It's a rather involved procedure. You would need to remove the right cam cover, turn the engine to a specified position, and use a dial gauge to measure the lift on (I think) the #1 exhaust valve. It is probably more work to check it than to simply replace the chain. I don't think that there is an offset key to correct valve timing if the chain has stretched.

The bottom line is that the chains do stretch and the guides wear, get brittle, and sometimes break. The tensioners lose their seal and start to leak down. This is a fact of life with the M116 and 117 V8's, and there's nothing hugely different about the design of your engine.

Obviously, the advice you got from the dealer was motivated by the desire to make some money. But that doesn't necessarily mean it was bad advice. If you are going to keep the car for a while and want it to be in primo condition, it's something to consider.

Personally, if I was looking at this car, I would be inclined to pay a premium if the chain was replaced. But your average used 129 buyer could probably care less.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe