How do I find my gear ratio??

Medic574 · 04-04-2010, 08:55 PM

I have a 1997 28' Z276 with a 7.4L motor and the Bravo III outdrive. I am going to bite the bullet and replace the lower unit due to corrosion. I am having trouble finding the gear ratio. There was a stamped serial number on the upper unit which has since fallen off. Does anyone have a similar setup to advise me the proper gear ratio?

Thanks in advance

WetWilly · 04-04-2010, 10:18 PM

Hi Medic574, found this info and since you're taking off the outdrive, "B" should work.

Hope this helps and would you mind letting me know if this info was correct?

Thanks,
WetWilly

Here is the slightly "edited" version.

Drive Unit Gear Ratio Identification
All Bravo drive unit gear ratios are identified on each drive in two places:

A) The first place to look is on the decal on the port side of the drive housing. It will have a number such as (1.50R) and then the seal number.

B) The second place to look will be on the universal joint splined yoke. It will be identified with a letter such as (F). This method is explained in the following chart and is true for new or un-tampered with drive units. A drive unit could have had the gear ratio changed for high altitude, which would void out any application of the following chart.

The gear ratio then would have to be determined by counting the teeth on the drive gear and the driven gear in the drive shaft housing and using the following chart for reference.

Bravo Three

U-JOINT IDENTIFICATION:
RATIO U-JOINT SHAFT MARKING
2.43:1 N
2.20:1 K
2.00:1 B
1.81:1 G
1.65:1 C
1.50:1 F
1.36:1 P

NUMBER OF TEETH PER GEAR - DRIVE SHAFT HOUSING:
RATIO – DRIVE / DRIVEN
2.43:1 - 23 / 30
2.20:1 - 23 / 30
2.00:1 - 27 / 32
1.81:1 – 27 / 29
1.65:1 – 27 / 32
1.50:1 – 27 / 32
1.36:1 – 27 / 29

Medic574 · 04-05-2010, 01:11 PM

Thanks for the reply....

The decals are LONG GONE! I don't see any numbers. I am only removing and replacing the lower unit so I don't think I will see the universal joint splined yoke.... or will I?

Anybody have any recommended sites to replace my Bravo III lower unit with either a new or reconditioned unit at a resonable $$$......

Medic574 · 04-15-2010, 02:44 PM

Will I be able to see the 'universal joint splined yoke' by removing just the lower half of the unit or do I have to remove the entire unit to visualize the 'universal joint splined yoke'?

WetWilly · 04-15-2010, 11:46 PM

Hi Medic574, unfortunately, the "U" joints are located inside of the upper bellow and can't be seen without removing the ourdrive from the gimbal.

I do have an idea, I just dont know if it will work since I've never tried it, but it should so here it is:

What you're basically going to do is turn the motor over by hand and count the prop revolutions for 1 crank rotation. We can't use the starter to turn the motor over because the neutral lockout switch prevents cranking while in gear.

1) So get a friend or two and start by disconnecting the battery (just for safety).
2) Now make a reference mark on the prop and the crank pulley (chalk or a crayon works well).
3) Find a socket that will fit the crank pulley bolt and a breaker bar for leverage.
4) Put the shifter in gear and using the socket, turn the motor a little to insure the gears are engaged (you might want a friend doing this, that's what they're for, right

).
5) At this point, you want to turn the motor over for one full rotation while someone counts the prop rotation. If the motor does not want to turn over, don't force it too much otherwise, you risk sheering off the crank bolt! One last thing, I can't remember which way the bolt tightens so you may end up loosening it, if this happens, just go the other way which I think is counter clockwise, but don't quote me on that...

Since it's a direct motor to outdrive coupling, it should work and you should be able to match the rotations with the chart I posted earlier.

If you try this method, let me know how it goes!

Thanks,
WetWilly

P. S. To make turning the motor over much easier, remove the spark plugs first (no compression = less resistance).

Medic574 · 04-16-2010, 02:23 AM

SWEEEEEEEEEEEEET!

I will give it a try in the near future and will let you know. Thanks again.

WetWilly · 04-16-2010, 04:45 AM

Hi Medic574,

I think I made an error when I said the following. "What you're basically going to do is turn the motor over by hand and count the prop revolutions for 1 crank rotation." and for item #5. I believe the motor turns faster than the props. I my case, our B3 has a 2.00 to 1 which means 2 crank rotations to 1 prop rotation.

So this is how #5 should read -" At this point, you want to turn the motor over until the prop makes one full revolution, while counting the crank revolutions as they will be greater than the props (this will be your ratio)." So if the crank turns around twice to the props one turn, you too would have a 2.00(crank) to 1(prop) ratio.

Sorry for the mix-up,
WetWilly

Medic574 · 05-09-2010, 06:08 PM

I cheated; I spoke with someone at a boat yard who ran my Motor serial number and that provided several statistics including the gear ratio. Still looking for a good deal on a lower unit. Is it worth replacing the entire unit since the newer Bravo's supposively have less of a corrosion problem???