Strange Measurements

One of today’s jobs has involved attending to the king pin bushes of the Singer.  I have some new king pins and bushes in my possession with which to complete the job, and thankfully the new hardened and ground kingpins (which are deceptively expensive to manufacture in small quantities) have been made to perfection.  I’m afraid to note, however that I shall not be using the new bushes which is something I had entirely expected and which I was able to decide upon within seconds of unpacking the new components when they arrived.

The tell tale clue lies in the fact that the new bushes were an easy fit over the new kingpins.   The kingpins themselves are made of hardened steel; manufacturing them is quite involved because it neccessitates machining the metal stock to an oversize, cutting the grooves in them to allow passage of grease, drilling and threading them for grease nipples and cutting the locating flats for the taper cotters which prevent them from turning in the axle beam. After this they are hardened and then surface ground to an accurate final size.  Axle Assembly The Kingpins are shown here in the illustration – they fit into the  bushes which in turn are fitted to the stub axle assemblies, and this is where the wheels pivot in order to steer the car. Even the very smallest amount of wear between the kingpins and bushes is magnified many times at the wheel rim and leads to excessive tyre wear and to poor handling of the car. Providing a precise and correct fit between kingpins and bushes will result in no discernible “play” at the wheel rims and (if greased regularly) will last for an astonishingly long time before replacement is needed.

This point is where we stray into the realms of the eponymous “strange measurements”:  Machinists in the 1930s measured in thousandths of an inch, known universally as “thou”.  I too measure in “thou”, not because I was about in the 1930s but because I am an automotive dinosaur and very proud of the fact.   As it happens most of us choose inevitably to ignore that “a thou” is 1/1000 inch and we regard it colloquially as a unit of measure in itself; therefore it is quite acceptable to us when we refer to “a tenth of a thou”. And so it is that the tolerance for fitting a kingpin to a kingpin bush is “two tenths of a thou”. Yes, we all know that really means 0.0002″ or  1/5000 of an inch but calling it “two tenths” is traditional and tradition is strangely pleasant in this context.

Many people are surprised that the tolerance is this close, and although it is quite possible to measure and to manufacture to sizes this accurate it is only meaningful if it is done in a temperature controlled room.  In practice this is done by trial and error:  The kingpin bushes are fitted to the stub axle assemblies and they are then progressively enlarged using an expanding reamer and a pilot.  The pilot ensures that the two bushes (one at each end of the kingpin) are kept in perfect alignment, and the reamer is progressively expanded by tiny amounts until the kingpin is the perfect fit. For this reason I recently commented to another engineer that we know the tolerance is “2 tenths” but no one knows precisely which “2 tenths”.

The important part of all this is that the bushes should be manufactured to an internal diameter which is too small to pass over the kingpin.  They are .002″ (two thou) larger than the bore in the stub axle assembly and when they are pressed in we can then be certain that they will no longer be perfectly round or of perfectly predictable internal size; neither will they be in perfect alignment with each other.  The line reaming will correct all these faults and will leave a superior surface finish to that which could be obtained by machining in a lathe.

The new bushes which I have manufactured today have been made from Phosphor Bronze.  This material would have been considered a luxury in 1935 and was seldom used for this sort of application.  Certainly the replacement bushes which are currently available are made from “bearing quality brass” which theoretically is the correct specification.  Sadly “bearing quality brass” is not what it used to be – presumably because there is no longer any call for it in modern automotive engineering and the use of Phosphor Bronze ensures a job which will give a decent length of service. How long is a decent length of service? The kingpins in my 1930 Alvis need to be re-bushed some 100,000 miles since my father replaced them; when I mentioned this to him he looked disappointed and wondered whether he had fallen in some way short or whether I had neglected to grease them properly.

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3 Responses to Strange Measurements

  1. Fabulous post, Pin, which has given me more of an insight into the precision and loving detail of your engineering: it is nothing short of miraculous to a lay person like me. It has brought home to me the tiny margins upon which rest the quality of performance and reliability of the working car.

  2. Ernest G Wright says:

    Very interesting post I also measure in Imperial far more accurate than that foreign stuff, I always fit the new bushes to the stub axle before condemning them as the radial pressure will “close em up a bit” as was the case on my uncle Tom`s FWD 1250 Alvis many, many years ago at the dreaded MOT time, not a job for the faint hearted I can tell you, the new bushes were a slip fit over the king pin or chalice that had been built up and ground to size we fitted them and had to take a whisker out “technical term” with an adjustable reamer, the old boy had a set of original bushes “in stock” in the kitchen pantry along with a spare supercharger, block, head ,BTH mag, gear box parts, brake drums and God knows what else an Aladdins cave for FWD bits…..Happy days in GO 1591
    p.s. I`ve an XK8 on the ramp with code p0706 thats how I ended up here great web site.

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