Yes thats fine 100 is better than 120. It means that the cooling system has to build up 100 before it opens to allow coolant to overflow.

100 caps are hard to find, most people just use the 120.

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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg