First of the L67 rods showed up, time for a little comparison:
On the top is the L36 rod from the motor, on the bottom is the L67 rod. Construction is similar, using a bronze bushing on the wristpin end. The L67 rod is shorter and heavier in section and mass.
L36 Rod: 633g (rod, cap, and bolts only..no bearings)
L67 Rod: 688g (rod, cap, and bolts only..no bearings)
Here'* an L67 piston that came on the rod: Note the teflon coating on the piston skirts as well as the coating on the crown.
L36 Piston w/wristpin: 497g
L67 Piston w/wristpin: 554g
And last, here'* the replacement pistons I am going to use: Federal-Mogul Sealed Power PN H871CPA 0.030 overbore.
Replacement L67 Piston w/wristpin: 560g
Hope this makes it pretty clear why the crank will need rebalanced after doing this substitution. The change in mass is going to be 118g per piston/rod assy, or a total change of approx 708g for the entire motor. That'* like adding and entire extra piston to the motor'* rotating mass.
Here'* a little factoid for you: The factory GM L67 piston was actually cast by Alcoa Aluminum. Inside the skirt, opposite the casting number 23523E is the stylized "AE" casting mark that Alcoa uses. The L36 piston was also cast by Alcoa. A forged product made by Alcoa would have the same markings, except for a stylizes "AF" mark.
Ok, cocktail party conversation aside, the reason for going with Federal-Mogul cast pistons over aftermarket forged race pistons is pretty simple. $$$$$ and actual need. The quotes I got for custom pistons in a 0.030" overbore were hovering around $800.00 for the set, with a long delay before they were made. The Federal-Mogul pistons were $275.00 and on the shelf. Spending an extra $500.00 on a motor that would probably spend it'* life bagged on an engine stand didn't make any sense. This motor is just a spare should the other factory assembled L67 die and need an overhaul.