Some thoughts on babbiting, by Mr. Mike Flanagan
The following is an article I was asked to write for Restorer Magazine regarding my thoughts on Insert bearings in the Model A motor.
I was not asked personally to write an article but the request was framed in a question that asked if I knew anyone thick skinned enough and knowledgeable enough to write such an article and live with the consequences. I can't for the life of me imagine why I was approached but so be it.
After writing the article it was then rejected as being "inflammatory" which I would not think all bad given the number of fireplaces owned by Model A drivers. Inflammatory or not this is the way I feel about inserts, there is more but to go there would indeed be inflammatory and contrary to popular belief I do attempt to retain the civility at all times:
My concern stems strictly from the sense of the mixing of two completely different technologies that I fear are not mixing as well as some would have us believe. Admittedly appearances would suggest that they will work well in an A engine and I have seen some that will literally scoot, albeit a little noisily for my tastes, but some do run well, but at what price. The research I have done on insert bearing shells, their design and their construction has led me to the following conclusions.
Federal Mogul states in their literature that inserts "are designed to be used in fully pressurized applications". They don’t seem to be equivocal in this regard… they state it as fact not conjecture.
Their statement was not made in response to folks using the bearing inserts in unpressurized situations, as that had not started when the words were written. It was a statement made in explaining the science behind the design. In other words design criteria combined with empirical data from testing led the manufacturers to the conclusion that pressurization of the hoiling system; wherein the bearing never actually contacts the journal during normal use, was essential to the effective life of the insert.
Inserts have several things going against them for use in the Model A engine. One of them is called eccentricity.
Eccentricity in this case has nothing to do with my demeanor in a social sense but rather in that the insert shells are not made uniformly in section. That is to say the thickness of the outer supporting shell plus bearing material bonded thereupon is not the same thickness at the parting line at both sides and the midpoint or the “U”. This dimension is greater by .0015 at the center of the shell than at the ends or parting line.
This eccentricity is to allow clearance for what is known in the industry as “pinch” which occurs primarily when the rod is pulling the piston down on the intake stroke. In the “four-stroke engine the two down strokes are “power” and “intake”.
On one of the down strokes, the power stroke, the piston is being helped by combustion. Pinch is not a concern on the power stroke. However on the intake stroke the rod cap must do all the work of pulling the piston down in it’s bore.
With the friction of the rings and the resistence of vacuum pressure as it tries mightily to pull in all the air/fuel mixture it can get there is considerable pressure exerted on the rod cap. This resistence to downward movement is what causes the “pinch” which is nothing more than the rod cap deforming and drawing the two sides of the rod at the parting line with it towards the center of the circle.
This is what causes the “wiping” at the parting line on modern inserted engines that have the clearance set too tight. The theory is that the clearance set at .0025 in the center, as on the 454 Big Block I recently built, actually has much more clearance at the parting lines or the sides of the rod. This allows the rod to pinch yet no contact is made bearing to journal at the parting line because of the dimensional differences.
Another problem is the inserts consist of a steel shell with an application of lead/copper alloy applied to it that is from .013 to .015 in thickness. On top of this copper/lead alloy is a layer of Babbitt that is .001 of an inch thick “for embedability”. In our non-filtered engines we need just a tad bit more Babbitt for “embedability”.
The insert, as designed, was made to run with a thin “pressurized” film of hoil keeping the insert from contacting the crankshaft journal. In the A this pressure is simply not there. We rely on what is called Boundary Lubrication. Boundary Lubrication is a free flowing coating of hoil with three boundaries or layers of the lubricant when viewed in section.
In cross section there is one boundary or layer that is next to the crankshaft journal and loosely adheres to it moving in the same direction as the journal in relation to the bearing surface.
Another boundary or layer is adjacent to the bearing surface and moves in the same direction as the bearing itself does in relation to the crankshaft journal. This layer adheres loosely to the bearing surface.
The third boundary or layer is between these two and moves at half speed to the other two and must also move in two directions at once as the other two boundaries are opposing each other. This turbulence is just about our only source of pressure.
Expanding on the clearances I mentioned previously; to accommodate the “pinch” the clearance settings in modern engines is commonly .002 to .0025. This allows the “pucker” of the rod to occur with no bind of the insert on the journal.
The clearance in excess to the .001 we are accustomed to is kept secure by the pressurization of the film of lubricating hoil. A rule of thumb in the industry is that there should be 10 pounds per square inch of pressure for each thousand revolutions per minute. The Model A would obviously need to run a constant 20 psi at the bearing when cruising at speed.
Setting the inserted Model A engine at .001 leaves over .002 at the parting line that is not supported by any pressurization. As the “flash” Babbitt is worn off and the insert begins to loose the copper/lead alloy this clearance would seem to become rapidly unacceptable.
Another consideration and perhaps the most telling is that when a Babbitt motor dies it is a protracted, often noisy, clanking death. But it does take a while. Once the death throes begin there can be as much as 100 miles involved in this demise. And it is never truly terminal. Pour some more solder in and machine it out and you are ready for more fun.
It has been my experience that when an insert goes it does so suddenly, noisily and either welds itself to the journal or grinds itself to death on the journal, either way is not necessarily good for the journal.
The engines I have been involved in converting back to Babbitt have exhibited all the above, each needed a new crankshaft or buildup of the offending journal. One was caught in time but barely.
You be the judge. If you use inserts check them often and when the wear becomes severe enough that you can begin to see a considerable amount of copper color or the steel backing it is time to re-insert.
I have been told that a set of inserts is good for about 10K miles as a rule of thumb. In my mind that is simply not a worthy duration cycle. Replacing the mains requires pulling the engine and doing that every 10k miles on a tour car hardly seems worth it… especially if it occurs in a Motel 8 parking lot.
Remember, the Model A was never a car that was meant to be a marathon driver. There are cases of them being driven seemingly impossible amounts of miles.
I have disassembled engines that the
original Babbitt was worn to the point that the grooves were completely gone. I know personally of a Model A engine with 40,000 plus and counting.
The biggest consideration I see is when these insert wonders start “shelling” out… who is going to work the overtime to get everyone back on the road?
And some engine builders pouring Babbitt do not have the expertise to do a block that has been opened up to allow for inserts. At my shop the line can get as long as it pleases, my limitations are such that I cannot control them and when they say no it is no.
If you are waiting until I change to inserts before doing so yourself have a seat… it will be a while. I would not want to drive an inserted motor any farther than I would want to walk….