Found a cheap one with a "sudden knock" coming from the engine.. Guy says it happened over night and he thinks it's a spun bearing. I don't claim to be a master mechanic, but I can rebuild a subby motor no problem... Real question is, would a spun bearing really cause this knock? I haven't heard it yet, plan on going to check it out this weekend, but I've never had a spun bearing cause a knock.. In my experience the motor just quit working all together... He's asking $2000 for it, my budget is around $2500, so I figure worse case I can always do a mild rebuild on it and be ok, any thoughts about the spun bearing?

Yep, a bad rod bearing can come on suddenly, and be really noisy. Motor will still run, but knock like hell with a loud nasty banging sound. I have not ever gotten into engine rebuild, but know that crank shaft, rod, and bearing will need work. Others will tell you more about having the crank "turned" etc........Motor will need to be pulled out to be bench rebuilt.

 

Does make me wonder why the bearing went bad. Maybe driven low or out of motor oil?? If so, repairs may dictate a new or used motor.

they do loose bearings so it's not terribly surprising, but make sure it's not the timing belt tensioner either.  they can get weak and they're slamming into the bracket can cause a noise that has been mistaken for rod knock before.  it's surprisingly loud and sounds like "knock".

Lots lately 4 in a row all knocking I get the cranks turned down and replace the bad rod and away you go after endless hours of getting all the debries out of oil passages and cams tubes ect . I think this happens to these engines because coolant gets in oil thru bad headgaskets and end up with coolant additves / sugars in oil if not enuff oil changes wipes out berrings Also some years came with a 7mm oil pump was to small and could not keep up as it gets worn a bitt pump cant mantian preshure

Had 2 engines with knocking bearings. 1 bad enough to not reuse the crank (I had a spare) and 1 where the crank was just fine. Miked to spec and just polished up the journals. It will be louder than you think it should be.

Somone brought it up that not all engines came with std berrings so very important to measure crank and confirm berring size. I send every crank to machine shop now just to cover my butt and sometimes they may find something iv'e missed

If it's the belt tensioner, you can pull the timing cover, start engine, and watch the tensioner. If bad, it'll be bouncing around whereas it should be still and controlled. I wouldn't pay $2k for a knocking engine though. Personally, I'd look over EVERYTHING else on the car (let it idle a bit (if he lets you) to see if it's over heating (meaning HG issues, more involved engine repairs). Check the oil. I'd rather see darker oil than a "fresh" oil change as it might be hiding coolant, so darker oil with no coolant would put me at ease a little over fresh, clean oil in a knocking engine. Make sure everything else works i.e. windows go up/down, heater works, lights work, no excess play in steering or front wheels, etc. etc. Anything you find wrong, point it out to him, but in a polite manner. Chances are he doesn't know, or if he does you caught it and he thinks you know a lot about the cars. Rattle off the extra repairs it'll need, and ask if he can work with you on the price. Chances are he just wants it's sold and will come down a bit. If the entire engine is toast, it'll be cheaper to swap in a junk yard one, which around me can be had for $150-200 all day long.

Also suggest pulling out the dip stick if the Forester has an automatic transmission. If ATF is black in color, then tranny fluid is really old and prolly burnt. If rose color or reddish, then ATF fluid is in good shape. Dip stick is way up against the firewall on driver's side. Can be hard to find, as dip stick gets buried under hoses.  This test will also give you an indication of how well the vehicle has been maintained during its life time.