I am in the early stages of rebuilding a rolling chassis for my 57 XK140 OTS. I am replacing the original torsion bars with upgraded ones from Classic Jaguar in Texas. I installed one of the bars but am not sure how to set it up.
The factory manual says to use a tool to ensure that the distance from the top of the shock tower on an XK120 to the bottom bolt is 15 3/8 inches. My understanding is that the adjusted number for a 140 with its differing shock mounts on top is 14 3/4 inches. That is what I set it at. My quandary comes in setting the rear adjusting lever. According to the manual:
“If a new torsion bar is to be fitted it will be necessary to engage the muff on the correct torsion bar spline to enable final adjustments to be correctly made at the barrel nut. To do this ensure that the adjustment barrel nut is just entered on the threads of the bolt . . .”
When I did this, the reaction lever locking bolt was at the bottom of the arched slot through the frame, and the barrel nut was under no tension at all. This seemed to leave me with two choices: (1) tighten the barrel nut only to the point where it took up the slack with the locking bolt still at the bottom of the slot, or (2) tighten the barrel nut to a point where there was room for the locking bolt to move up or down in its slot during final adjustment. I chose the later as it seemed to leave me with the ability to adjust the ride height it two directions at a later date. In doing this, however, the barrel nut is about 1 1/2 inches from the bottom of the bolt which is quite different from what is specified in the manual.
So here are my questions:
Should the torsion bars initially be set upon replacement so that the locking bolt is at the bottom of the slot in the chassis or higher. If higher, how much higher?
Does the fact that these are upgraded (stiffer) torsion bars alter the 14 3/4 measurement between the shock bolts? In other words, is the car more likely to sit higher once the torsion bars are stressed, than it would with regular torsion bars and does this alter the set up procedure in any way?
Since nobody else is weighing in, I will share what I know - not sure it will be helpful.
I recently set the bars on my 120, and what I recall is that I really did need the suspension to be fully released when setting the muff, in order for there to be adjustment left when there was weight on the car. Even so, my adjusting nut was pretty far up the threads. What I would expect from a stiffer bar, is that you would have less turns on the nut than on a standard torsion bar. I don’t think it matters where the nut winds up, as long as your ride height is correct, and you have the ability to adjust up and down (weight off, of course). Seems like you got it right to me.
It also follows that the torsion bar stiffness is not what you are adjusting - that is fixed, based on the bar and the weight of the car. The only thing you are setting is the ride height. Btw, there is a measurement to the crossmember to the floor for ride height as I recall.
After writing this note, but before I could hit the send button, my friend Alan Gerard called - (you may know him from his Seattle/Bellevue days - he now lives in Shelton), and confirmed my impressions. He knows more about old Jags than anyone else I’ve met.
There is a lot in the archives about torsion bars.
Here is one example.
You can’t really set them until the car is fully assembled. In your case with just a rolling chassis, you want to hold the 14 3/4 as you assemble the suspension parts, so your rubber bushes are not stressed when at the fully assembled correct height.
So your locking bolt is initially at or near the bottom of the slot, but it moves up as you add the engine and body weight and crank up the adjustment nut.
Hold to the 14 3/4 when you are fully assembled.
Be sure your torsion bars are stamped Near Side (or Left) and Off Side (or Right) and never put them on the wrong sides.