Piston Ring Rotation
 

I've been doing some reading out of curiousity about piston ring rotation, and I think it's an accepted fact that rings rotate and are encouraged to do so. This NASA study (albeit an old one) estimated that ring rotation is on the order of 1 RPM per 1,000 engine RPMs.

Total Seal on it Web site says that cylinder cross-hatching determines ring rotation speed: "Cross-hatch angles can also vary depending on the application. These angles determine the critical ring rotation speed, generally a 45° cross-hatch angle will do a very good job."

Anyway, when I disassembled my 3.0 engine, the rings were all over the place.
Given that rings rotate, why do we worry so much about staggering the ring end gaps when replacing rings? It seems to me that we're ensuring the best possible conditions at start-up, but beyond that do we have any control over where the gaps may end up at any point in the start/shutdown/start engine cycle of life?

Brian

good question - subscribed

+1 on good question...

If I remember correct GM had a recall on may engines because someone on the line installed the rings with all the gaps lined up, didn't work very well and will not correct its self.

I heard of someone doing that on a Lycoming engine. The plane ran out of oil on the run up.

Maybe the orientation is most critical at break in and then, when they are broken in and moving all over the place on their own, it's not an issue? I plan to stagger mine according to the book when I put everything back together, just to be safe.

I staggered mine, too, but I imagine they won't stay put for long. Maybe, as you say, it's critical to stagger rings for that first start-up and break-in. The rings definitely move around, because otherwise we would see specific cylinder wear issues, right?

Brian

Probably not, unless they are all lined up. Most of the older engines I have taken apart were pretty gummed up behind the rings. Not likely that they had rotated for many tens of thousands of miles. I would assume ring rotation would only occur in a perfectly round cylinder, as a slightly oval cylinder is going to cause the ring to break in in a stationary position.

But what do I know. I'm an armchair quarterback.

I did have a friend who rebuilt the engine on his Triumph motorcycle with the rings all lined up, and it used a lot of oil. he drove it about a thousand miles before tearing it down. The problem did start to rectify itself, but only partly.

I will be interested to see what the real deal is.

You got it...The break in is critical, so better to give him all the help it deserve.