In the August issue of TTT 2 we looked at modifying aftermarket front springs, including the desirability of bushing them at the front ‘eye’ and rounding off and chamfering the edges. This article looks at mounting the springs to the chassis and suggests a material specification for the shackle pins as well as giving dimensions of the pins and the bushes.
Quoting from the Instruction Manual, “The front ends of the springs are attached to the front end of the chassis by hardened steel suspension pins, which run directly in the spring eyes, being lubricated through grease gun nipples fitted in the outer ends”
At this point the reader may find it useful to look at a couple of photos to illustrate the text.
The above photo serves two purposes. If you enlarge it you will be able to see that whoever was stamping the chassis numbers at Abingdon in May, 1946 must have had a momentary lack of concentration having not realised that he had already stamped all of the 0740s. The ‘4’ is just about visible “behind” the ‘5’, if you see what I mean.
However, the main point of the photo is to show the front mounting point for the spring. The hardened steel pin goes through the hole in the chassis dumb iron (or knuckle), through the spring ‘eye’ and screws into the threaded portion of the front chassis cross tube. The following photo (photo 2) shows the pin which has been passed through the spring eye. Note the tab washer which locates between the ‘nibs’ on the chassis front knuckle (see photo 1) with the other tab being bent over the ‘nut end’ of the pin.
A spare pin and tab washer (but not the grease nipple!) can be seen in the second photo; the pins I have are made from EN351 steel, case hardened and ground, the tab washers I keep have been laser cut and cost £0.70 each, compared with £3.60 each charged by MOSS Europe.
The diameter of the pin is ½” and the overall length is 3.569 inches. The pin should not be over tightened as it acts as a pivot for the spring.
The drawing of the rear shackle assembly above has been scanned from the TC Instruction Manual.
The suspension arrangement for the rear of the front spring is a little more involved, albeit, quite straightforward. If you refer to photo 3, the top shackle pin (shown without its bushes) is inserted (with its two bushes) through the tube in the chassis shown in photo 4. The bottom shackle pin is inserted (with its two bushes) through the ‘eye’ of the rear spring. The top and bottom shackle pins are then joined by shackle plates – one on the outside (as you look at the car) and one on the inside.
With the help of the photos above we can now put the jigsaw puzzle together. The front ‘eye’ of the spring is offered up to the space between the two sides of one of the chassis dumb irons, the tab washer is held in place with one of its tabs located between the ‘nibs’ at the side of the dumb iron and the pin is pushed (normally with a twisting action) through the hole in the tab washer, through the first side of the dumb iron, through the spring ‘eye’ and through the second side of the dumb iron until it locates the threaded insert of the front chassis cross tube from when it can be screwed in and tightened.
DIMENSIONS FOR FRONT SHACKLE PINS AND SUGGESTED MATERIAL SPECIFICATION
The overall length of the front shackle pins is 3.33 inches. The critical length is the measurement between the shoulders, which should be 1.75 inches. This leaves 1.58 inches for the threads on both sides of the pin, so 0.79 inches each side. Of the 0.79 inches, 0.615 is threaded (7/16 BSF) and 0.175 is left unthreaded (these latter two measurements are the best I could get and may be slightly different). The diameter of the pin between the shoulders is 0.5 inches and 0.434 inches at the unthreaded portion.
The pins I keep are made from EN19T. Originally they were lightly plated but mine aren’t – the manufacturer recommends a liberal application of grease or copper-slip before assembly.
KEEPING THE SHACKLE PINS PARALLEL
The tube in the chassis which takes the top shackle pin measures 1.5 inches across (and is 0.875 inches in diameter), however the rear ‘eye’ of the leaf spring which carries the bottom shackle pin is only 1.25 inches across. To keep the shackle plates parallel a 0.125 inch ‘washer’ is brazed to one end of the shackle pin (see photo 6). It would have been helpful when taking photo 6 to have placed the shackle pins the other way around so that the thicker ends were at the bottom (as they are when fitted to the car). Sorry about this!
THE SHACKLE PIN BUSHES
The original bushes were made by Harris Flex and were patented. A couple of these original bushes, which were removed from TC0750 are shown in the photo below.
There are two different part numbers for these rubber bushes:
ACA 5242 (a BMC part number, I believe) is the bush (one of a pair) which goes through the tube in the chassis (see photo 4). The Harris Flex part number is CW8505. Part number 99557 is the bush (one of a pair) which goes through the rear ‘eye’ of the leaf spring and is used for the rear ‘eye’ of both front and rear leaf springs. The Harris Flex part number is CW719.
The chassis tube bushes should measure 0.75 inches in length from inside the flange – the thickness of the flange should measure 0.125 inches. The spring ‘eye’ bushes should measure 0.625 inches in length from inside the flange – the flange (thickness) should measure 0.125 inches.
Bushes are available commercially both in rubber (the original spec for these bushes) and in polyurethane. Polyurethane is the way to go, since I am told that the rubber bushes “do not last five minutes”. As long as I can get a favourable price, I am going to have moulds made for these bushes and also for the large bush on the lower shackle pin at the rear. An added bonus is that ACA 5242 fits the rear ‘eye’ on the rear TD/TF leaf spring.
© JOHN JAMES September 2010