Keeping it on the straight and narrow – Aspects that affect TA/TB/TC steering (Part 4)

Eric Worpe delivered a superb presentation at the MGCC ‘T’ Register’s ‘Rebuild’ seminar in March 2013. Eric used flip charts to aid his presentation and I have been working with him to ‘flesh out’ the flip chart notes to produce a series of articles for inclusion in TTT 2.

Eric divided up his presentation into seven headings which he termed as “Seven Deadly Sins”. We have so far covered the first three ‘Deadly Sins’ i.e.

CHASSIS – is it true? – Issue 19 (August 2013).
FRONT AXLE GEOMETRY – Issue 20 (October 2013)
FRONT SPRINGS – Issue 21 (December 2013)
In this issue we’ll look in depth at the fourth ‘Deadly Sin’: KING PINS

King pins or swivel pins have had a longish history from the earliest cars with simple beam axles to some quite recent cars with independent front suspension. Despite being subjected to considerable stress, king pin assemblies seem to give a reliable service when regularly maintained.

One story about Henry Ford tells how he would send inspectors to breakers’ yards to assess what were the causes of terminal car breakdowns. King pins were the only items that emerged unscathed, so he ordered their specifications to be downgraded. Fortunately, our king pins have escaped the machinations of the ‘bean counters’, as a good safety factor seems to have been built in to the king pin assembly.

Figure 1

Fig. 1 sketches out the component parts of the assembly, the original bushes were of the bi-metal wrapped type consisting of an internal layer of leaded bronze fused to a steel backing in the form of a split cylinder. Impressed in the bronze layer is an oil/grease groove with a spur take off that feeds lubrication to the thrust faces of the beam axle’s eye. The main thrust washer has either an eccentric groove or radial channels to distribute lubrication around the thrust faces. Sitting on top of the king pin is a felt washer covered by a dished cap to keep moisture/abrasive dirt out. The felt washer should be soaked in heavy oil such as EP140, before assembly.

The cotter pin is, I believe, an unappreciated critical item as all it seems to have to do is wedge the king pin in the beam axle’s eye. How well this is achieved is crucial to the robustness of the assembly as the eye is heavily stressed and any slight tendency of the king pin to rock in the eye becomes exaggerated by the constant stress reversals on the road wheels.

Cotter pins made from easily machinable steel tend to “ruck up” when their flat flank is obstructed by the leading edge of the notch on the king pin.

This in turn prevents the cotter pin wedging the king pin really tightly in the axle’s eye. Cotter pins need to be as hard as high tensile bolts; the ones that John James had made up were from EN19T grade alloy steel. They should be hammered home and only then should the nut be tightened; pulling the cotter pin through by just tightening the nut is not effective enough.

Check that the diameters of new king pins are within an acceptable tolerance, some have been known to be 2 thou under, as an estimate 0.750” + 0.0001 to – 0.0003 seems reasonable, although I would prefer the high end, especially if the eyes are worn. They should be a snug fit in the axle eye and not able to drop out under their own weight.

Beware undersized king pins as these will hasten wear of the beam axle’s eye and a sloppy fit of the king pin is a serious problem and not easy to resolve.

One method sometimes used to overcome a worn axle eye is to “heat shrink” by heating the eye to a dull red and then hammering the forging to close up the eye. However, the axle is made from a carbon alloy steel and heat treated for strength, so that reheating the eye would alter its crystal structure and reduce its strength. Mark Jablonsky has analysed the beam axle to be similar to EN17 (C=0.36%, Si=0.25%, Mang.=1.88%, Moly.= 0.64%). Hardness testing also confirmed that heat treatment had been applied. If “heat shrinking” were to be used, then the whole axle would need to be re-heat treated afterwards, a somewhat complex procedure.

Where the amount of play in the eye is marginal it might just be possible to close up the eye using “cold shrinking” with specially shaped dies and a powerful hydraulic press, not as straight forward as it may seem though.

Other alternatives exist, such as boring out the eye to about 21mm and then pressing in a toughened steel sleeve with a wall thickness of just over 1mm. and then boring out to size. This would weaken the axle eye, however some mitigation of this problem might be possible if the eye was only bored out to say 20mm and then fitted with a split sleeve of 20thou/0.5mm shim steel which could be fixed in place with Loctite 603. A king pin could then be used to hold the sleeve in shape whilst the Loctite cured.

