Determining your Gearcase "Gear Ratio"- Without removing Gearcase

[EuropeanAM]

Several have asked "how do I determine which gear ratio I have"... and don't care to pull their gearcase to find out. Here's a simple method to determine the ratio "roughly":

Ok... I know this is gonna sound too simple to be true... but it is

- Make a mark on the flywheel, and the prop hub, with masking tape. You'll need a buddy to watch the mark on the prop for you.

- Remove your spark plugs to make turning the engine easier.

- Place the engine in Forward Gear.

- Connect a flush attachment and provide "a little" water to lubricate the water pump impeller (just so you're not turning it totally dry).

- Rotate flywheel "clockwise", and count the turns. While you're counting the turns of the flywheel, have your buddy do the same at the prop.

- Your buddy needs to tell you to "stop" at precisely "10 revolutions of the propeller".

Now compare your flywheel revoltions. Examples you may find:

16 1/4 flywheel vs. 10 propeller= 1.62

17 1/2 flywheel vs. 10 propeller= 1.75

(essentially, you're multiplying the ratio by 10 to make the difference broad enough to recognize it without special measuring tools... for instance, 1.62:1 becomes 16.20:10)

Hope this helps some of you out...

[1outlaw]

JMO but i can pull my lower and install in 15 minutes...Seems easiest way to me

[EuropeanAM]

I'd prefer to pull the gearcase too... but some folks are a bit "anxious" about pulling their own...

[Wildcat Dude]

Good info thanks - glad mine is marked though but people ask all the time.

[prbullet]

DON would this work for a mercruiser eng. Also?
But I can get to the flywheel, would have to be on crank pully?

[EuropeanAM]

Yes, it would also work for a Mercruiser Drive as well.

Gets a little trickier.... as they've VERY SLIGHTLY altered the final-ration numbers over the years (example: 1.81, 1.82, 1.84).

[hooken203]

Just curious if I have a ratio of say 1.50 or 1.75 or 2.0 can't I just mark the propshaft at 12 o clock do the same on the flywheel and see how far I turn the crank for one turn of the prop shaft. I understand that Dons way is more accurate so you can sort out the ones that are odd numbers but it seems to me that if I have 1.75 to 1 then the crank turns 1 and 3/4 times for one turn of the prop.

[EuropeanAM]

That would be correct... although it's a little trickier, when you consider there may be 1.5, 1.62, 1.8X, 1.98, 2.XX, etc ratios at play.

In the 3L gearcases.... 1.62 and 1.75 are close enough that "multiplying by 10" helps to easily clarify between them.