I started the more formal side of my engineering studies in 1984 and this involved spending one day of each week in a college workshop and four days in the lecture room. Friday was “practical day” and was looked forward to by all the engineering students, most of whom would end up at the stores counter. Those who were not queuing with bits of paper requesting a new bubble for a spirit level, a square drill or a bucket of steam were instead at the side of the queue having been sent down there with a ticket for a “long weight”.
I’m not quite sure that victimising 16 year old students is considered acceptable practise any more but I am sure that knowing how to deal with a long wait is an essential skill for any sort of career: The wait for a quality white metalling job has always been a pretty considerable one and at last I have collected the re-metalled bearings and connecting rods for the Singer Le Mans engine rebuild. White metal, properly known as Babbitt Metal now differs a little from the original 1839 alloy developed by Isaac Babbitt in Massachusetts. In the case of the Singer it is cast directly into the steel connecting rods and also into the bronze main bearing shells in order to provide a bearing surface for the crankshaft journals and pins. One of the important duties of the bronze backing shell is to conduct heat away from the babbitt and in to the engine block. (How’s that for precision terminology? According to unspoken convention, the original stuff is Babbitt Metal and the generic term is babbitt.) If the babbitt is overheated it begins a transition from solid to liquid and enters a plastic state. When it cools from this state the macro structure of the material will have been altered and the bearing will ultimately fail.
If the bearing is going to conduct this heat away then the bronze shell has to be a precise fit into the bearing housing with near total contact area. After the shell has been manufactured and fitted to this level of precision it is then heated up and “tinned” with molten metal in order to provide a key for the subsequent white metalling. The Shell is held in a mould and maintained at a suitable temperature for the molten babbitt to be poured into it and when everything has cooled down it will be looking rather lumpy! The excess babbitt is then carefully removed from all areas except the bearing surface and this is done painstakingly both by machine and by hand. The bore of the bearing is then machined with the whole unit assembled into the engine crankcase in order to maintain alignment between front and rear bearings. Finally oil grooves are machined into the bearing surface and the connecting oil holes are drilled. After all this work the near perfect fit to the bearing housing must have been maintained.
This all puts the ease of overhauling a post – war engine with its “thin wall” shell bearings into an interesting perspective and it certainly explains the usual long wait.
There was, incidentally a long standing tale (most likely apocryphal)which maintained that a student had once emptied a bucket of cold water over a lecturer, having apologised that his bucket of steam had condensed on the way up the stairs…….