If your crankset is less than 20 years old, it will have the standard 130mm BCD, and the 39-tooth ring will mount onto the spinder. Your biggest concerns will be shifting in front with the larger jump, and the ability of your rear derailleur to take up the slack in back. If you have only 6 or 7 cogs in back, you may also pick up some redundant ratios. My suggestion: replace the 53 with a 50 when you replace the 42 with a 39.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069