Buy a used spring off a 6 cyl car.

The spring is still available from Mercedes as far as I know.

Mercedes W108 Steering and Suspension Parts they show the "kit" for $209, but I've called them before and they were out of stuff still listed on their site.

Also, if you go the rebuild route, Mercedes will only sell you one if you have a core, so you'll need to pull yours first.

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68 280SL - 70 280SL - 70 300SEL 3.5 - 72 350SL - 72 280SEL 4.5 - 72 220 - 72 220D - 73 450SL - 84 230GE - 87 200TD - 90 190E 2.0 - 03 G500

Nissan GTR - Nissan Skyline GTS25T - Toyota GTFour - Rover Mini - Toyota Land Cruiser HJ60 - Cadillac Eldorado - BMW E30 - BMW 135i