chaos.lrk.ar.us!dave.williams (Dave Williams)
hotrod  16 Feb 93
- -> i was wondering why con rods need to be reconditioned.
  -> do the big ends get out of round or does the metal stretch?
  -> what is the limiting measurement before the rod needs to be redone?
- If an engine is run for a long time under detonation, it'll eventually
  pound the big ends out of round.  If it has been run without oil
  pressure the bearing shells may spin.  If either happens, the rebuilder
  takes a little metal off the cap, then rebores the rod round.  A
  correctly rebored rod may show an unmachined patch up to 1/2 inch wide
  at the parting line - this is just fine.  Some shops wind up shortening
  the whole rod by a detectable amount, trying to get a clean cut all the
  way around, since some customers bitch iffen it ain't all shiny.
- The shops buy manuals with the limits for this sort of stuff, or the
  numbers can be found in the factory service manuals, or sometimes in the
  bearing catalogs.

chaos.lrk.ar.us!dave.williams (Dave Williams)
hotrod  21 Feb 93
- -> : correctly rebored rod may show an unmachined patch up to 1/2 inch
  -> wide : at the parting line - this is just fine.  Some shops wind up
  -> shortening                      ^^^^^^^^^^^^^^^^^
- > Why is this fine?  I would think that it would indicate the hole is
  > still not round, true?
- The part you're most interested in is the top and bottom.  In fact,
  many bearing shells, such as the original Moraine ones in the 350 Chevy
  I just overhauled, are actually thinner at the ends.  After 3000 miles
  you could see the dark area where the surface overlay had not been worn
  off.  This is mainly to ensure you don't have a misalignment for some
  reason, with one shell acting as a wedge to scrape the oil off the crank.
- -> Maybe this is the answer to the above question?  The "within round"
  -> tolerance is loose enough that it doesn't *have* to be perfectly
  -> round?
-  That too.  You'd be surprised at some tolerances - for example, Ford
  allows .003 out of round on the main bores and .0015 on the crank, for a
  maximum possible runout of .0045.  The oil clearance is only .0025.  You
  can see why paranoid engine builders don't just slap the bearings in and
  torque it down.

[email protected] (Dave Williams)
fords  29 Nov 1994
- ->> How would that help a low RPM cruiser?  His 6.0 is most certainly not
- -> If the engine is properly built, I think the ring sealing and valve
  -> job will deteriorate to unacceptable levels before the bearings.
  -> Is there some other reason?
- Probably 90% of the engines I've torn down (or, since I've been hanging
  around a couple of shops, seen torn down) have been in the shop due to
  rod bearing problems.  The other 10% had something else that killed 'em,
  but the rod bearings were usually in pretty sad shape anyway.
- The rod bearings get, at best, oil half of the time, as oil is only
  passed through when the oil holes are lined up with the grooved upper
  half of the main.  Given the dynamics and small groove, I'd guess the
  actual, effect oiling is under 90 degrees of crank rotation.
- Every builder has his "magic bullet", oiling is mine.  An oiling
  problem will quickly progress from "noisy" to "spun a bearing" to
  "ventilated a block."  I don't much care for that sort of stuff.

[email protected] (Dave Williams)
gnttype  27 Nov 1995
- -> The rod bearings were a mess!  The top halves were all pounded out
  -> and looked pregnant--for lack of a better technical term!  *My*
  -> theory is detonation beat the hell out of the upper halves which in
  -> turn opened up the bearing clearances
- That is a classic example of detonation-induced bearing damage.  The next 
  step, had you gone that far, would have been for the bearings to beat the 
  locating tangs down, letting the shells spin and gore the big end.  As the 
  clearances open up, the piston will start slamming into the head on the 
  exhaust stroke, until it breaks the piston into pieces. Then the rod and pin 
  will beat a hole in the cylinder wall, and things deteriorate rapidly from 
  there. 
- I've disassembled probably a dozen engines that went like that.  How someone 
  could stand the noise and commotion is beyond me. 

[email protected] (Dave Williams)
gnttype  09 Dec 1995
- -> OK, quick re-cap, last year when I changed heads I swapped out the
  -> rod and main bearings,#1 rod bearing were slightly worn(1 side
  -> polished) Put motor back together,rod knock after 3,000 miles @#$%@$#!!!
- One side worn generally indicates a bent rod or (less often) a tapered
  crank journal.  A tapered journal is usually that way from the factory;
  caused by not properly dressing the wheel when they grind the journal.
  Though it's possible to make a rod with nonparallel bores by any of
  several machining errors, most rods get bent in service.  Either way,
  you had a problem that needed to be corrected.