XK120 Leaf Spring Questions

Can old spring leaves be interchanged with new ones? I’m asking because I have a set of old springs that are still fine with the exception of one middle leaf from the driver’s side that has about 3" broken off an end. I’d rather swap leaves in favor of still using the old set as the new springs – sold specific for the '120 and sourced from “the usuals” – appear to be of careless manufacturer. Okay, I’ll be honest here: they look like third world crap. I will only use the new springs if I have to after doing some remedial work such as taking an angle grinder to clean up the leaf ends, removing paint and greasing between leaves, using bushings at each end that actually fit the original bolts and replacing the center pins to more match the dimensions of the originals. And I don’t want to do all of that knowing that I may still be fitting substandard springs.

Attached are pictures of old and new springs. The set on the left are the questionable new ones. The set in the center are early-day 8-leaf replacements for the 7-leaf originals on the right. Too bad the originals were prone to breakage early on as the manufacturer obviously took some pride in their product. Rounded edges and they taper in thickness on all ends. Nice.

Finally, I’ll add that three springs are pictured but I actually have a total of six – a match for each example pictured.So I wouldn’t be doing this to just one side of the car.

Any thoughts here from those who have done this before?


You can interchange leaves, but you have to determine the final spring rate, between the two sides, is reasonably equal.

Spring rate. I’m not sure how I’d accomplish that, Wigs. I’m still factoring in variables. The new springs with 7 leaves are 1.500" at their thickest point and meet the specs then of C3661. Correct for my March, 1950 car but as they were prone to breakage, the factory then fitted subsequent cars with C5721 with 8 leaves measuring about 1.687" in thickness. Which is what I pulled out of my car. Unless a good case can be made otherwise for this driver, I think I’m just going to wing it by taking apart the C5721’s, pulling the bad leaf off the one, and it’s counterpart on the other side, then replace them with equal thickness/ close-enough-arc matches from the new springs and trim any excess to length with a cutoff wheel. I’ll then slather plenty of anti-seize between leaves before bolting everything back up.

Does this sound do-able or, out of ignorance, am I disregarding too many other factors I’m unaware of?

You could use this…

The equation below can be used to estimate the spring rate of a given leaf spring pack:

Spring Rate = (E x (2+nfull/ntot) x ntot x w x t3) / (0.75 x l3)

ntot = total number of leaves
nfull = number of full length leaves
E = Young’s modulus (29,900,000)
w = leaf spring width (inches)
t = leaf spring thickness per leaf (inches)
l = spring pack length (inches)

…or wing it, and ride height will tell ya most of whatcha need to know.

I had one leaf replaced here…

Thanks for your additional input Paul and for sharing the video Jim. Spring rebuilders hereabouts are something of a long drive and I don’t know of anyone who does this to the level of Jones Spring like in Jim’s video.

Fascinating video, Jim - thanks for posting! Very timely for me, as I’ve just managed to source an original pair of XK120 springs which look to be in good order, and am trying to decide what to do with them next. I’ve always been somewhat offended by the square-cut ends of the aftermarket springs that my car currently has, and am pleased to see these ones have the proper rounded and thinned ends to the leaves. I’m considering just taking them apart, cleaning them up and re-greasing and trying them out, as a first (and hopefully, last) step. Does anyone have any recommendations about what paint to treat them with and a suitable lubricant? I won’t be going the whole hog with leather gaiters.

You certainly can replace individual leaves. I did it on mine about 20 years ago, and I consulted this forum about the correct lengths on each leaf, so it should be in the archives. I ground round tapered ends on the replacement leaves to match the ends of the originals.
I re-arched each leaf, starting with the top leaf according to this spec.
Set it across two wood blocks like a bridge, then jump on the middle til you get the right camber measurement. Then do each succeeding leaf until it matches the curvature of the preceding leaf.
The result was the car sits at the right chassis height, 7-1/8" to the ground measured at the front end of the rear springs.

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A man after my heart…:smirk:

Well, it worked.
One could also set the leaf up in a hydraulic press frame with a pair of jack stands.

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I like your method better…:smirk:

Thank you for the reply, Rob. Since my last post and after a long mental back-and-forth, I’d gone ahead with using the new springs. But due to your post and others, I’ve since saved my old springs from the scrap metal bin – so I may revisit this in the future.

Great video, Jim.

Not done typing apparently, I need to hit 20 characters. 20. And done.

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Nice stamping on XK120 spring.
A car buddy of mine told me what they used to do in the old days with leaf springs, as regards protection/lubrication. There is a product called Denso tape which is used for binding up underground cables and pipes to keep the water out. Basically, it is a woven fabric tape, available in various widths, which is impregnated with a very sticky grease, which works its way in between the leaves. Apparently you don’t even need to bind the end, it just kind of melds to itself. Not pretty, and very messy to handle (rubber gloves are a must) but does a great job, according to him.

And this is what was (is) an a August 1955 XK 140. Looks identical to me apart from the fact that the manufacturers logo is up side down on one of these…Should we start a research on the production date numbers on these springs???

Bob K.

My December '53 120 OTS

I’ll have to take a scraper to my springs and see what I can find. I believe mine are the earlier C3661, but haven’t measured the leaves yet.

Chris (and others),

Various sources indicate that the heavier C.5721 version was introduced in December 1952. So 1953 and 1954 manufactured XK 120s will have them (but continued on the XK 140 models).
Did a (very quick) scan on basis of the numbers seen on these springs so far. No logic found yet. There are 2 different number types: the first is a 4 letter/number code typically 1-C-23, whereas the second code is typically a 2 digit number.

Figures/letters that are not 100% clear are indicated in red.
Some speculation:
The C is the only common indicator in the first code. The last 2 digits of the first code may indicate the year of manufacturing. The first digit might be a month indication. In that case the Spring brought forward by Chis is a later replacement (1962) as is one of the Springs of my former XK 140. As both are indicated 1962, that may be the year an additional batch C.5721 has been made by Jaguar.
No clue what the second code stands for.

Bob K.

Springs and pictures


![12 197|666x500]


Thanks for these pics. Not all readable but plenty of new info. New table now comprises all XK’s. The coding system is consistent, meaning the 2 last digits give the year code. Unclear what the first digit stands for (month??) and also what the second code means (2 digits). Could be (the man at) the production line??

Bob K.