So, I picked up this 1989 F-250 off Craigslist yesterday. Got it because I wanted the 5speed zf and transfer case for my 1979 F-150 build project (build thread coming soon). Anyway, pickup has a 460, and was advertised as having a spun rod bearing, which I was okay with, wasn't wanting the motor anyway, and the pickup was priced accordingly. I was going to winch the ol' heifer on my trailer and not think twice about it until I got home, but I thought I would hear for myself how bad the knock was from the spun bearing (and, it's 10x easier to drive a vehicle on a trailer than winch it on) while I could ask the original owner if it was what he heard. So I hit the ignition, and she fired right off. Great oil pressure, no knock, but it's shaking the hell out of things, like the engine is out of balance or something. But no knock. I asked the guy if this is what he witnessed, and he affirmed that was his "knock".
So to my question: it's not a knock like a bearing is out. It's not a "rough idle" vibration. It's a "wheels are out of balance/lug nuts are gone/bent wheel/going to shake the truck to pieces" vibration that maybe gets a little better if revved up, but not much better. I know the guy put new plug wires on very recently (old ones are in the bed), so my first step will be re-confirming firing order and timing with a light. Next step will be a vacuum test. What should I look at next? The pickup looks good enough that if this is a relatively cheap/easy fix, I may not part it out, and in fact use it as a farm truck. Let me know what you guys think. TIA

 

Btw, all these symptoms were while sitting still. Didn't change whether clutch was engaged or disengaged. Moved it 30 feet at most loading and unloading, didn't seem to change moving or parked.

 

Small possibility that the rod has given up altogether and you are running on 7 cylinders. You should hear that though.

I would think it would have sawed it's way out of the block if it broke. Strange things happen in the dark confines of a failing motor.....

 

I have that in the back of my mind as well. I will add a compression test to my list of things to do...

 

The junk yard 460 I put in my truck had a violent shake and it turned out the be the clutch in the fan. It was seized off-center. I changed the fan and it has been fine for the last two years.

 

it should make a fine workhorse.
if the seller can confuse a 'violent shake' with a 'knock', there's a decent chance he did install those plug leads wrong.
FWIW my initial feeling was engine mount(s).

 

That's how my 460 ran after some mouse mayhem to the plug wires. New wires and it ran like new.

 

1989 460 is externally balanced (has a weighted crank spacer), but ...

If someone wasn't aware and installed an internally balanced flywheel and/or harmonic balancer from a 1968/78 or early 1979 460), a vibration will result that will eventually tear the center from the flywheel.

 

So I had time yesterday afternoon to do some more investigation on the vibration. Timing was correct, harmonic balancer didn't appear to have come apart or have slipped. So I started it up and let it run for 1/2 hour or so. After the motor got warm, I used an inferred thermometer on the exhaust manifold/head. Cylinder 3 is for sure a dead hole, and I believe cylinder 2 is as well. Shortly after this, I could hear the valve train knocking badly. So, back to parting out the truck. Thanks for all the replies.