Yet another alternative is to bore out the eye to just clear any ovality and use oversized king pins. Machining, case hardening and accurately grinding to size king pins to match is not for the faint hearted, especially as solid bronze bushes might also be needed as the bronze layer in wrapped bushes is quite thin.

Figure 2

The traditional way of reaming king pin bushes was with a stepped reamer as shown in Fig. 2. The leading reamer section acts as a pilot for the final reamer section, which should leave a smooth surface with enough clearance to allow for a film of lubricant. Tested dry, the king pin should just about slide down under its own weight through the bushes; when lubricated with light oil, it might need a gentle push. A tight king pin will tend to score surfaces as any boundary lubrication could be displaced resulting in metal to metal contact.

3⁄4” stepped reamers in good condition are rare so alternative methods are available using a 3⁄4” hand reamer with a parallel shank and square drive as opposed to a machine reamer which would have a No.2 Morse taper drive.

Hand reamers have an advantage due to their cutting flutes having a tapered lead in for about 1/3 of their length which helps alignment through the two bushes.

A special technique is recommended, remove the most worn bush and press in a new bush. Then, using the existing old bush as a guide, ream out the new bush. Replace the second old bush, and then ream it out using the first new bush as a guide. Plenty of Tallow needs to be used as a lubricant.

Figure 3

Yet another way suggested by Peter Cole uses a reamer mounted in the chuck of a lathe (Fig. 3). A mandrel needs to be made up that just slides into the newly pressed in bushes. One end of the mandrel is then mounted in the tail-stock with the stub axle in place. This is then slid along the mandrel onto the slowly rotating reamer by holding the stub axle’s spindle with gloved hands. Again, plenty of tallow is needed. A dull reamer will transmit considerable twist force to the stub axle, so extra care is needed.

Adjustable reamers with their straight flutes can produce a rippled finish and should be avoided except for the final easing of a tight king pin. Smoothness of finish is all important as any high- spots might give the initial impression of a good fit, but as they soon wear down, play will develop.

Many king pins that I’ve seen are worn prematurely due to corrosion from a lack of lubrication. I’m inclined to suggest more frequent lubrication than the recommended 500 miles and using a mix of EP140 oil followed by LM grease. This is especially true with new bushes as only a small clearance exists to act as a reservoir for any lubricant which should be pumped in quickly to ensure it spreads through out. Jacking up the front wheels should help lubrication to flow around the thrust washer.

The recommended free sliding movement of the stub axle along the king pin is about 4 thou (0.1 mm). As most thrust faces have worn, some packing out may be needed, either by using a thicker bronze thrust washer or using shim washers on the non-thrust face of the axle’s eye.

Ed’s note: I thought it would help to make Eric’s description of the eccentric grooved thrust washer and the bi-metal wrapped type king pin bush “live” a little if I were to take a few photographs of these items. How on earth did we manage before the digital age?

Thrust washer with eccentric groove – material SAE 660 leaded bronze.
Bi-metal ‘wrapped’ king pin bushes – note the oil/grease groove which has a spur take off that feeds lubrication to the thrust faces of the beam axle’s eye.
Bi-metal ‘wrapped’ king pin bush showing the spur groove that feeds the thrust washer.

Eric has added the following comment subsequent to me sending him the finished magazine article:

“Some ground finishes on new king pins can be less than smooth, so you may feel that it is worthwhile buffing the bearing surfaces of the king pin.

This may seem a bit OTT, but having bought a buffing wheel I’m on the look out for any applications and polishing the bearing surfaces does seem a really nerdy thing to do. I blame the medication!”

In the April issue we’ll look at the fifth ‘deadly sin’: TRACK ROD & DRAG LINK ENDS – worn balls and cups poorly set up.

In the June issue we’ll look at the sixth ‘deadly sin’: TRACKING – set up, tyre tread and pressure.

In the August issue we’ll look at the seventh deadly sin’: ‘THE BISHOP’ – Bishop Cam steering box.

One thought on “Keeping it on the straight and narrow – Aspects that affect TA/TB/TC steering (Part 4)

  1. Bob Schapel says:

    Great article! amazing that the deeper one gets into the theory of this stuff, the more issues emerge. Good to see it all laid out. I guess compromise between the “perfect” solution and what is able to be done within budget and time constraints, is usually the track taken.

    Yours oTCagonally, Bob (South Australia)

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