<VV> Re: Divergents

[Louis C. Armer, Jr.]

Hey Bob, I have a couple of questions for you. Is the GM crankshaft "fix" 
for all
engines produced after 1961 or just for 145 cu engines? If the "fix" is for 
all engines
after 1961, what causes the  #5 and #6 break? Is it the deformation or 
viabrations
and why wouldn't  "pounding out" still happen even though it might be 
slower and
less severe than before? If whipping action is caused by a flywheel or torque
converter wouldn't it still exist regardless of the bearing clearance or 
crankshaft
location? Finally are you suggesting that crankshafts prior to the "fix" 
would break
somewhere other than at #5 and #6?

Chuck Armer

ps.

>Wow, you sure must have a bunch of spare time to write these.
>Regards, Bob Helt

!!! <GGG> Chuckster
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At 08:11 PM 2/21/05 -0500, you wrote:
>In a message dated 2/21/05 1:02:19 PM US Mountain Standard Time, BobHelt
>writes:
>
> > HERE IS A SPECULATION
> > (on the number four main bearing)
> >
> > Putting two and two together usually arrives at four.  Here is an 
> attempt to
> > do just that. But remember there may be some hidden pitfalls that might
> > upset our attempts. So divergent tangents can upset this logic at any 
> point. What
> > we are going to discuss is the change to the #4 main bearing and the
> > implications. For what it's worth, here we go.
> >
> > Bob Kirkman in his Tech Guide Supplement article relates how some early
> > Corvairs develop a crankshaft "Thump" at idle. He tries to identify the 
> dates
> > that this occurred, and when the fix was implemented. He also describes 
> the
> > method that was used by Chevrolet to correct the thumper problem. In 
> all of the
> > Corvair literature I know of, Bob's is the only reference to the Thumper
> > problem.
> >
> > The problem, seen on an oscilloscope setup, was a sudden movement of the
> > crankshaft across the #4 bearing clearance caused by cylinders 4-5-6. He
> > reported that this thump could not only be heard, but also felt by 
> touching the body
> > anywhere. That has to be a serious problem!
> >
> > He references an investigation of the problem during 1962 and 
> implementation
> > of the fix for the 1963 model year. The fix he reports was a lowering
> > (dropping) of the crankshaft by 0.0015" on just the #4 (the front) main 
> bearing.
> > This resulted in a new pair of bearing shells for #4.
> >
> > OK, so far so good. Except for one little-noted item, we would have to 
> leave
> > the subject right there. That item is a small article in the April 1960
> > Chevrolet Service News, that tells about the new #4 main bearing with 
> the lowered
> > centerline (it doesn't mention why the change was made though). That tells
> > us the fix to the thumper problem was determined in 1960, not 1962. And 
> now we
> > start adding our second two (to get four).
> >
> > If the new #4 main bearing was released to production around April 1960,
> > that would mean it was probably implemented during the later half of 1960
> > production, and not 1963. That would also mean the Corvair Shop Manuals 
> and other
> > literature would show these changes for the 1961 model year.
> >
> > The correction of the thumper problem as reported by Bob Kirkman, was as
> > previously stated to be a lowering of the #4 main bearing centerline by 
> 0.0015
> > inches. But wait! The thump was reported to be caused by cylinders 4-5-6,
> > which would have caused a sidewise deflection of the crankshaft, not an 
> up/down
> > deflection. So how does lowering the crank solve a sidewise motion 
> problem?
> > And doesn't lowering only one of the crank bearings place a strain on the
> > crank? This sounds like a strange kind of fix if that was all there was 
> to it, in
> > my opinion.
> >
> > But now there are several other things to think about. The 1961 and all
> > later Shop manuals all show an increase of 0.0005" in the crankshaft 
> journal
> > diameters for bearings #3, &#4. In addition, the spec clearance of 
> these two
> > bearings was tightened by the same 0.0005". Why was that done? That 
> puts the
> > minimum clearance on these two bearings at 0.0007", which is quite 
> small by any
> > standards.  Finally, the parts manuals offer undersize bearings (U/S) 
> to allow
> > for selective fitting of the main bearings on a standard crankshaft.
> > Bearings #1, #2, and #3, all have available one and two thousands 
> undersize
> > bearings. But #4 also has available a 0.003" undersize as well as the 
> one and two U/S
> > bearings. Why would a 0.003 inch U/S be offered for only the #4 bearing?
> > There must be some connection to that thumper problem!
> >
> > So now we start adding things up. The centerline was lowered, but also the
> > bearing clearance was tightened up too. And changes were made to allow the
> > factory and Dealers to set this clearance as low as possible.
> >
> > But let's speculate further. The crank has either a flywheel or torque
> > converter mounted to the #4 bearing end. Is it possible that either 
> unbalance of
> > these two heavy items, or a "whipping" action (remember the reasons for 
> the
> > flexible flywheel) caused an operational increase of the bearing 
> clearance?
> > Possibly this bearing was under-designed in its ability to handle the 
> heavy
> > flywheel or torque converter loads. Maybe the heavy loads caused a 
> "pounding-out
> > of the aluminum web supporting the bearing, increasing the clearance and
> > causing the thumping (remember, that would be before the fixes).
> >
> > So now, let's review the fixes: A new #4 bearing with its centerline 
> lowered
> > by 0.0015"; Journals #3 &#4 on the crankshaft increased by 0.0005"; and 
> the
> > clearance spec for these two bearings tightened by 0.0005", with a minimum
> > spec clearance of a "tight" 0.0007".
> >
> >       Let's digress for a minute and look at how these bearing clearances
> > are measured. The Shop Manuals all like the Plastigage method, whereby 
> a thin
> > strip of plastic material is squashed between the journal and the 
> bearing when
> > the crankcase halves are tightened to spec. But think where the plastic
> > strip is usually placed. It's NOT at the crankcase parting lines, but 
> about 90
> > degrees away in the center of the bearing shell. That means we are 
> measuring
> > the side-to-side clearance, and not the up-down clearance. I wonder 
> what the
> > up-down clearance might be, at the top of the #4 bearing (at the top of 
> the
> > lowered portion of the bearing)? I haven't measured this clearance, but 
> logic
> > would indicate it to be very small if one considers a perfectly round 
> journal
> > and bearing, using the spec of 0.0007" side-to-side clearance. The crank
> > journal has got to be rubbing on the top of this bearing! To what effect?
> >
> > We now arrive at the final speculation. What are we doing to the 
> crankshaft
> > with this bearing arrangement? We've got bearing #3 using a standard crank
> > centerline position, but with a tight clearance spec. Thus, bearing #3 
> holds
> > the crank tightly in position. But bearing #4 pushes the crank down by 
> 0.0015",
> > thus deforming it to some extent. As the #4 journal rotates, it gets 
> pushed
> > down slightly, and held down. So, could there possibly be a relationship
> > between this crankshaft deformation and the fact that many broken 
> crankshafts
> > break between throws #5 and #6, both of which are between the #3 and #4 
> main
> > bearings? Who knows, but it's something to consider.
> >
> > Bob Helt
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Corvair Atlanta, BOD
CORSA MEMBER
CORSA Tri-Membership Chairman
http://carmerjr.home.mindspring.com/