Shrinking cast iron onto aluminum

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.

=20 Dan

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

Speak to Nick M=FCller. Not sure if he visits this group anymore, but he builds lots of small engines. His website is here:

Chris

I can't speak authoritatively on this, but I'd use ductile iron rather than cast.

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

-- it also requires more skill in getting a good fit.

I wouldn't use either one. As Dan said, this is an invitation to crack the iron sleeve.It's true that ductile iron's coefficient of thermal expansion is much higher than that of gray cast iron, but it's still around 2/3 of aluminum's coefficient.

As someone else said, the better solution would be to find a way to retain the ends of the wrist pin in an all-aluminum piston.

As for supereutectics, a lot of cast automobile parts are made from it. But I don't think I'd go to those lengths. Any casting grade of aluminum should do it. The more silicon, the lower the coeffieient. But most intricate cast aluminum parts are made from relatively high-silicon grades.

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.

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.

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.

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.

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.

How big is the piston? Can you machine it solid out of a salvaged AL truck piston which would give you a proper alloy? Your cylinder could also have witness marks and a copper crush washer to ensure clocking.

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.

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.

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-)

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.

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

(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)