Yes, the pin location on the stock Bullet is fairly low on the piston, and it can benefit from being set higher, near the lbottom of the oil ring, but not intruding into the oil ring groove.
The Harley piston shows the pin intruding into the oil ring groove, and that is a fairly extreme measure which potentially can affect oil ring stability.
Basically, the observation made of the mass being located far away from the pin is correct, and this contributes to piston rocking action. In the stock form, it is not much of a dome, or mass, so it's not much of an issue. In the Hitchcock's piston shown, it is significantly more of an issue, especially in light of the overweight mass of the Hitchcock piston to begin with.
The reason that the "piston kits" offered for sale all have the pin locations the same as stock is for easy installation into the stock engine platform. However, in a racing engine or specialty build, the pin can be placed anywhere it needs to be. We have produced pistons with several different pin locations for different piston applications, even in standard height barrels, in order to achieve different compression ratings for our piston. We can make our pistons, or custom racing pistons in any configuration, including pin height, and we have. We also have the lightest piston(396 grams) available for the Bullet, and that there isn't such a heavy mass up high with our superior crown design.
Here's a couple pics of our forged ACE 9:1 ultralight piston.
A shot of our under-dome machining for lightening the piston.
Now, the issue of the longer rod is one which we have studied, regarding some of our more high performance applications. A longer rod has some benefits, but also some drawbacks. In this case, the benefit would be allowing a shorter and lighter piston with less rock to be used. However, the longer rod gains mass in the beam with longer length, and the rod is steel, and the piston is aluminum. The reciprocating mass would be affected in a way we would not want.
So, the way to handle this is to move the pin up to just below the oil ring groove, and cut the barrel down to the proper height to get the compression height needed for this piston. This way you get reduced "dead weight" of the barrel, and you get the advantage of the lighter piston with the higher pin height, without paying any weight penalty in reciprocating mass, and the rod length stays the same.
That, in a nutshell, is the way to do it.
However, we can offer Carrillo rods for the Bullet in any length, and also custom pistons with whatever pin height or crown shape or compression rating that might be wanted. And we also can design chamber modifications for new chamber shapes and crown designs of a more modern "squish" type design, which we have done for others.
Bullet Whisperer has taken this to an even further extent on at least one of his racers, and used a shorter rod length, and cut the barrel down quite a bit, and also used a taller pin height on the piston. This would only be done on a Bullet aiming for very high rpms, and the lightest reciprocating mass possible. When making larger scale changes like this, the big change in rod length does affect the breathing characteristics of the engine, and may require different camming to optimize for the much shorter rod.
If you have interest in the lightest and best pistons for the Bullet, either custom or our normal style, or custom work on your cylinder head porting or valve train, please contact me.
Topic: Merits Of Longer Rod And A Piston With Wrist Pin Closer To Crown (Read 2744 times)