Surface rust is not usually a problem. The only way to be sure whether or not they need to be replaced is to remove and disassemble them, examining each leaf for deep rust or wear. This probably needs to be done periodically anyway as part of normal maintenance - disassemble, clean, paint, lubricate and reassemble, replacing bushes as necessary.
However, it is possible to get a good idea as to whether they need replacing without this (although it needs to be emphasised that this is not definitive). Have a good look at the spring, looking to see if the leaves are all parallel. If they spread apart (pushed apart by rust) as you move away from the middle where they are clamped by the U-bolts, then they are probably in need of replacement. If OK in this respect, go to the front of the spring and, using a thin wedge such as a screwdriver, spread the front of the third leaf away from the second leaf, and observer how deep the wear is as shown by the step in the second leaf where the end of the third leaf rubs. If it is more than about 0.5mm, then the spring probably needs replacing. Also check for any broken leaves.
Hope this helps.
John
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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