Cast iron piston substitute

[hacklordsniper]

Hello,

im close to finishing my little IC by Jan Ridders called Debbie. The design calls for cast iron cylinder and piston, and i built CI cylinder on my last lathe. However i dont want to ever again put cast iron in my new lathe, so what could substitute CI piston in this engine?

Could i use teflon for piston? Or alluminium with teflon rings?

 

[mrehmus]

Hello,

I'm close to finishing my little I.C. by Jan Ridders called Debbie. The design calls for cast iron cylinder and piston, and i built CI cylinder on my last lathe. However i don't want to ever again put cast iron in my new lathe, so what could substitute CI piston in this engine?

Could I use Teflon for piston? Or aluminum with Teflon rings?

Cast iron is the best and if you clean the lathe up afterwards, will not hurt it.

Aluminum pistons are used all the time but Teflon is not a good idea as it flows under pressure like cold grease unless it is mixed in with some sort of fiber. Any type of O-Ring on much more than a hit'n miss (higher speed) will quickly fail.

Make the cylinder out of leaded steel (12L14 in the US.) and the piston of cast iron. Lap the cylinder and then lap the piston to the cylinder.

Otherwise you will have to make piston rings (a much bigger job) either using the Trimble method (Strictly I.C. articles) or one of several other articles on piston rings. Most model piston rings are cast iron anyway.

Machinists cut cast iron all the time with no bad effects on their tools. If you just don't want to clean all of the tool, cover the parts you don't want dirtied with the plastic film used to wrap food.

 

[digiex-chris]

Tough to beat cast iron and steel combinations for wear and longevity. Steel liner and a cast piston is a really good bearing surface.

That said, most commercial model IC engines use a brass liner that's been hard chrome or nickel plated. The smaller ones use an aluminum piston, and lap a slight taper into the nickel plated surface. Rather than piston rings, it's a lapped fit between the cylinder and bore. It needs to slightly bind at the top of the cylinder in order to maintain proper clearance when hot. It works ok, but to get enough taper in there you'll either need to lap the taper in, then plate, then finish lap, or get someone to plate a good solid 0.002" onto it and then lap the taper in.

I like making those kinds of fits. You can basically just stop 0.001 or .002" undersize on the bore, and fix both taper and surface finish problems using the expanding lap. It doesn't depend on an extremely accurate lathe or process. then finish off by lapping a slight taper by delaying a little longer in the bottom end. Then lap a piston to JUUST jam at TDC. Also because the bottom end is looser, it's really easy to sneak up to a perfect piston fit.

 

[Jasonb]

As this engine uses a piston that is lapped into the liner for the fit then you really only have the option of a steel piston if you don't want to use iron.

As the others have said cast iron is a really nice material to machine and its self lubricating properties make it ideal for a lot of parts of model engines, you will be restricting your choice of future projects if you don't want to use it.

 

[kf2qd]

A cast iron liner and aluminum piston with cast iron ring is really the easiest. There is a reason why so many engines have been produced this way over the years (and still are...)