1/15/2007 Rev .6
What I write will probably start some controversy. Please feel free to correct what I say with facts.
Babbitt has been an interesting subject. I can remember back as an 8 years old (or so). My father and brother were heading down the path to going in to the Model A Engine rebuilding business. Now you have to understand my brother is gifted in the mechanical arts so the quest for understanding takes a different path than most. What became apparent is the people doing engine work were not operating on scientific facts or a technical understanding, they were just pouring babbitt. Some were having better luck than others, but finding someone with some vague concept of what really was going on was impossible. Oddly, that still has not changed. I have been on a quest to separate fact from fiction. I have found several people that seem to understand some areas of babbitt quite well, but I for what I want I need strong facts to reference. I like stuff that is properly researched so I can what out known truths. Easier said than done so I will try to present as much referenced information as I can and try to pull together some facts to hopefully get a better understanding of just what is happening with babbitt. This will hopefully help you figure out who really knows how to pour and bore babbitt and who is just lucky.
What is Babbitt?
Babbitt is a tin based bearing material with two other elements. The two elements are copper and antimony. There are a few lesser elements, but I will only focus on these three as they are most important. Originally, Babbitt did not have ANY lead in it. As any popular trade name Babbitt soon became any bearing material, so lead based bearing material is included in the term Babbitt today.
Tin is a great metal for a bearing. It is soft, but not too soft. It will let dirt embed and it coats it so it will not score the shaft. If you run out of lubricant, the tin will melt some and liquid tin is a lubricant too. But it really is too soft to be used as a bearing in engines and it melts at too low a temperature (449* F)
Copper is used to make the tin harder. As the percentage of copper increases so does the melting point of the tin copper alloy. Copper melts at 1984* F.
Antimony also makes tin harder and increases its melting point. Antimony brings another unique property it shares with water. Antimony expands as it solidifies. Most metal shrinks as it cools. Shrinking means the bearing that was just poured in wants to pull away from the block. Antimony melts at 1167* F.
Copper and antimony also affect the malleability of the alloy. They modify the proprieties of the tin and allow it to handle the impact force in the engine without cracking or be squished out like butter.
The following text in red was what my original conclusions were, but now I have some better information.
You must consider the bigger picture with alloys. (I am researching this further so this may change, I have read about this in one place but need to check the science on it) I currently understand that the higher melting temperature copper and antimony alloys will tend to crystallize (solidify) first as the Babbitt cools. So you would expect higher concentrations near the cold block. Now think of how inserts are made. They have a brass or steel shell with some copper then some babbitt. So if you have the copper and antimony in higher concentrations near the block you will have a strong base for the tin above to hold onto. This also means the bearing surface will tend to be more tin which has better lubricating and bearing properties. Ford did pour into cold blocks and the KRW documentation (see The Restorer V39,I1,p8) also indicates this is the proper procedure. Now consider cold is a relative term. With something that melts at 700* F then 500* F is is cold enough for it to be solid. This is one of the very debatable issues of babbitt. Do you pour into a cold block? If the copper/ antimony portion stratifies on cooling than the answer would be a solid Yes you should pour into a cold block for a better babbitt. Someday I hope to have a solid answer to this question. Now for spun pour rods you will find it is critical to cool in a certain manner to allow for stratification of the harder alloys which solidify at higher temperatures. There are some web sites showing how they spin pour large round bearings like what are used on power generators. (I need to put some web site links in here).
Recently I was able to talk to a long time metallurgist and he gave me some insights that make more sense. This person work in the steel industry so he could not be more detailed, but what he said made more scientific sense. Metal is all about crystals. You need the right kind of crystals for each type of job the metal is expected to perform. You introduce other trace chemicals into a crystal matrix to modify its properties.
Now consider a large chunk of steel at the foundry. They pour the block and let it cool. If the block is left to cool would be a useless chunk of metal. The problem is the slow cooling. As it cools from the outside in the block will form into large crystals and the large crystals are not very strong and have other undesirable properties. So the metal is now further worked at temperature to make lots of smaller crystals. The higher quality steels tend to have a finer grain structure.
Now consider the babbitt. If you allow the babbitt to cool slowly it will form large crystals. I believe that large crystals are more likely to start micro cracks which will lead to later failure. So if you pour into a relatively cool block (300* is a lot cooler than 900*) than the babbitt will cool rapidly causing finer crystals. This fast cooling would also keep the mixture of metals uniform throughout the bearing preventing hard spots.
The spin pour babbitt documentation I have read may pertain more to large thick bearings like used on huge power generators. The thin shell A rods still need to be cooled, but I would believe the idea would be to cool the babbitt rapidly to get the fine grain structure.
Please keep in mind this is more of an educated guess, but I believe it is more realistic. This is a tough subject to get reliable information.
Lead in babbitt is a bad thing. Lead should never be in a bearing that has compressive loading like in an engine. The problem with lead is multifold. First it is too soft. Lead babbitt can be scraped with your finger nail. I learned about this when I was like 8 years old. We have like 30 or 40 engine blocks (yes we are block hogs) and I have had the experience of touching a lot of bearings and real original Ford babbitt does not scrap off with your finger nails, lead stuff does. Do you really think this will hold up in an engine? Lead has another bad property, it work hardens with impact quickly and cracks. It is interesting how many of the bearings we found could be scraped with you finger nails were cracked too. Now you will love this, according to a study by a German company, even lead at .03% concentrations in tin babbitt can lead to micro cracking. To make lead harder they add nickel. Nickel has this nice property of scoring you crankshaft if it is too soft. Ford did not harden the cranks a lot because he selected a certain grade babbitt and did not need the extra hardness. If you plan on driving your a than you want to stay way away from any lead babbitt. Most babbitt produced today will have not more than .03% lead in it and then is not so bad as it may really have no lead in it.
Ford used Genuine Ford Babbitt which was 86% Tin, 7% Copper, and 7% Antimony (see The Restorer V39,I1,p8). Today most shops pour grade 2 Babbitt which is not Ford babbitt and, contrary to what several people in the business have told me, it is softer than Ford babbitt. Today the only off the shelf babbitt that is close to Ford babbitt is grade 11. There is at least one company that has the exact Ford mix available for purchase. For practical purposes I must add there are many cars out there done with grade 2 babbitt with thousands of miles and no problems.
Consistency of pouring is another concern. Back to the crystals again. It appears that if you do not mix the babbitt correctly before you pour you can get hard spots. Any none uniform distribution of hardness in babbitt will lead to fracture sites. You can get severe changes in babbitt chemistry if you do not follow the rules. The pot must be clean, nothing left from previous pours. You must cut up the babbitt to quickly melt it. If you plan on reusing babbitt (you are allowed to add a small amount to new babbitt) you must pour the babbitt into smaller pigs. The smaller pigs allow for quick cooling and quick reheating. If you left it in the pot it would cause the mix to crystallize into large crystals and not re melt back to the original form properly giving you hard spots.
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