bit of mild steel flat with a hole at each end, and some long bolts. That will give you enough room for boring the crankcase. To offer you some more ideas, I show several faceplate setups here:

http://www.charleslamont.me.uk/Seagull/crankcase.html

see also the sump, cylinder barrel and con-rod pages.
 
Brian,
I assume you intend to make the center bore on the lathe. Have you considered doing it on the mill with the part centered under the spindle and using a boring head to make the bore accurately?

Or are you trying to accomplish something different to what I assume?

Peter J.
 
Brian,
I assume you intend to make the center bore on the lathe. Have you considered doing it on the mill with the part centered under the spindle and using a boring head to make the bore accurately?

Or are you trying to accomplish something different to what I assume?

Peter J.

My plan at present is to do the cylinder bore on the lathe. It will be a "second operation" after the crankcase bore for the crankshaft is finished. that way I won't be boring into a blind hole, the drill and boring tool will break thru into the cavity already machined in the first operation. As you can see in some of the model cross sections, the cylinder bore is not a smooth stepless bore all the way through. There is an enlarged area part way down the bore for the water jacket, which again would be difficult to accomplish on the milling machine.
 
This quest is taking me in new and different directions than I have ever gone before. The consensus seems to be that what I want to do regarding the piston/cylinder fit with no rings is indeed possible, but I am going to have to learn lapping skills that I currently do not have. So---I have today ordered a 15/16" internal barrel lap (which will easily expand to 0.945" or 24 mm) and a tube of 8 micron to 12 micron "light" diamond lapping compound. I looked at external manual hones, but as they cost upwards of $400 I will probably be making my own external lap, (pending some information from Ramon on HMEM) from a piece of brass with a hole bored thru it and a pinch bolt. Wiser heads than mine are suggesting that I use leaded steel for the liner, and as I have a piece of 12L14 left over from my last engine I will probably use it for the liner with cast iron for the piston. My course of "step by step" actions will be to first make the cast iron piston, turning the outer diameter of the piston to about .001" to .002" oversize from my desired 24 mm (0.945"). I will then set the piston aside. I next machine the outside diameter of the piece of material for the liner to ensure it's roundness over the full length, including an inch or more to be held in the chuck jaws. Next I will bore the liner and chucking stub to about .0015" to .0001 undersize and then turn the outer diameter to finished size, turn the "lip" at the top of the liner to finished size, and still leave the liner attached to the "chucking stub" held in the lathe. Then over to the rotary table to cut the ports and sparkplug hole in the sides with the mill. Then back over to the lathe to lap the inside bore to exact size, using a lap mounted in the lathe and the cylinder liner in my hand (I try for a tighter fit at the top of the liner than at the bottom--this is sort of a "by feel" thing.). Now, assuming the cylinder liner is exactly finished to "On size", I put the chucking stub end of it back into the lathe. No farther machining operations will be carried out on the liner, except to part it off from the chucking stub after the piston is fitted. I then use the external lapping tool on the piston by hand, and bring the piston down to a point where it will just begin entering the bottom end of the cylinder liner. At this point I attach a T handle to the piston, using a temporary brass "wrist pin" and using a solution of very little fine diamond paste with a lot of kerosene, lathe not running, I wring the piston in and out of the cylinder by hand, untill I feel it enter freely up as far as the exhaust ports ,then with increasingly more friction as it reaches top dead center in the cylinder. This can be adjusted by just how much lapping I do of the piston into the cylinder. When I am happy with the fit, I then part off the liner from it's chucking stub.---Have I got the sequence right???---Brian
 
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These two parts are going to be the most critical parts on the engine, in terms of me being able to make them well enough to give sufficient compression for an engine to run, so this is where I am going to start. Contrary to what I said earlier, I haven't made them yet, so don't bother to copy them. They may change. I will post a download link to all the drawings when the engine is finished and running.---Brian

 

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