Loud Knocking Diagnosed - Spun Bearing:(

[Charlie Smith]

Earlier, Jim Green wrote:
>
> Well on the advise of Javad and Phil, I dropped the
> oil pan, and found a spun rod bearing in cyl #4.
>
> http://www.geocities.com/jeg1976/bearing.jpg
>
> Well that lasted a whopping 40 miles.  The crank is
> still in pretty good shape, but needs polished.  The
> picture looks worse than it is.
>
> http://www.geocities.com/jeg1976/race.jpg
>
> I dug up a post in the archives that suggested using
> 600 grit, then finishing them off with fine
> Scotchbrite.  I like this idea much better than
> pulling the engine out again.  Anyone see any reason
> why I shouldn't try it?  I'm going to check to see if
> it's out of round, but I doubt it since I only drove
> it about two blocks.  All the other bearings look
> fine.

You really don't want to do it that way, unless you are planning
to immediately trade the car.  It won't last and the next "have to
take it apart" will probably be worse.

First, I'll bet that a careful measurement with a good micrometer
will show 0.0005" out of round or more.  Something like that just will
not last when you put it back together.  Measure that journal and also
the rest of the rod journals; if any are 0.001" or so smaller that
the 'new' spec, consider turning it undersize if you can find a really
good shop.  You'd be amazed how much oil pressure you will lose with
even an extra 0.001" clearance.  Plus, if the journal isn't mirror
smooth you can't really measure it very well and it won't hold a
bearing for long anyway.  The picture of your crank looks terrible.

Secondly, you can't get the inside of the engine anywhere near clean
enough unless you take it all apart.  I can tell you from personal
experience you can't get the inside of an engine (or transmission)
too clean when putting it together.  Years ago I finally built a
workbench with a sheet steel top for this type of work, and an engine
assembly stand to keep it off the floor.  This way, I could wash the
top of the bench with solvent before I started to make sure it was
about surgically clean.  Taking the engine apart to clean things should
include taking plugs out of oil passages to clean them with a gun barrel
brush.

As a final point, after you get it back together and running, change
the oil and filter after about 30 minutes of road driving.  That will
help make sure the grit you missed on re-assembly doesn't stay in the
engine to continue grinding and polishing things.

I'd plan on replacing the rod that went out.  It is certainly not round
any more.  If the big end of the rod isn't round, it won't hold a bearing
any time at all.  You ought to take all the rods including the new
replacement rod to a good machine shop and have them checked for roundness,
length, and twist.  Since the piston in the bad cylinder was traveling
higher in the cylinder, courtesy of the huge extra clearance in the rod
bearing, look carefully at the top of the piston to make sure it wasn't
hitting the head or valves.

I'll quit carrying on, there are books written on this.

    - Charlie


  Charlie Smith   charlie at elektro.cmhnet.org  614-471-1418
  http://www.elektro.com/~charlie      Columbus Ohio   USA
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