Looking for Graphite Piston

[Javin]

I'm trying to make a Lamina Flow engine, and everything I've read says to use a graphite piston. However, I can't seem to find any place ANYWHERE online that would have one pre-machined. The extent of my tool shop is a dremel, so I couldn't tool one myself.

-Javin

 

[mklotz]

The chances of finding a graphite piston machined to your specifications are probably vanishingly small.

The only thing I can think of that might work is an Airpot. These are commercially made dashpots used for damping mechanical motion.

http://www.airpot.com/html/dashpot.html

All the ones I've encountered have a graphite piston sliding in a plastic-like cylinder. The plastic could be problematic in a (hot) laminar engine but perhaps it would work. The other alternative is to use just the piston but, since you're limited to the sizes they manufacture, you would need to make a cylinder to fit and that will be difficult if you don't have a lathe.

I see this is your first post. Let me invite you to post a bit about yourself in the Welcome thread. Also, it's worth including some clue about your location in your profile so people can make more relevant suggestions about suppliers.

 

[Javin]

Hrm. I've tried posting an "intro" post but it just gives me an error saying I've "already posted" it... Even though it's not showing up.

 

[Javin]

I'm not sure as to how the magic of the graphite works (never done any work with it, or with pistons for that matter). Just how precise does it have to be tooled to make a Lamina Flow engine work? I would imagine even the slightest air leak would "break" the engine, no?

 

[mklotz]

Traditionally, graphite is used in this application because it is both heat resistant and self-lubricating. Because of the miniscule power these engines produce, even very light oils may be too viscous to allow operation so a piston that requires no (external) lubrication is highly desirable.

The ususal test for a piston cylinder test is to insert the piston and seal one end of the cylinder (e.g., with a finger) and the piston should not drop under the force of gravity. When the seal is broken, the piston should descend slowly. For a laminar engine, I would shoot for a piston about 0.001" smaller than the cylinder or, equivalently, a 0.0005" clearance.

 

[Javin]

Ah. That being said, I wouldn't be capable of machining one myself, even if I had the tools.

 

[mklotz]

Making a piston without a lathe is admittedly problematic but don't be scared off by the numbers I quoted.

For one-off model engines the piston is typically made to fit the already made cylinder. This eliminates the need to be able to measure precisely tiny intervals like the 0.001" I quoted. Typically, the piston is roughed to a size close to the final fit and then, in the case of graphite, abraded by hand while testing against the cylinder until a satisfactory fit is achieved. Graphite being a soft material, the abrading is done with a roughish paper, such as the brown paper used in some supermarket grocery bags.

 

[Tin Falcon]

the airo pot is one way to go to make a LDT sterling the cylinder and piston are pre made and pre fit. Another trick for precision machining of pistons is mentioned in one of James Senfs books you set the compound to a shallow angle such that advancing one thou on the compound dila will dial in .0001.
I can did out the numbers if you like. also a free cut or spring cut. Ie take a pass without advancing the cross slide at all will often take a tenth or two off the diameter.
Tin

 

[shred]

If you could live without graphite, two telescoping sizes of K&S brass tubing give a decent piston/cylinder fit; probably good enough to make a non-LTD sterling work, but I've no idea what the design of the one you're trying is.