How to determine the upper unit gear ratio for mercruiser 4.3L v6

[mckavity]

I'm looking at replacing my upper unit (R rotation) on a 1990 mercruiser setup (Doral Monticello), how do I determine the gear ratio that I need? I see units available with 1.62 or 1.81 etc The boat has twin mercruiser v6's. Engine (starboard) serial number is 0C767853

Cheers and thanks

 

[achris]

If you post the drive serial number, we can determine what the ratio was when it left the factory. The alternative is to remove the top cover (off the drive) and count the teeth. 24:24 will be the 1.62:1 ratio and 17:19 will be the 1.81 ratio.

But being 1990, the ratios are more likely to be 1.65:1 or 1.84:1. The difference if because of the tooth count change in the lower housing. Earlier lower housings had a 17:28 count, later had 13:21... A difference of 1.6%. Not enough to make a difference to a single engine set up, but it could be for a twin rig. If you're only replacing the top housing, no bother...

Chris..........

 

[mckavity]

Ended up getting a 1.84:1 full unit (top and bottom). This problem solved, now on to the next