1976 Mercruiser 120

[mirage2521]

Hey All, I have got a 1976 Wellcraft V20 with a 120 Mercruiser, the lower unit has cracked, I talked to a mechanic he says it was caused by corrosion ...any way, I have 2 other complete outdrives, waiting to be installed, what I need to know is how to determine which one I should use, they are both from the same era, one is a 120 the other is a 140, I know there may be a gear ratio difference in the upper housing, all I really need to use is the lower but I would like to check serial numbers to make sure they will work, I cannotfind a number on either of the 3, where should I be looking. Thanks in advance

[whywhyzed]

either one should have the correct ratio. The only 100% sure way to determine ratios is to turn the input yoke, and count how many turns it takes to turn the prop 1 full revolution. Any gears could be put in any case, so stamps and numbers on the case are not worth much.The other reliable way is to disassemble and count teeth on the gears.