Check to see if the load on the engine makes a difference. Pull, "float", and coast conditions will sometimes make a difference in whether a knock shows up or not. This is particularly true of a piston slap, where there is a little too much clearance between the piston skirts and the cylinder walls. Piston slap usually shows up on a float condition, neither pulling or coasting. The comment about the harmonic balancer and/or flywheel being loose is definitely a good one, too. A couple of months ago, I encountered a 235 with a loose balancer which was rattling back and forth on the Woodruff key, and had worn the keyway in the balancer. When we replaced that one, I used the drill and tap jigs I've built to thread the crankshaft snout and install a bolt and washer. We also used red LocTite between the damper and the crankshaft when installing the new damper to resist any slack . Simply bolting a slightly loose damper on doesn't solve the problem.
Jerry
