[SEL] piston rings

[jbcast at charter.net]

---- curt at rustyiron.com wrote: 
> Arnie,
> When wiring the rings closed there is no gap. Turn the OD to the cylinder
> diameter. Then remove from the fixture. I surface ground them to get the
> desired width, making them .0005 to .001 narrower than the ring groove.
> Just before sliding the rings on, I file the desired gap. I actually do a
> modified step gap, cutting the gap only 1/2 way across the ring width on
> one side, and the opposite on the other side. Did that make sense?
> If you buy step gap piston rings there is considerable overlap when the
> ring is installed in the bore.
> On the modified step gap, there is no overlap when the ring and bore are
> at the same temperature. As the ring temp exceeds the bore temp it grows
> and the stepped halves begin to overlap slightly, essentially making for
> an air tight gap.
> Probably not a big difference over the traditional gaping method across
> the full width of the ring, but I figured it couldn't hurt. I've done 3
> engines this way now, including the Alamo and they have loads of
> compression and run nice and slow.
> Curt
> 
> 
Stepped ring gaps look good but don't help. Compression doesn't travel "line of sight". As the piston is coming up on compression the ring is forced to the bottom of the piston, compression is coming over the top and against the back, it actually helps to expand the ring. The overlapped step has the same or usually a larger gap at the bottom half, compression will find it's way to the gap and out. I've seen pinned rings with the pins on the bottom side of the land. This is a poor design, the area around the pin equals the gap, 1/4" is not uncommon. If the pin is on the top, the bottom half of the ring can be gapped as a normal ring. End gap counts for a small loss of compression, the fit of the ring to the piston land is one of the most critical areas. If a ring wears, the piston wears with it, maintaining a seal. When a new square edged ring is put into a worn groove, compression leaks all around, it would probably take years of running for this to "seat in".
J.B. Castagnos