<VV> Fwd: followup on the #4 main bearing (very long)

[BobHelt at aol.com]

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
>