Pistons problemo!

[graham-xrf]

Here we are talking vintage horizontal twin engines - like 1930s through 1940s vintage.
The bores had corroded, and one piston somewhat scored above the top ring. The poor thing needed the re-bore!
Step 1 was to discover a place that supplies Douglas T35 pistons. I found one in UK that caters to the vintage motorcycle restorers. They had them all, even for a ancient Douglas T35 engine, and they could do it in a +0.020 to suit a re-bore. Bore and stroke and wrist pin size matched all exact.

Step2. The piston supplier regularly supplies the re-bore firm, (Saunders in UK), so they delivered them there direct, saving me a long road trip. The bores are now all beautiful, bored and finished with cross-hatch hone, but that is where I got my first look at the new pistons. Oh dear! This does not look right!

It turns out that the Douglas engines all looked much the same from the outside, but they varied what went on inside depending on the year, and the application. The same 350cc engine was used in everything from generators through to the motorcycles range. Simply changing the crankcase to mount in a bike, using two carburettors instead of one, changing the compression ratio to 8 to 1 with different internal head shaping and piston, gave them a range of low power generators through to as much as 28bhp for the motorcycles. Most of the other parts were interchangeable.

So now, when I compare the new +0.020 pistons to the originals, I think we have a problem! The compression ratio is already low enough without then losing the hemispherical dome!

The hunt is on to find a piston with 5/8" wrist pin, and (freely mixing units), 60.8mm dia + 0.020, with a dome top. i have no idea what could be done to "increase" the compression ratio to run with the flat top. This engine is not for performance anything. It only has to run occasionally, deliver maybe 4bhp or so, for display days of an historic French flying death-trap called Le Pou du Ciel, which will never be allowed to fly.

Now I just don't know what to do!

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[ PS: The last picture was from back when I had it assembled and running, from pre-lockdown days, before it got neglected. ]

 

[matthewsx]

Here in the states we use these guys.

Wiseco - Forged Pistons | Shop Automotive & Powersport Pistons For Sale

Shop forged pistons online at Wiseco. Explore our extensive range of powersport and automotive pistons to maximize your engine’s potential.

www.wiseco.com

I'd give the a ring and ask for something with the specs you need, they might have it off the shelf. If not they have been known to do custom stuff for pretty reasonable prices.

John

 

[sdelivery]

I would suggest you make what you need.

 

[matthewsx]

What could possibly go wrong?

 

[graham-xrf]

Many thanks Matt. I will be hunting out everything. The supply firm actually imports the range from Australia. They are also looking for what they can source, and they are happy to take the flat top pistons back, since they are still unused. These pistons were not exactly cheap. £225 for the pair, with set of new rings. ($281.25).

 

[sdelivery]

View attachment 486787

What could possibly go wrong?

He said it was for a static display that only gets run occasionally.
Besides making a piston is not that difficult.

 

[graham-xrf]

I would suggest you make what you need.

I am challenged enough making stuff end up round. Pistons are not round! They are a bit "oval-shaped", and they also have a taper. The special "cam-lathe tool" trick is not what I can do!

The display stuff is not quite "static". I expect they will taxi it about to show the visitors.

 

[sdelivery]

I have no problem turning a taper or out of round! lol
I have a friend who has made several pistons but I was skeptical, until I went into a customer's shop and he was making pistons, rods, girdles and more!
It was all diesel pulling tractor stuff but the same applies to smaller gas engines as well.

 

[graham-xrf]

View attachment 486787

What could possibly go wrong?

Yes - as per the "Jimmy's World T-shirt. Quite a lot actually. The one in your picture was a modified design that made it safe, although all are a bit prone to problems if the wind is a bit more than "slight". It was banned in France until the design fix could be found. Henri Mignet's first design is the one I am dealing with.



It had the unfortunate characteristic that if one had an engine with that little bit more power, or otherwise one went a little faster by pushing the stick forward. it would obligingly nose down into an unstable state. If one pulled back on the stick, it would nose down further, instead of coming back up. Pull back hard enough, and you crash into the ground!.

So look at it. There are NO ailerons. The entire fin at the back is all rudder, but there are NO rudder controls for the feet. The way it banks is by secondary effect from the rudder, (worked by the stick) . At least the yaw is in the correct direction, because of the lack of ailerons.

Look at the lower "wing". Is that a (sort of) biplane, or is that what would be the tailplane elevator made huge and moved forward? Is it a "tandem wing"? The way the movement of the stick made it go up and down was by tilting the entire upper wing, hinged at the leading edge. The air from under the wing, in the slot, going over the "rear" wing, gives that "rear" wing enough lift to tip the whole plane forward, and the faster you go down, the more it tips forward.
The problem was, Henri Mignet (designer) was utterly crap at conventional plane controls coordination. He was out to make the controls "simpler", so he settled for only a stick - NO ailerons, and NO separate rudder control.
Pull back => UP.
Push Forward = DOWN
Stick left => TURN LEFT
Stick right => TURN RIGHT
Bank in a turn comes as a bonus secondary effect from the big fin at the back. The cables from the stick that, in any other airplane, would work ailerons, were connected to the rudder instead.

Modern versions
There are hundreds all over France, and some have been modified to have a real tailplane elevator at the back. They are slow, and safe, but not easy if the wind is more than about 8mph. Originally, folk built them at home from a magazine article series.
After the safety design change, here is what they used to look like..



And then, they evolved..

_ _

This one has a canopy..



And this one even folds up to stash in the garage!



BUT - I need to find a solution for the pistons. Whatever specialized synchronous tool cam movement lathe used for making a "oval" in a piston, and simultaneously a non-linear taper is something I do not have. I have only just started the hunt. All this, by the way, interesting as it might be, is a total distraction from the stuff I would would rather be getting into.

 

[sdelivery]

Wow just wow. I don't think I could ride in one. But they are so cool.