Shrinking cast iron onto aluminum

I honestly can't remember if I asked this already.
I'm working on a model airplane engine. It's a Cox 09 crank case on
which I'm replacing the crank, rod, piston and cylinder, for the purposes of learning to build engines.
I want to use an aluminum rod and a traditional wrist pin, instead of the Cox-style ball-and-socket rod connection. But because the cylinder is screw in, and you can't really count on the ports ending up clocked in any particular position, I pretty much need a smooth piston in order to not screw up the porting.
So I'm thinking of making an inner piston of 6061 aluminum that acts as a carrier for the rod, and shrinking an outer piston of cast iron onto it. I'm envisioning a 30mil wall thickness for the cast iron. (If you're not familiar with the Cox 09 engines, we're talking about a 1/2" bore here).
Am I totally out to lunch here? Am I going to get this whole thing assembled, then hear a "tick!" as temperatures are equalized, telling me that my nifty new piston design is really just a method for making short, broken cast iron tubes?
And how much do you think I can expect the piston to warp as the engine runs? Can I expect the cast iron to control the swell of the aluminum much with heat, or can I expect the aluminum to just expand the piston as if the cast iron weren't there, leading to either seizure or the above- mentioned broken cast iron tube?
Is it even reasonable to expect to be able to machine a cast iron tube to a 1/2" OD with 1/32" walls?
It's a steel cylinder, but apparently it's pretty common to have steel pistons running in steel cylinders for this size of engine -- that's my backup plan which makes me worry a lot less about tensile strength, without really alleviating my worries about distortion with heat.
Another alternative that occurs to me is to dredge up some hypereutectic aluminum from someplace, and use that for its favorable expansion properties. But I'm not sure if (a) I can get it, or (b) if this whole shrinking thing has a chance of working.
Comments appreciated.
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Tim Wescott
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If you shrink cast iron over aluminum, you already have the cast iron in tension. And then heating it up increases the tension. Cast iron is not all that great in tension. So I think the cast iron will crack.
How about threading the inside of the cast iron and outside of the aluminum. Maybe use a center punch to to keep the assembly from unscrewing. That way the cast iron could not be in tension even when heated. It would be slightly heavier.
Dan
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well to answer the machining question of the cylinder:
yes it can be done. the out come of course will be tooling and skill dependent. but it's not that difficult of a problem at all. i'm making the assumption that tolerances will be within the +/- .001 range.
but i have to ask, why not focus on some buttons for the wrist pin holes? i assume that's the issue?
sounds like an interesting project.
good luck
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Speak to Nick Mόller. Not sure if he visits this group anymore, but he builds lots of small engines. His website is here: http://www.motor-manufaktur.de/
Chris
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Tim Wescott wrote:

I can't speak authoritatively on this, but I'd use ductile iron rather than cast.
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On Thu, 19 Jan 2012 10:15:45 -0800, Jim Stewart wrote:

I don't have any ductile, or I'd certainly use it.
I'm thinking that I'm just going to bite the bullet and use steel. I made the cylinder out of fairly soft steel, and I have a good long length of some high-alloy stuff that certainly counts as "dissimilar". The biggest reason that I hesitate is that -- while a steel on steel fit is actually supposed to last longer (http://www.modelenginenews.org / techniques/materials1.html) -- it also requires more skill in getting a good fit.
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wrote:

It isn't clear to me why you aren't using aluminum, Tim. I thought that you were worried about wrist pins in relation to ports.
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On Thu, 19 Jan 2012 13:33:34 -0500, Ed Huntress wrote:

Because I can't count on the cylinder clocking, and because of the spacing of the ports around the cylinder and the size of the pin, if I leave any gaps in the piston for things like wrist pins then there would be an opportunity for a transfer port to leak to the exhaust as the piston pin swept by.
The recommended "easy" way to do an engine is to use a cast iron piston in a steel sleeve -- but that's also for when the sleeve is clocked in a known relation to the piston, and there's no place for any given port to leak into the pin sweeps by.
I'm not sure if this is why Cox chose the piston/rod retention method he did, but Cox pistons are certainly featureless as you go around the circle.
I suppose that if worse came to worst I could go ahead and make ball and socket, and peen the piston onto the rod. Ick.
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wrote:

The little Cox engines were extremely clever pieces of design. One of my former co-editors, Bob Hatschek, wrote at least one article about their manufacture and was quite authoritative about the engineering. They were a screw-machine job. Bob holds, or held, some model-glider records and travelled all over the world to fly them, so he was really into models.
So now I don't understand why you're thinking of steel or iron. You still have the wrist-pin issue, unless you put a sleeve over the piston or plug the ends of the wrist-pin holes.
If it were me, I'd consider shimming the cylinder in the crankcase until it lines up perpendicular to the wrist pins. I'd turn out a tool to punch shims out of copper or stainless, whichever worked best.

They swage them with a custom tool. Clearance between the ball and socket is critical, as you probably know if you've run them to destruction. I had a piston in an 049 come loose when I was a kid. But I always mixed in some extra nitro, too. <g>
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On Thu, 19 Jan 2012 14:16:57 -0500, Ed Huntress wrote:

Putting a sleeve over the piston is exactly what I'm proposing to do, and putting up for review. I think I've been talked out of using cast iron for the sleeve, though.

