MC oil pressure: thrust bearing wear
veetesse at hotmail.com
Mon Dec 23 14:12:09 EST 2002
Before completely removing the MC engine in the 4ktq, my friend and I
decided to test the oil system for pressure bleeds with compressed air this
weekend. I was skeptical of the idea at first, but it was educational and
cheap to do.
The pan, pickup tube and valve cover were already off of the engine, so we
added compressed air to one of the oil pressure switch holes in the radiator
side of the block using an air gun. We then watched, listened and felt
around the crank and head area of the block for _excessive_ air leaks.
Obviously, some air will come out from various oiling holes...we were on the
lookout for something glaring that would explain this engine's low oil
Instead of finding a huge hissing leak somewhere, we noticed that there was
a little bit more air coming from around the main thrust bearing area than
the other mains. We then pulled both sides of the thrust main bearing from
the engine (by pushing the one half around the crank) and looked at the
Strangely, it seems that the front side (towards pulleys) of the thrust
portion of the bearing is already down to the copper material of the
bearing... only 60 miles after complete rebuild (!). The flywheel side of
the bearing seems fine as does the portion where the crank actually rotates.
Immediately, we suspected excessive crank play, reinstalled the bearing and
checked clearances again with a feeler gauge. We found the crank play to be
within the spec limit found in the Bentley just as previous checks had
Upon reinstalling the bearing shell again around the crank (the half that
goes in the engine), my buddy noticed that it seemed like one edge of the
thrust bearing surface had slightly more clearance (a couple thousands) than
it's other side when fully seated in the engine. See below for horrible
ascii art. you'll have to imagine that the l's represent the lip or thrust
portion of the bearing that overlaps onto the bearing saddle in the block.
You can see that only ONE of the four shows the clearance.
l l flywheel ->
ll l l
Has anyone ever seen this before? What would cause something like this?
Thrust bearings really only go in one way, so I don't think it's a block
machining problem... defective bearing? Worn out crank thrust surfaces?
(even though the clearance is within spec). We checked to make sure all the
rods were installed correctly and the cap was centered. Kind of an
obscure question, I know... Although this is a strange problem, I doubt this
is the source of the oil pressure bleed...
We then pulled the head and manifolds off of the engine to look for problems
in the head or block (cracks, etc.). Nothing looks out of the ordinary that
we can see. The rest of the engine will come out very soon for complete
teardown. I wish there was some other way to do this, but at the present
time, ripping everything out seems to be one of the only options.
Another MC engine has been sourced and is being shipped here (thanks to
Chris Semple at Force Five) and will be built as a replacement while I try
to find the problem with the first engine. Hopefully, I'll be able to make
some progress over the holidays...
86 4ktq (in pieces again)
81 Coupe ITB, 83 CGT
88 80 Fwd
83 GTI FSP, 83 GTI 16V
90 VW Cabriolet
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