Candidcameron

CandidCameron

Q Why do the piston rings in fourstroke engines sometimes rotate in their grooves? Five years ago, I personally rebuilt the engine in my 2002 Honda XR650L, and I was very careful to set the three ring gaps at 120-degree intervals to minimize the likelihood of blow-by through the gaps. I worked in a small-engine repair shop when I was younger, and I was taught to align the gaps this way when installing new pistons or rings. Recently, I tore my 650's top-end down to find the cause of a light smoking problem, and I found all three of the ring gaps in almost perfect vertical alignment. Obviously, the rings had reoriented themselves during the 22,000 miles I had ridden since the rebuild. This was not the first time I have found evidence of rings rotating in their grooves.

I have thought about this for quite some time and can come up with no rational explanation. I assume that ring movement is a known phenomenon, since the pistons in two-stroke engines use pins at the ring gaps to prevent the rings from moving to where their ends might get caught in a port. Can you explain how and why rings often move in their grooves? David McClure Austin, Texas

A l can't claim to have a clear answer, either; I only have some notions. I know, for example, that a couple of engines over the years have had torsional failures of the con-rods, indicating that torsional motions do exist in the rods. One of the engines in question was the early version Allison V-i 2. Also, in some cases, a person

can see what looks like waves in the ring track near TDC, suggesting that stick-slip friction may occasionally occur there. Of course, we all have in mind the smooth motion of all these parts, and there’s not much in that motion to suggest anything asymmetric. But when the piston changes from one cylinder wall to the other at TDC as the rod swings over, the ring has to slide on its bottom surface to continue sealing. Can it always do so symmetrically? Does the location of the ring gap cause friction on one side to be slightly greater than on the other? I not only don’t know but have never even seen any speculation on this matter. Such a motion could rotate the ring, and I’m sure if we had an afternoon to think about it, we could dream up other scenarios. -Kevin Cameron scenarios.

Kevin Cameron