Still scratchin my head tryin to figure out the knock in my '55 265, the original engine in my truck. It was rebuilt sometime before my dad bought it in 1985. It's a VERY dull single knock, too low-pitched to be a rod or valvetrain and only when NOT under load. I have the engine torn down now and while I have not taken any measurements, nothing feels loose and nothing is out of place.
Maybe a wrist pin in one of the pistons? Those can knock sometimes for years without really doing any damage...and it goes away sometimes after warmed up / under load.
small EX manafold leak can do that,or fuelpump pin if pump is old with wear on arm and pin. Just ideas.
I'd check the rod bearing thickness with a 0-1" ball end micrometer... I happen to own one with a digital readout...
It never went away once warmed up, and the engine would spin to 5000+ before the cheap chain-store points would float (I was never kind to it)... The noise never got worse in the 3 years I drove it. The engine is now on a stand gettin ready to hit the machine shop to prepare for a rebuild.
With a small block Chevy, it's probably a cracked piston skirt. Hold the rod horizontal and let the piston swing down and tap the rod. A dull clunk will show which one is cracked, a good one kinda rings. The cracks aren't always readily visible. Usually they are radiating out from the wrist pin, in the unmachined part of the piston. There's a steel plate built in to the aluminum casting, that's probably why they will run for years making that rattle.
back in the day, I saw many 265's with broken piston skirts held in place with the steel reinforcment in the pistons.
The Jalopy Journal
