Thread: Engine Knock

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My 74 two bolt 400 SBC developed a severe knock over the past couple of days. Could only be heard at 3k+ rpm but then moved quickly into the lower ranges until it could be heard at idle. Sounds like it is coming from the mid center or top center portion of the engine although it is hard to tell. Video link below shows that it comes and goes which is what I can't exactly figure out. I'd like to have a better idea for a diagnosis before I just break into it. Any theories?

http://www.youtube.com/watch?v=IleBauFOnS4

Try the easy stuff first, take a look at your flexplate (assuming it's and automatic) and see if it might be cracked? Pull one plug wire at a time and see if the knock quits on a certain cylinder or not...

Flexplate would be my first guess too.

I don't know I think I'd pull the valve covers sounded like a rocker to me but not being there its hard to tell

Take a short piece of broom stick or PVC pipe one end to your ear and the other different places on the engine and see if that helps to localize the noise.

ok, i've had a little time to mess with it this morning. The knock is the loudest under the oil pan. Broken piston skirt?

actually, i'm thinking more and more that it is the flexplate. time to get to work and actually look.

initial inspection of the flex plate turned up nothing. Of course it's still in the vehicle so I may have missed something. I pulled the oil pan and all the rods look ok, all are still attached and there are no noticeable stray marks on the underside of the engine, no debrie in the oil pan. I'll likely head back to the flexplate and tranny bolts. Anyone know any rough torque specs for this area? If that doesn't turn up anything I'll pull the valve covers.

on a whim I filtered the oil I just drained and I found some copper shavings/flakes. Sound like bearings? Where is there copper in this engine?

hmm... I think #1 rod bearing spun due to oil starvation possibly caused by a clog as the others look ok.

#1 and #5 rod bearing spun for sure. Initially I thought it was likely due to a clog. Now, I'm wondering if it's due to the engine's slant. It is off level with the front higher by about 5 degrees, maybe less. Could this be the reason? I'm thinking that if the oil pump is functioning properly that it should not matter, I'm just wondering.

With a pressurized oil system the 5 degrees shouldn't matter.We once lost a very expensive race engine due to pcs of lint from some shop towels clogged a oil passage.See what you can with cleaning engine brushes,but sometimes after the fact it is hard to tell if the stuff you might find is a result of the failure or the cause of it.