Piston Rings and
Cylinder Bore Finishing

Performance and Technical information on piston rings and finishing cylinder bores for Harley-Davidson motorcycles.

Piston Rings

The job of the piston rings is to create a seal between the combustion chamber and the crankcase of the engine. To do this the piston rings must form a tight seal between the rings and the cylinder wall. The better the seal, the better the air/fuel mixture and burnt exhaust gases will stay above the pistons. This will also keep the crankcase oil vapors below the rings and out of the combustion chamber.

Finishing the Cylinder Bore

The idea is to provide a fine grit hone finish to the cylinders, preventing the crankcase vapor from traveling up and the burnt exhaust from traveling down. HEAD QUARTERS, one of the premier Harley-Davidson performance shops in North America has developed procedures that help assure a good seal between the piston rings and the cylinder walls. The best results are obtained using a Sunnen pin hone with an AN-600 attachment. Two methods are used depending upon the cylinders:

  1. When honing to an oversize the cylinder is roughed out with #70 grit stones, with the last .003" taken out with #280 grit stones. A final clean up is done by plateau honing with #600 grit cork abrasive stone. If you use a boring bar, take out the last .004" with a hone.
  2. On new cylinders, they are given a few strokes with #220 grit stones, then a plateau hone with Sunnen's #600 grit cork abrasive is used to knock off top off the cross hatch.

HEAD QUARTERS has tried several brands of honing oil, but found Sunnen's to be the best. A 60 degree crosshatch is used instead of the the standard 45 degree pattern. The 60 degree crosshatch does a better job of trapping the oil and holding on the bores where it is wanted. Honing should always be done in torque plates on aluminum cylinders. Don't forget to use cylinder head and base gaskets with the torque plate. Use you dial bore gauge several tines during the honing process to insure that the hole is kept straight and round.

In addition to finishing the cylinders, remember to install new valve guide seals be installed to prevent oil from being pulled down into the combustion chamber. Harley-Davidson engines should also have the breather gear timing checked and blueprinted if needed.

The procedures described were developed by Doug Coffey of Head Quarters of Ontario, Canada for use with the installation of Total Seal Gapless Piston Rings.