Too many resources have shown the rod length to be 5.9" for both motors. We have known for a long time that the 3900 LZ9 rod is 5.827" long from measurements, but the LZ4 was kind of a mystery even now in 2023. Maybe its out there, but most people want to build the bigger motor. However, there is something more to these motors than to think of them like bigger 2.8/3.1 motors. Unlike the 2.8 and 3.1, the difference is the stroke and rod, not stroke and pistons.

Having both rods, measuring the big and small end and dividing each by 2, then adding the beam, the LZ9 was 5.827", and the LZ4 was 6.020". Measuring from the bottom of the pin to the top of the piston and subtracting half the diameter of the pin came up with 1.3035" on my caliper. Math for the 6.020 rod comes out to 1.303. This puts the 3900 piston down the bore about .0345" according to the math. 1.76:1 Rod/Stroke Ratio for the 3900 LZ9, and 2.01 for the 3500 LZ4. Both are interesting ratios to me, from Larry Widmer preferring 1.75:1 and Smokey Yunick preferring 2:1 or higher. You can research those if you want to know what that means, but I wanted to mention it since both options are presenting themselves to us from the factory.

Other imporant specs are the .943" pin (floating) and a 2.375" bore for a 2.25" bearing.