I have been engaging in a bit of an old fashioned art today: The hand scraping of big end bearings belonging to the Singer Le Mans engine.
I talked about the white metal bearings from this engine in the last post and fitting them correctly calls for a rather different approach compared to the bearings found in most post – war cars and in all modern cars. Perhaps the most important thing to consider first is what constitutes a correctly fitted bearing. Bearings come in various different designs but the ones being discussed here are “plain bearings” meaning that the bearing is simply a round hole which runs on a round shaft with no interspersed rollers or balls. The fact that these bearings operate entirely on a frictional basis with no rolling components gives them some very specific requirements indeed: Firstly they generate much more heat than a bearing which uses a rolling component and secondly they rely much more upon oil pressure. Both of these factors require a certain standard and tolerance of fit: If the plain bearing is a separate component it must have a fit to its housing which is near perfect to ordinary workshop methods of measurement in order for it to conduct this heat away. The plain bearings of the Singer big ends are cast directly into the connecting rods so this is not a consideration for this engine, however the second factor is very much a consideration: The bearing must have the correct fit to the crankshaft and be of a smooth finish which gives a perfectly even clearance for the whole area of the bearing.
The oil pressure upon which these plain bearings rely has very little to do with the pressure read by the car’s oil pressure gauge. The pressure from the engine’s oil pump is only relevant to tell us that a suitable amount of oil is reaching the bearings; after that the bearing and crank pin pump their own much higher pressure which is called “hydro dynamic pressure”.
The shaft running in a plain bearing is of course not going to run in the centre of its clearance: It will move to one side of the bearing hole depending on which direction forces (ie: either gravity or other mechanical loads) are acting on it. This will allow the rotating shaft to pump pressure (hydro dynamic pressure) against the oil surrounding it and to form a hydrodynamic wedge of oil as shown in the illustration. This means that as long as the bearing is supplied with oil, the shaft will never actually touch the bearing metal but will instead be supported by the hydrodynamic oil wedge which can be as little as one molecule in thickness.
Imperfections in the fit of the bearing will interfere with the ability to produce a satisfactory hydrodynamic wedge and bearing life will be shortened.
The fit of bearings to shafts is usually measured with a marking compound variously known as “Engineers Blue” or “Prussian Blue”: The shaft is coated with a blue, greasy compound and the bearing then assembled around it and rotated before removing again. In areas where the shaft is a correct fit to the bearing there will be a blue “witness” left on the bearing. In areas where the bearing does not fit closely enough there will be no witness and in areas where the fit is too tight the blue will have been transferred to the bearing and then bear signs of heavy pressure. The ideal situation would be to find an even witness of blue all over the bearing which is referred to colloquially as “100% contact area”. Clearly this is technically erroneous since as we have demonstrated, bearings do not contact shafts but it’s an elegantly simple description and I shall continue using it!
Of course, if you looked at the surface of a bearing under a microscope the “contact area” would be far from ideal but for white metal bearings 100% to the naked eye is the required fit.
This is the bit where I am rather happy to be very old fashioned in my methods: These days most people finish white metal bearings by machining them to the required size and then fit them to the engine. Here is a picture of a connecting rod bearing belonging to the Singer after machining to size and obtaining a witness with marking blue. A very poor contact area indeed. There is a very good reason why machines don’t do better than this on ancient designs but that will wait for another post. Since I can’t entertain the thought of assembling a bearing with this sort of fit I have machined a little metal from the “butt faces” where the bearing halves fit together which has closed the bearing up and made it temporarily slightly oval and too tight on the crankshaft.
The next stage in the proceeding is rather repetitive: Coat the crankshaft pin with marking blue and assemble the big end bearing to it before rotating it to obtain a witness; remove the bearing from the crank pin and then scrape away white metal from the high spots using a bearing scraper by hand; wash the bearing clean. Then repeat until the desired contact area shown here is obtained. Incidentally, if you “blue” the bearing shells on a modern engine you won’t get contact area this good because they are designed to put up with a lesser standard of fit.
I’ll end this rather long post with picture of two bearing scrapers. These are “half moon” scrapers and are used for removing metal from round surfaces. The one at the top of the picture is the one used on these Singer big ends and was purchased some time in the 1990s. The cutting edges of these tools must be maintained perfectly sharp using an ordinary oil stone of the type used for sharpening wood chisels. The bearing scraper at the bottom of the picture was the first one owned by my father – he must have purchased it around the time of the Second World War and it would have been identical to the one above it before it became too sharpened away to use any more. These tools which are identical right down to the grooves in the handles are separated by at least 50 years…. thank goodness that they are still available!