Sunday, January 3, 2021

About That Small Engine I'm Building

Small?  Tiny?  How does 3/4 cubic inch strike you?  To me, that's closer to tiny than small, but definitely in the range.  3/4 cubic inch is bigger than the tiny model plane engines and smaller than any lawnmower I've owned.  

The short update is that it still doesn't run. A longer update is I haven't tried to run it since finding the fuel bottle destroyed back a bit over two weeks ago.  The parts I need to fix the tank were here by the day after Christmas, much sooner than I expected, but I haven't tried to put gas in the tank and start it yet.

I've been doing other things.  I noted in that linked post that it was time to take that valve stack apart. For openers, it didn't look like the intake valve spring was moving right, so I did something I should have done all along and tapered the valve guide per print.  First the intake and then the exhaust.  Because I didn't think it possible to remove the valve guide from the intake block without ruining the guide, I set up my four jaw chuck on my manual Sherline lathe and used their compound to cut the taper.  

Then I lapped the valves against their valve guides again. This time with the valve stem in the three jaw chuck on the lathe and pulling the block with the valve guide (pictured) against the valve.  I've done more troubleshooting steps trying to convince myself the engine is OK.  

I wanted to make sure the valves would drop freely through the valve guides.  Intake fell, exhaust would bind sometimes.  I made a little valve holder to hold it in lathe chuck, then held 240 followed by 500 sandpaper against as it spun in the lathe.  After this, it fell freely through the valve guide as well. It was time to put it all back together and see if it looked any better qualitatively.

When I ran the flywheel with my starter drill, I didn't see any motion of the intake valve.  I want to see it opening and closing, but I've been told by a couple of guys who have made this engine that it might not be perceptible.  On the other hand, I've seen videos where the intake valve's movement is very obvious.  There's a potential problem with the intake valve spring in that I made it from music wire I had on hand (.015" dia) not what the plans called for (.013" dia). If there are two identical springs made with those wire sizes, the one made with bigger wires is stiffer than the one made with the smaller.  Springs are complicated. The strength comes from the wire diameter, the pitch (turns per inch) and the diameter of the spring. I could have used the wrong wire but made up for it by making too many turns.

Strangely, I have around three feet of .013" wire in the house. It's called the second string in a set of Ernie Ball Slinky guitar strings.  Even stranger, it's cheaper to sacrifice a set of those strings than buy a few more feet of .013" music wire from the usual sources.

I'm just about at an impasse where I don't know what to do.  One possibility is to rewind the intake valve spring.  I have considered sending the whole engine to Hydraulic Press channel on YouTube to let him crush it into a round slab of mixed metals.  A better idea is to back away for a few days.  I could use to clean up and organize a bit better in the shop anyway.


  1. Can you open the valve by hand? How much distance is there between the turns of the spring? Since there's no intake lobe for the cam, and the valve is opened entirely by suction, the maximum amount of valve lift is only controlled by the suction and the amount of travel the spring has before it (or something else) goes solid, aka "coil bind". It could be the spring is a design-point and must have certain specs. If I had the wire, I'd remake the spring.
    You're getting there, and you're undergoing the same trial-and-error learning curve the pioneers of Internal Combustion went through. Good to see you sticking with it.

    Just spent the last three days cleaning up the Electronics Shop in the basement. I went to fix some little thing for SLW, and couldn't find a clear spot. Then I saw that I had FIVE distinctly different projects in various stages of non-completeness, and realized I've gotta hit the brakes, stop, and clean things up. So I sorted, bagged, boxed, and otherwise separated all five projects. Reorganized my shelves for wire and chemical storage, and started another "eBay" pile.....

    1. The valve opens easily with finger pressure. I could probably rig up something to measure how much force it takes to open it using fishing sinkers, but it doesn't feel stiff to me.

      I have some projects I'd like to do in the electronics end of the house, too.

  2. Sometimes it is a good idea to just step away from something for a few days. It's amazing what a different day will bring.

  3. Sometimes it is a good idea to just step away from something for a few days. It's amazing what a different day will bring.

  4. Do I see a steamer next in your life ? And I dont mean a steaming wife. Fits and clearances are looser for a repiprocating steam engine and there is no need for refined fuels. In fact during this discussion I just thought of something, almost too late for me to build for Winter Field Day- A small steam powered electric generator to power a QRP rig. Im sure someone will beat me to it but im an open source kind of guy. Never give up. Although sometimes the clock doesnt allow enough time to complete that bucket list. Then its priorities first.

    1. Steam engines are very popular with engine builders and I don't think it's an exaggeration to say I see something I've never heard once a week. I've only build one of those wobbler type engines that are pretty fun to watch.

      Most people run them on compressed air because boilers can be scary, but I'm sure that's what you would need to run a rig on FD.

      I thought that a 1 HP engine would be fine for running a generator. 1HP is 750 watts, and one hour of running that (750 Watt*Hrs) would run a 100 W rig for a week. I think one of those 49cc engines they put on mopeds and scooters could do 1 HP and be cool to build.

  5. Well I hope you don't give up. It is oddly comforting to see you work your way through this challenge. Call it the triumph of hope over experience. Thanks for sharing the work with all your readers.

  6. I had a truck come in the shop once that would start and run cold and lost compression warm. The valves were too tight.
    Another time while homeschooling, we discovered in science lab that ohms law didn't calculate right based on circuit off measurements. Temperature rise in the light bulb changed the impedance.
    Are there any variables not questioned in your engine?