I've considered that, too, and may well do it if the sleeved piston idea doesn't pan out.
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wrote:

I believe it would be a lot easier and much more reliable. But you're having fun, and I never like to get too practical when someone is having fun making something.
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On Thu, 19 Jan 2012 14:55:52 -0500, Ed Huntress

Hmmm....I haven't looked at one of those engines for maybe 50 years, but how are the transfer port(s) and exhaust ports displaced relative to each other? Most loop-scavanged engines have two transfer ports at 180 deg., and exhaust port(s) at 90 deg. to them.
I remember two exhaust ports on the little Cox engines, but I don't remember how the transfer ports are arranged. If they're at 90 deg. to the exhaust, you may have a problem with wrist pins no matter what you do.
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On Thu, 19 Jan 2012 15:00:24 -0500, Ed Huntress wrote:

Yup.
That's how a Cox cylinder is arranged, yes. The replacement I've made just has one exhaust port (it's an experiment), but it's got the same problem of the wrist pin passing by a port no matter what.
If everything were fixed in place rotation-wise, I'd just use a wrist pin that is held in place, one way or another. But because the cylinder can rotate (and because I _am_ having fun), I want to see if I can do it with a sleeve.
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wrote:

Oh, boy. I've been thinking about this, and I thought I had an answer for you, but I wasn't aware that the *cyinder* can rotate. That's a fly in the ointment.
The thought was this: Lots of loop-scavanged engines have been built over the years, and I'm sure that most of them have wrist pins. I can imagine that some, or most, are pressed-in rather than free-floating, in which case leakage is not a problem.
In fact, even if they're free-floating, held in with circlips or whatever as in many 4-stroke engines, it still isn't a problem if the ends of the wrist pin always faces the exhaust ports. When the pin crosses the bottom of the exhaust port, there's little pressure in the crankcase. Maybe no positive pressure at all. There's no pressure anywhere to cause a leak.
But if the wrist pins are floating in the piston and they cross a *transfer port*, there is going to be some crankcase pressure before the wrist pin crosses the bottom of the port (if it has a bottom -- see below).
But where would the gas go? When the ends of the wrist pin are exposed to ("communicating with") the pressure in the transfer port, the pressure is the same on both sides -- the inside of the piston is at crankcase pressure-- and there is no communication with the exhaust port at that point.
So it may not be a problem at all, even with the Cox transfer ports, which, IIRC, are just milled slots on the inside of the cylinder and they extend from the crankcase to the top of the port -- again, IIRC after 50 years.
Give it some thought. Don't trust my memory. I have trouble remembering things in a week, let alone over a half-century. d8-)
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On Thu, 19 Jan 2012 17:40:05 -0500, Ed Huntress wrote:

The cylinder isn't really free to rotate, except that it's screw-in, so it's kind of installed at an arbitrary angle. It's been suggested that I shim it so that angle isn't all that arbitrary -- and I'm taking that suggestion under advisement (I'd like to avoid it if I can).
What my real concern is that, since it's only a 1/2" bore, and since I (ehem) got the exhaust ports closer to the transfer on one side than the other, that a wrist pin might span the difference between the exhaust and transfer, or nearly so, thus creating a momentary leak from case to exhaust, where none should be.
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wrote:

Aha. Ok, I'll pass on my final thoughts and leave you to your fun:
1) Machne another cylinder, and get it right this time. d8-)
2) Use the shim method and key the cylinder into the crankcase if you're worried about it rotating. A tiny capscrew with the head turned down somewhat (or ground down) should do it. I don't remember the .049s rotating in use, but I don't know if I ever looked. Then you can just line up the pin ends with either the transfer or exhaust ports.
3) Press the wrist pin into the piston bosses, leaving the con rod to bear in rotation on the pin (many engines have been built this way). If you want a close fit of the pin ends, press them in, then cylinder grind them flush on your lathe. A Dremel held in the toolpost would be adequate for that.
4) If you must put a sleeve on the outside of the piston, make it from the same material as the piston. You could even make a two-piece piston if you want to get fancy.
Mostly, have fun. And stand back when you run it. d8-)
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I haven't made a model engine yet, just collecting ideas.
Instead of tightening to a shoulder could the cylinder be threaded further up and seat against a locking ring, allowing it to rotate to the proper angle? The outer edge of the metal gasket washer could be folded into the spanner notches to keep the ring tight.
Then measure the piston height at TDC and machine the cylinder or head to adjust compression.
jsw
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Tim Wescott wrote:

The only real way to make a wrist pin style piston work easily would be to mill your own cylinder, Install it in the case, then mark the locations for the ports so the pins avoid them. I think pretty much all of them are made that way.
What you could do is machine the piston in three parts that thread together and capture the wrist pin between them.
A hollow body with a milled recess for the wrist pin, then a thin cap that sets on that with a second set of recesses, then over that you screw the piston dome. Sort of like an acorn nut on a hollow bolt with a shim under the nut.

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wrote:

Greetings Tim, I'm familiar with the Cox .049 engines. And the way the cylinder threaded into the crankcase. I assume your engine works the same way. Since you're making the cylinder too why can't you thread the cylinder into crankcase to a specific torque and mark the cylinder and the crankcase before the porting work is done? Then remove, port and reinstall to the torque and mark. I'm not questioning your methods, I'm wondering what I'm missing, because the above method is what I'd do. Eric
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On Fri, 20 Jan 2012 00:04:57 +0000, etpm wrote:

(a) The cylinder is already made. (b) Not a bad idea; I could do it with shims. (c) I want to do it my way. (d) If (c) doesn't work, see (b)
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