Normally, the rails are chainged with the chain. Ask your mechanic if he changed them. You shouldn't have to check very often. It is an expensive proposition to check the chain stretch. Ask your mechanic for a price. It requires pulling the valve covers and 2 hyd lifters. then a solid "lifter" goes in and the #1 intake valve is taken to 2 degrees of lift and then the position of the crankshaft is noted. This is done on the #6 cyl as well. There is more...

I wouldn't check for 20,000 miles then 30,000 miles thereafter. and monitor the strech. It really lasts. Try not to worry about it.

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Steve
1985 Mercedes 500SL Euro (Gray market)
1995 BMW 520i Euro (Gray market)
1992 BMW 525it Wagon
1994 Honda Del Sol Si