Good question...the spring pads are idientified by small "lugs" about 3mm wide and maybe 4mm long.The number of lugs(usually 1 thru 4)indicates the thickness of the spring pad. These lugs are visible around the outside edge of the spring pad with them installed. You can have the thickness deciphered with the use of a TDM(Technical Data Manual). Or just tell us what kind of car and how many lugs are on the spring pads. Somebody will have the information you need.
Regards,
Randy D.

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W.R.Durrance
ASE Master Tech
Mercedes Tech 20 Years
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