Shrinking cast iron onto aluminum

Tim Wescott · 2012-01-19T04:09:14+00:00

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...

Jan 19, 2012 26 Replies

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Ed Huntress 14 years ago

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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Jim Wilkins 14 years ago

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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John B. 14 years ago

Years ago pure racing engines usually had ABC cylinders and pistons - aluminum, Brass, chrome. Have a look at

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you haven't seen it.

-- John B.

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etpm 14 years ago

I guess c is most important. "I did it my way". Totally reasonable. Eric

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Steve W. 14 years ago

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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Steve W. 14 years ago

One problem with shims is compression loss. Unless this one has an adjustable head?

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Tim Wescott 14 years ago

ABC is cool, but I suspect you need a surface finish that's at least as good as, if not better than, steel on steel, _and_ you have to find a chrome shop that can do hard chroming in small quantities.

ABC engines are pretty common production items, these days.

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