Thread: need timing set info

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replacing the timing cover due to it corodid,gone. im replacing all i can while im in there timing set, water pump, fuel, etc etc. my ? is the timing gears i took off are s-420 for the cam and the crank is s-493. all the sets i find around here at the local parts store are the same for the crank but the crank is a s-421. they both look the same other than the new gear the s-421 is off about 1/2 a toth if i was to put it on. my guess is that the old gear the s-493 is retarted abit. is it worth my time to seekout the 493 or is there much of a differance. gas milage, low end tork, etc etc. this motor is in a large box truck and heavy. kinda shooting for the gas milage. thinking of taking parts back and getting a 3 key set to advance or retart the timing. this new gear i gotten looks as though it will turn the crank clockwise a 1/2 tooth.is this advancing the timing. and what are the efects on power and gas with advancing or retarding.