225TXRP: How does one determine gearing?

[carboncow]

I have a 1991 225HP and we know the lower unit has been changed. It was suggested it was for a V4 as the stock anode didn't fit.

I was looking at prop calculators and they always ask gearing...how is this determined for my engine?

[boscoe99]

The Yamaha parts catalog sometimes will provide the number of teeth for the pinion gear and the reverse/forward gear. Divide the bigger gear teeth number by the little gear teeth number and you will have the gear ratio.

For instance, the F225 gear box below has a 15 tooth pinion gear and a 30 tooth forward gear. Gear ratio is 2:1.

Or, check the applicable service manual for this information.

Now for some reason the older models don't show the number of teeth. What you can do is to count the number of turns on the drive shaft while rotating the propeller shaft one turn.

See illustration below. re the parts catalog way of getting the gear ratio.

[carboncow]

Great info. Thanks.

[rodbolt17]

an easier way you may ask.
I will tell you cause I am sheet lazy.

roll it up to the TDC marks on the flywheel.
take a marker and make a line from the prop hub to the gearcase,mostly a straight line if you can.

place it in FWD

now have someone rotate the flywheel ONE turn while you count how many times the prop/gearcase marks move.
1:1 would mean that for ONE revolution of the input shaft(flywheel) you would get ONE revolution of the out put shaft(prop).
1.5/1 means that that to make the shaft rotate ONE revolution you have to turn the input(flywheel) 1.5 times.
2/1 means you would have to rotate the flywheel 2 turns to get one turn of the prop.
if you can figgue out there are 360 1* marks in a circle the rest is a fairly simple calculation.

[madharchod]

for this particular engine its when you turn the flywheel two times your prop will turn one time
should make sense now

[rodbolt17]

depends.
poster stated he suspects the gearcase has been changed.
there were seveal V6 ratios and at least two V4 ratios.
the V4 gearcase will fit the 2.6L mid section.