Jan Ridders Coffee Cup Stirling

[AjohnwParticipant @ajohnw51620]

I'd try measuring the tap as they are often over size, subtracting the pitch and adding on 0.1mm for the tapping drill Dave. Sounds like it was back cutting.

John

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[pgk pgkParticipant @pgkpgk17461]

The ridders engine I built also used a graphite piston (whch i ended up swapping for bronze cls i changed froma glass cylinder to brass) BUT I ddn;t have any real problems tapping the graphite..just go gentle because it is softish stuff and if you wind the tap in too far there is little 'feel' before you strip it. I used a single machine tap by hand.

(I had a PPL for some years..glad to say i never had a real engine failure, just a couple of hard bouncy landings in cross winds and a radio comms problem needing an unscheduled landing and temporary arrest on a high security airbase in denmark)

[Ian S CParticipant @iansc]

I would not tap the graphite myself, I would drill it through the crown (I imagine this is where you want to tap it), and make a metal insert with a thread, make it Top hat shape, push it through from the crown, and screw the forked fitting for the little end into this. I'm assuming that the piston is to some extent hollow. Personally I'm sticking to cast iron for now.

The only crash in aircraft that upset me for a while was one about a month after I started in aircraft maintenance, a Piper PA-18A, it crashed and burned when the pilot over shot his landing, and tried to climb over some high tension wires, instead of going under. The aircraft was the first one that I worked on, cleaning the hopper out(aerial topdresser), and cleaning and testing the spark plugs. That's 50 years ago, and I'll fly in most things. There was a bloke in the USA in the middle of the 19th century who said man would never fly, he had two sons, Wilber and Orville.

Ian S C

[SillyOldDufferModerator @sillyoldduffer]

Thanks again for the advice everyone.

I now have the engine running with a new graphite piston. This one is a little too tight and it makes a scraping noise. As I can't see any sign of any damage to piston or bore I hope it will sort itself out in due course.

I failed again to tap the piston. It doesn't matter much because the screw protrudes through the piston and I can bolt it together. But I like a challenge.

I drilled a 2.6mm hole for the M3 tap and took a lot more care. It went well until the very last moment when all the threads stripped. I wonder if the tap is blunt at the end of the taper and tears the graphite when it should be completing the cut. Next time I'll use a new one.

Now I've had so much practice it only me about 10 minutes to make the piston, with another 20 or so polishing it carefully to a tight fit in the cylinder. Then I spent 2 hours cleaning black muck off the lathe – nearly as messy as the time I used an angle grinder to cut a chunk off a cast-iron block. I wonder where I can get an apprentice to bully into doing the dirty work?

The Wright brothers were a clever and determined pair. It wasn't just that they designed and made the plane, they also designed and built a lightweight engine with the necessary power-to-weight ratio.

Cheers,

Dave

[Ian S CParticipant @iansc]

How are you polishing the graphite? The correct way to make the piston is to turn it until it's just too big to enter the cylinder, then cut some strips of paper, and back it with your 6" steel rule, and use this as you would use carborundum strip on steel (graphite, paper, pencil, paper).

Ian S C

[SillyOldDufferModerator @sillyoldduffer]

Posted by Ian S C on 19/04/2016 12:34:56:

How are you polishing the graphite? The correct way to make the piston is to turn it until it's just too big to enter the cylinder, then cut some strips of paper, and back it with your 6" steel rule, and use this as you would use carborundum strip on steel (graphite, paper, pencil, paper).

Ian S C

That's almost how I did it Ian, except I used a bit of wooden strip. Using a steel rule is a better idea though – if nothing else it would be easier to handle.

I've been out most of the day but I did find that the scraping noise was because the crankshaft was misaligned. In the coffee-cup design the crankshaft is supported by a single pillar that's held in place with a grub-screw. It's quite easy to mess things up by accidentally bumping the flywheel, and I am on the clumsy side of slap-dash.

Duty calls tomorrow so it will be Thursday before I get to play with the engine again.

Thanks again,

Dave

[Ian S CParticipant @iansc]

Just don't let oil near it, I think it might turn to mush.

It doesn't have to be a rule, but anything similar, and flat.

Ian S C

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