Cast Iron piston ring gap

[scooby]

The way I've been doing the piston rings for the Howell V4 is I turned to .002 oversize and made sleeve and mandrel like on the JEH piston ring page. I made mandrel with cover and pin like shown in strictly IC article using specs from V4 Plans (.131 pin, .827 mandrel dia, etc

I made the cleaver in SIC article and cut the rings, placed them on mandrel, heated them using kiln around 950 degrees, held for about 40 min. I put them in the sleeve/mandrel to turn the .002 off to finish the ring.
When I removed it I notice the gap has shrunk.

I made a lot of extra rings, so on another batch I heated to 1000 degrees and held for 3 hrs and I'm still having the same problem.

Anybody know what I'm doing wrong? Or is the loss in gap normal?

here is the kiln I'm using

 

[Tim Wescott]

If this is the style of making a ring where you compress the ring, clamp it into the mandrel, and then turn it round, I have two guesses on what's happening:

1. Just the action of compressing it into the mandrel could be doing it. You can test this yourself by compressing it onto a mandrel then immediately taking it off.
2. When you turn the thing down, you're turning off material that's in tension. What's left hasn't been stretched as much. So you're removing some of the material that would otherwise be pulling the thing open when you take them off the mandrel.
   1. If I'm right, then making them a bit more to size and turning off less material would reduce the amount that the spring-out changes.
3. When you turn the thing down you're stressing the metal. I've found that repetitive stress on a piece of metal that has built-in stresses (like a spring) will tend to relax it. Maybe that's it? Solution to the last one should apply to this, too.

On the bright side, even in its not-so-open state, it's still going to put some force on the cylinder walls.

 

[scooby]

I think your right. I put one in sleeve and on mandrel, and left it for awhile.. Gap is like it should be, didn't change...

Could I heat treat and set gap again after turning to correct diameter? If I had enough cast iron stock, I'd just make new ones and turn to exact size then heat treat.

 

[Tim Wescott]

I think you probably don't want to go there, but that's a guess.

If it isn't already clear, I only know about making piston rings from reading about them. Here's the end of the string to find out what I've read: http://www.modelenginenews.org/techniques/piston_rings.