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]

Re: 1976 Mercruiser 120

either one should have the correct ratio. <br /><br />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.<br /><br />The other reliable way is to disassemble and count teeth on the gears.