Always replace piston rings when servicing pistons.
Piston rings are of two types, compression and oil ring. The two compression rings are positioned in the two upper piston ring grooves. The dot on the second compression ring must face upward. Ring sets are available to fit standard and over-size pistons. Piston ring sets must be properly fitted to the piston and cylinder.
Average Piston Speed
Stroke, RPM and rod length affect piston speed, and 4,500 feet per minute or less is recommended. Stock EVO at red line is 3,683 feet per minute. Piston speed over 5,300 feet per minute causes excessive skirt wear and possible seizure. Average piston speed equals stroke multiplied by RPM divided by six. This means that 3,500 feet per minute equals excellent reliability, 4,000 feet per minute is good reliability, 4,500 feet per minute equals fair reliability and 5,000 feet per minute is poor reliability.
Piston acceleration is the rate of acceleration from a dead stop, at Top Dead Center (TDC) or Below Dead Center (BDC), that the piston achieves. Piston acceleration normally occurs at 75 degrees Before Top Dead Center (BTDC) or After Top Dead Center (AFTC). Excessively high piston acceleration causes ring flutter and damage to pistons and rods. Select ring thickness based on desired RPM. Maximum ring thickness equals average piston speed divided by piston acceleration. Given an average piston speed of 4,250 feet per minute and a piston acceleration of 89,863, calculate the ring thickness.
Types of Pistons
There are two types of pistons, cast and forged. Cast pistons are the most commonly used and also have a longer history. In earlier Harley-Davidson engines, the cast pistons were made with inadequate alloy which expanded too much with heat and required a looser fit. This, in turn, resulted in excessive engine noise at cold start-ups. Harley then redesigned the piston with steel reinforcements to prevent expansion and engine noise. Forged pistons were also made with inferior alloy and usually broke when struck hard. Forged pistons were manufactured as high-performance, with denser grain, a thicker wall and less silicon, which caused more expansion.
The purpose of piston rings is to seal the combustion gases, conduct heat from piston to cylinder wall and control cylinder oiling. The compression ring, closest to the crown, does most of the sealing. Materials vary and must be appropriate for cylinder material. The oil control ring is closest to the skirt and regulates oil in the cylinder. There are two styles of oil rings. One-piece oil rings are thick, brittle and not very flexible. A multi-piece oil ring has an expander and rails, is not very flexible, conforms to a worn cylinder and is emissions-oriented.
Piston Ring Clearance
Piston rings should be installed so end gaps of adjacent rings are a minimum 90 degrees apart. Ring gaps are not to be within 10 degrees of the thrust face center line. Harley-Davidson specifications are as follows: Compression ring gap (top and 2nd) 0.010 to 0.023 inches, oil control ring rail gap 0.010 to 0.053 inches, compression ring side clearance top 0.0020 to 0.0045 inches and 2nd 0.0020 to 0.0045 inches, and oil control ring side clearance 0.0014 to 0.0074 inches. Use a feeler gauge to check the clearances.