Re: Big-end bearings gone... any ideas?

["Fred Munro" <munrof@isys.ca>]

-----Original Message-----
From: nikolay_nesterov@cc.littonatd.com <nikolay_nesterov@cc.littonatd.com>
To: quattro@coimbra.ans.net <quattro@coimbra.ans.net>
Date: Friday, April 24, 1998 8:20 PM
Subject: Big-end bearings gone... any ideas?


>
>     Hi,
>
>     just resubscribed to the list after I was dropped because of some
>     list malfunction long time ago....
>
>     My 5K sat patiently since then with #2 big-end bearing spun, #1 and #4
>     badly damaged and #3, #5 bearings looking OK.
>
>     Main bearings look good, crank journals show some damage but the marks
>     are not deep, mostly bearing material deposited onto the crank.
>
>     I am wondering what would be the best way to proceed from here....
>        maybe polish those journals with some 600 sand paper put it all
>        back, check oil pressure and see?
>
<snip>
>     Still wondering why that bearing spun?...
>
>     thank you
>        -Nick
>

    If you have 3 damaged bearings, you probably lost oil pressure. I have
also heard of this type of damage happening if an engine sits for along
period of time and is started at WOT - the small amount of oil remaining on
the bearing surface can't withstand the load and there is not yet any oil
delivery from the pump. The bearing spins when the friction force with the
crank overcomes the friction force and lock tab force in the rod journal.
    In operation, the crank bearings are actually riding a "wedge" of oil
created by the hydrodynamic forces in the bearing and the constant
pressurized supply of oil. The lubricity of static oil is sufficient to
protect the bearing surfaces at low (startup) load for a short period of
time, but if you lose oil pressure under high load - say g'day to your
bearings.
    You can polish your crank journals with 600 grit. Finish them off with
fine Scotchbrite. They should be visually free of defects. Then get a
micrometer and thoroughly check each journal for out-of-round and
side-to-side taper. Remember the oil wedge? It's really, really thin - out
of round results in high spot contact and taper will squirt most of the oil
out one side.
    Judging by your description, you may need to get the crank turned. Then
you need non-standard oversized bearings. It is better to find a good stock
crank if you can.

HTH

Fred Munro
'91 200q  250k km