Packard Forum Index » Post-War (1946-54)

My best guess is that it's piston slap from a cracked piston skirt that was not distinguishable above the previous valve noise.

The fact that it occurs once per piston stroke means that it can't conceivably be in the valve train, unlikely also since the valve train was just redone.

It could also be a rod bearing knock but it's unlikely to "just have one" be problematic compared to a cracked piston.

Damaged pistons are not unheard of in high mileage engines, not just from overall fatigue, but also from detonation ultimately caused either by carbon buildup altering the compression ratio, or the timing/mixture drifting out of specific from lack of regular maintenance. Likewise if the engine was parked for a long time, some goofy things can happen with the piston or two with exhaust valves left open; the cylinders may flash rust over time, causing broken piston rings and/or excessive wear when the engine is restarted. Excessive carbon buildup can help to damage rings as well when restarting a dormant engine.