[gtofreek]

Here is a good article that explains piston pin offset.

http://www.motorcycleproject.com/mot...ws-offset.html


Here's another oft-misunderstood bit of techie trivia -- what is piston pin offset for? That is, what purpose does it serve? Let's examine that one.
Modern powersports engine pistons are not mounted on their connecting rods exactly on center. They are offset radially, that is, perpendicular to their rotational axis. In other words, in a vertical, single-cylinder engine, the piston is offset toward the exhaust side of the cylinder. That is, the piston pin hole is bored off center toward the intake side, usually less than 1 millimeter. It is frequently stated with a sort of bored complacency that the purpose of this offset is to reduce piston slap. The truth is, while piston slap is reduced through piston pin offset, but that is not the purpose of the technology.

The piston engine has three major parts: crankshaft, connecting rod, and piston. Each has a different job to do. The reciprocating part -- the piston -- makes the crankshaft -- the rotating part, uhh, rotate. The connecting rod is simply the part in the middle. It translates the piston's recip motion into the crankshaft rotary motion. The neat thing is that in the process, it shares in the motion of both. That is, the connecting rod is both a reciprocating part and a rotational part, at the same time. (In fact, when balancing an engine, it is common to divide the connecting rod's weight in two, thus permitting half its weight to be calculated as recip and half as rotation.) The point is, the upper half of the connecting rod reciprocates with the piston, the lower half rotates with the crankshaft, and this is important to understanding the stresses on all three parts, but especially those on the connecting rod.

The piston and its half of the connecting rod stops twice per crankshaft revolution, even though the crankshaft continues to turn. This means the piston and top of the rod also start back up twice. This stopping and starting imposes stresses on all three of the parts, stresses that increase with crankshaft rpm. To reduce these loads, the piston is mounted to the connecting rod slightly offset. This causes the piston to reach top dead center at a different time than the connecting rod, effectively spreading the shock loading over a greater number of crankshaft degrees. In short, the real reason for piston pin offset is that it softens reciprocal loading, permitting lighter more power-efficient parts to be used, and the engines to be capable of higher rpm.

However, there is another phenomenon at work also, a kind of side benefit. Because the connecting rod spends most of its time in the engine at an angle, the piston engine has what is called minor and major thrust. Major thrust refers to the downward-stroking piston's force against the cylinder wall during combustion, due to the rod being angled in that direction. Minor thrust is the piston's thrust against the opposite cylinder wall during compression, because the rod's angle is opposite also. These thrust forces push the piston firmly against the cylinder wall. The important thing is that at TDC, they flip-flop. Major thrust turns into minor thrust, and visa-versa. In older engines, this flip-flop caused the piston smack the cylinder, resulting in a noise. Fortunately, the piston pin offset in today's engines, besides reducing inertia stresses, does two things that reduce this noise. First, because the piston is mounted off center, the transition from major to minor thrust is less sudden. There is less impact. Second, instead of a sudden lateral shift, the piston actually rolls from major to minor thrust. That is, the piston shifts first at the skirt, then gradually the rest of the piston makes contact, instead of all of the piston at once.

To summarize, piston pin offset is the manufacturer's way of reducing stress on reciprocating parts. It permits these parts to be lighter, which results in more efficient manufacture and less power loss in the engine, as well as higher rpm. A complementary result of piston pin offset is reduced piston slap due to the more gradual shift from major to minor thrust. Make mine well done, please.