Luke:
I have 16" ET51 wheels that I want to put on my 25mm W126. I need spacers, but I worry about possible damage to the suspension or tire rub against the inner fenders. If Stuttgart thought the W126 needed ET51 offset they would have built it that way.

Are spacers a safe way to go over the long term? What spacers do I need for my application? Do you sell them?

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1991 560SEL (254K miles)
1980 Lincoln Mark VI - Givenchy
1979 Lincoln Mark V Collector's Series
1979 Lincoln Town Car Collector's Series
1979 Ford "Effie" Pickup (in gradual transformation to a show/street truck)
1969 Lincoln Mark III (waiting for "Overhaulin" to call)
1989 Lincoln Towncar
2001 Lexus GS300 (spousal unit's daily driver)

For spacers go to The Tire Rack.
51mm seems like a lot of offset.
Sounds like you may want to go with 15 mm spacers, that would make your offset 36 mm ( about right for a lot of M-B's.
But, as I said, the Rack will know more.

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2007 C 230 Sport.

You need H&R Hubcentric spacers. They work perfectly, are TUV-approved, and all MB and AMG wheels have 3 slots MADE for wheel spacers. The slots are recessed so one can remove the spacers using a small, stubby, flathead screwdriver.

A net ET of 21-31mm offset will suit you just fine.