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As a metallurgist who has specialized in failure analysis for over 40 years,
I'd like to guess with you and give you my opinions based on what you and
others have posted in this thread and also based on my own experience. Note
that these are opinions based largely on sight-unseen (but pretty complete)
descriptions. (That is, I haven't seen your engine.)
I have a hard time imagining that a properly fitted B20 rod bearing shell would
spin given constant lubrication and no undue contamination. Probably over 90%
of the B18 and B20 bearing shells I have removed are still usable. Being
Scottish, I'm not sure why I have bought so many new ones. I have a box full
of good used ones in std size. So you have a couple possibilities here, and
my first choice would be that the bearings were assembled too tight (maybe
undersize?) or that something blocked the oil passage somewhere along the way.
I wonder if whatever stuck the oil pump bypass valve might have also
contaminated the bearing.....
For some reason all my major failures have been on cylinder #2, including the
5/16" hex nut incident that slightly damaged the piston and head but destroyed
the big bore block by cocking the piston in the cylinder and splitting the cylinder from top to bottom, also the dropped lifter incident in a B30 where
it got trapped before falling into the pan and later got hit by the left side
#2 rodcap bolt, apparently breaking it off or at least initiating a failure
that went as far as allowing about 1/4" slop in the travel of the #2 piston.
(Never got into the lower end of that one but we did pull the head.)
As far as the rodcap bolts are concerned, unless they get cracks in them, they
do not get more "stretchy" with age. The elastic modulus for nearly all steels,
and certainly including steels used in Volvo rodcap bolts, is close to 30
million psi. That is the theoretical stress to double the length, calculated
using a much lower stress and extension. It doesn't change regardless of age
or any other condition. Naturally if cracks reduce the cross-sectional area
but you are not aware of them it appears less. But as long as the stress on
the bolt is in the elastic range, the same stress stretches the bolt the same
amount. Beyond the elastic range the bolt is plastically (permanently) deformed
and you can detect that by measuring its length with a micrometer. I'd be more
concerned with damage to threads, etc, since these are reduced-shank bolts, so
that the highest stress is in a smooth, unthreaded area.
My guess on the valve train is that it was overloaded. The motor oil industry
recognizes the B20 cam/lifter interface as a problem to the point that they have
what they call the "Volvo B20 Cam and Lifter Test" for motor oils. (My efforts
to get a copy of this procedure have thus far been unfruitful. If anyone can
help me get one I'd be deeply indebted.) So we know that this interface is a
weak link, and I have experienced a number of failures here on both B20 and B30
engines, most less severe than total loss of a lobe but on one B20 and 2 B30s
I have seen one lobe completely rounded off to where there was no valve lift.
One of the B30 incidents happened on a drive from Phoenix to Yuma, about 150
miles. My OPINION is that when you add to that an unknown cam, dual valve
springs and high-ratio rocker arms, the total is greater than the sum of the parts can endure over the long haul.
The suggestion was made to "Normalize" the crank. Normalizing is a heat
treatment used on rolled steel products where they are heated to around 1500° F
and allowed to air cool. Its purpose is to provide a uniform grain structure to
the steel following the extreme deformation and varying thermal treatment of
rolling, so that successive pieces of the same material will display fairly
uniform properties. This is something you do NOT want to do to a crank for
a number of reasons. The first is that the entire surface will be scaled and
possibly decarburized so that all the machined surfaces will have to be
re-machined (and will thus be undersized). It will probably also result in
substantial warpage. Finally, any heat-treatment given to the crank in
manufacture will be lost and it will be relatively soft (and thus weak, as
strength is proportional to hardness).
I'd recommend that you get out your mikes and examine and measure your lower
end and MAYBE have the crank and rods magnafluxed. If you are close to original
size on the bearing journals, and are still so after polishing out any
coppering or other surface effects (like still pretty round and within 0.001"
or so of the size you want) go ahead and use it. Rods likewise. Mike the
rodcap bolts for length and get rid of any that are long. Do a trial assembly
with plastigage, shimming the sides of the rods tightly so that they don't turn
on the crank throws while torquing down the caps. If clearance is within spec,
great!
Get a cam you like that has enough lift to do your job. I like the Chevy/IPD
lifters (as you are already probably quite aware) and whatever pushrods are
needed for the cam and lifter combination. I would go with original ratio
rockers and no more valve springs than it takes to keep the door shut at your
normal max revs.
If the ring grooves are OK I'd go ahead with the pistons you've got, with
whatever cleanup is necessary. Make sure your small-end bushings are OK.
I guess what it all comes down to is that there are street engines and race
engines. Race engines are built for the short run and will not do in the
long haul.
Note that these are predominantly my opinions and you are encouraged to do
whatever you feel necessary or whatever warms the cockles of yore heart.
But in any case, get it ROLLING!
--
George Downs, Bartlesville, Oklahoma, Central US
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