Loud Knock. Spun Bearing?

[tahoe_snow]

My car (74 911 w/ 3.2) has been in storage for a couple seasons awaiting a complete brake overhaul (it was a turbulent ride through the financial crisis) . This winter I finally bit the bullet and got the job done (story in and of itself), and the car was back on the road.

Well last night, I could be wrong, but I think a bearing must have let go. The motor began knocking loudly at all RPMs including at idle and in neutral. Although the motor didn't seize.

Previously I heard a light knocking under load, but thought it was something loose in the exhaust, as I had been messing around with the exhaust for SSI and M&K swap. The M&K had a failing weld on the tail pipe that was loose, so I didn't think much of it.

To be clear: I'm not qualified to rebuild the motor myself, but am willing to do some up front diagnoses.

I'm going to drain the oil tonight and see if there is anything on the plug and filter, but I'm sort of preparing for the worst.

(Note: updated to clarify current situation).

[4sd911]

First off you need to find the knock. Only under load, maybe a CV joint? Can you hear it rev'ing the engine in neutral at a stand still? Maybe a bad rocker arm? As you suggested check the oil for debris. If the oil hasn't been changed for a few years it will to be changed any way. Might need to open the oil filter to look inside.

[tahoe_snow]

Yes to be clear the problem happens when the car is idling in neutral, so it isn't a CV.

I did change the oil last fall before it went for brake service and didn't notice anything unusual at the time.

I'll drain tonight and cut open the filter.

If there is anything of note, I will post a picture.

Thanks for the help.

[tahoe_snow]

Hmm. Quite a bit of crap in my case plug:



I'll cut the filter apart tomorrow. Not off to a good start. I get it on stands and pull the valve covers as well.

One thing that is discouraging about this is that the car is very clean underneath. Not a bit weeping.

[TibetanT]

Subscribing for information purposes.

Sorry to see this happening.

[Geronimo '74]

Drain plug does not say good news...
Keep us posted.
sub'd.

[Tim L]

Well I don't think you need to do any more diagnosing. 99.99% it’s a rod bearing. I was in the same situation I decided to rebuild my engine myself. It was a fun project you will learn so much about these engines it’s well worth it.

[docrodg]

Originally Posted by tahoe_snow

One thing that is discouraging about this is that the car is very clean underneath. Not a bit weeping.

Something tells me you will be the weeping one.

Does not look good. if you find more crud you are going to need to consider the oil pump and cooler condition as well.

[jimbauman]

Here's what I would do... buy this engine and install it:

95 911 3.6 engine $6500 Portland, OR

Then take time to rebuild the 3.2 yourself - as others have said, you'll learn a lot and have some fun in the process. And your '74 will have a lot more oomph!

JB

[cstreit]

I think you'll probably find the remains of a rod bearing in your sump. Been there.. Sorry.

[tahoe_snow]

> Well I don't think you need to do any more diagnosing. 99.99% it’s a rod bearing.

At this point it more educational than anything. I want to see what is in the filter, then look to pull then engine.

On the 3.6. I've got to admit it I'm tempted, but that is a a fairly big project in and of itself. The car does have 993 brakes. I was considering trying to find a decent 3.2.

[john walker's workshop]

if a rod bearing spun, which looks to be the case, the crank is usually damaged. mic that journal in several places before reusing it.

[JFairman]

From your description and the shredded steel pieces stuck to the magnet it looks like the bearing spun and the motor was run like that for a little while.

The nonmagnetic lead babbit and copper bronze layers of the bearing shell are ground and worn off and the steel layer is shredded.

The crank is definately scored and it will probably be cheaper, take alot less time, and probably be more reliable to buy a good used motor and put it in.

You'll have to clean out all the oil lines, thermostat, oil cooler(s), and the oil tank too.

I hate it when that happens.

[cstreit]

Oddly despite repeated discussions of the babbit being non-magnetic, I scraped off a thin layer of an old bearing the other day using a razor-blade.

Guess what?

MAGNETIC! hm....

[JFairman]

Thats interesting. I've never tried that just heard the babbit layer is mostly lead or maybe it's a lead alloy with some ferrous material in it.

Or maybe from engine wear there was real fine steel particals like fine dust in the oil and some of it has been pounded into the lead babbit while it was running and thats what was magnetic.

[cstreit]

Well again, I always heard and assumed the same thing... ..so I thought to try it out. The Glyco's are definitely a tri-metal bearing.

A modern trimetal bearing infact has five layers: The nickel barrier plating prevents or limits diffusion of metallic components from the babbitt anti-friction layer into the copper/lead. supporting layer and vice versa.

The tin flash coating is, as previously mentioned to protect and provide a dry lubricant.

[docrodg]

I have noticed over the years that bearings become magnetic over time. Probably steel particles in oil being squezzed into it.