I did my first 5.3 rebuild and I am getting a rattle in the bottom of the motor. I had all of the machine work done and did not have to use any oversized bearings. the motor is completly stock with the exception of headers. Any ideas?

 

Could be a ton of stuff, from piston slap to loose collecter bolts on the header, to bearings, all your clearances measure within spec, mainly your crank end play, and rod side clearance? Got good oil pressure? Remember to bolt down everything with the oil pump and pickup tube? lot of things moving in the bottom of the engine none of it is supposed to be rattling or knocking. some piston slap is common with these motors but it goes away when the engine warms up. These were just a few thoughts,,,,

 

Checked all of the clearances with platic gauge. everythig looked good. all new arp blots through theh rotating assy. and in the heads. probably just piston slap but I want to make sure before I ruin the engine.

 

This is a longshot, but sometimes happens.... are you sure it isnt a small tool/bolt/something thats fallen into say, the top of the k-member or somewhere similarly out of sight and near the area of rattle? Like I said, its stupid but it wouldn't be the first part casualty to go unnoticed...

 

No, It doesn't sound like that. It is in the motor. It is kind of muffeled like a rod knocking.