I bought a freshly rebuilt 350 4 bolt main from a local shop. Looks like a good deal. Bring it home, install it, and then proceed to break it in. Use GM break in stuff, 30 weight oil, fire it off and run it between 1500-2000 rpms for 20 minutes. I have done a few engines in my time, so no big deal, until it starts knocking like a big dog. My old engine had a little noise, but was definitely on its way out. I had a new flexplate on it, and the harmonic balancer does not look bad. I transferred the stuff to the new engine. Now, this new to me engine is knocking, sounds lower end to me. Naturally I am very frustrated at this point that it is knocking. I pulled it. Took it back to the machine shop, and I watch them pull it apart, the bearings look good across the board, both rods and mains. Cylinders and camshaft are ok too. So the shop owner was unsure about what was causing my noise, but is going to mic everything and maybe even use a different crank. Anyone have any ideas? I don't think a fuel pump could be that loud, and again, I had the same one on my old engine and it did not have this noise. I just know the engine shop is going to call me back and tell me I am stupid.

 

Did it start slowly and get worse, or just start knocking?

At engine speed, 1/2, or 1/4 engine speed?

All hardware was tight I assume on taking it out like the torque converter bolts?

Did you have the trans full of fluid and torque converter seated properly?

 

Torque converter bolts tight, torque converter never left the transmission when I took the old motor out. Was knocking at 2000 rpms, and could hear light knock/feel light knock at idle. The old engine didn't make this kind of noise, so I don't think it has anything to do with the transmission.

 

1st report back from shop says "bearing issue" but couldn't talk to owner as to what exactly is going on because he was not there.