My new piston design

Going to meet a guy in town here at Proturn Engineering. He used to machine “blanks pistons”, but these days the “blank piston” company, Ross Pistons, is doing all the machining themselves, so Richard is doing the designs these days and sends them of to the States.

But before I was going to see him I thought, make my own design, and we will see what he things of it, but I would say its pretty close.

So what I have done, I made two spacers, 5.7mm thick and rolled a piece of Plasticine. Then I used the bottom of the cylinder that has the steel sleeve sticking out to cut my “Plasticine disk”


I put the disk on top of the piston, and marked the Piston pin orientation on it (solid line).

Then measured the distance from outside exhaust valve to outside mixture valve (left to right side). Cut that off, and made a 30 deg ramp going inward. That’s the angle the valves are positioned. This makes sure the valves keep the same clearance as what they have now!

Finally I marked the spark plugs position (dotted line) 30 deg from center line for piston pin. Looks like 30 deg was a handy angle when they were designing the cylinder about 90 years ago !!!  🙂 The spark plug doesn’t really protrude into the combustion chamber, but this way there is some room for the combustion to start. The shape might change a bit possibly.

So this is what I ended up with (see below)

Now all the off cuts, I made a nice round boll of and decided to calculate the volume. And instead of sticking that somewhere, I calculated if I make the “disk” 0.2 mm higher, then the volume decrease of the combustion chamber is what I need to get  a compression ratio of 1 – 7.

Looks nice and clean doesn’t it !!  Will be interesting what Richard will say about it.



Exhaust analyzer

Started to make adjustments to the SU needle last week, but its hard to see what’s happening, so I borrowed Wayne’s exhaust analyzer to check the mixture.

I Noticed that at lower rev’s it was running very rich and at high speed it was still lean I think. It’s a bit hard to say as the readings are very erratic, but I think that’s what it was.

So I adjusted the low speed mixture with a screw on the carb, and adjusted the needle a bit more to give me more fuel at higher speeds.

After installing the needle back in the SU carb, I was thinking “I don’t really know how far the piston goes up at full throttle, I need to find out somehow“.  This is so I know where to make the needle thinner, to get more fuel.

And that’s when Brian, showed up, he’s in our club. He loves photography, so with his massive tele-lens he made some great photos. Thanks Brian!!

SU piston 60{7d16e8204fe0e6723feb9d00795e42e38ac880e6f88f519e81a8ab927d02eaa1} up

As you can see the piston is up, about 60{7d16e8204fe0e6723feb9d00795e42e38ac880e6f88f519e81a8ab927d02eaa1} or so. This is at full speed !  🙂

Now if you do the calculations, the carb has an 2″ opening, the engine has an 1.5″ opening, that means that the carb needs to open 56{7d16e8204fe0e6723feb9d00795e42e38ac880e6f88f519e81a8ab927d02eaa1} to get all the mixture into the engine, so that’s looking good.

Just not sure yet if that’s a good thing or bad. Doesn’t matter I suppose for the fuel / air mixture ratio.

I think its still running a bit rich at low speed now and about 14{7d16e8204fe0e6723feb9d00795e42e38ac880e6f88f519e81a8ab927d02eaa1} at high speed which is what is required, so maybe that’s it.


Here is me looking at the rev counter and the exhaust analyzer:

Checking exhaust analyzer

So at the moment, the engine is running really nice, and people say it runs a lot nicer, so that must be good, just not sure if I have more power….

First proper flight

Yes this morning was perfect, no wind, very high pressure weather system and cold.

Evan made 3 test runs on RW07, tried all the controls and played with the engine. On the 4th run from RW07 he kept on climbing, not that fast but definitely no more runway left to land on. Evan made a gentle turn to the right and landed safely on RW28.

Have a look here for more video’s, a map and some great photos  🙂

Today is the day

Just got out of bed, clear skies, millions of stars, and absolutely no wind. Plus it’s 0 degC.
Getting ready to go to NZRT for another go at getting this plane airborne. But this time I am very confident it’s going to fly.
Hopefully Evan is happy enough to go around the circuit, maybe even a few times, that would just be superb 😊😊😊

More power

The engine started as usual with one swing. I hopped in and waited for it to warm up a bit and slowly opened up the throttle. All looked good. Then after a few minutes I opened up the throttle and YES making about 200 rpm more.

This increase in engine speed equates to a theoretical power increase of :

( 1900rpm / 1700rpm ) ** 3 = 1.40 = 40%

That should be enough to fly !! 🙂