whats the gear ratios? 18hp vs 25hp evinrude

[Dauzat]

is there a difference in the gear ratio of a 1971 evinrude 18hp fastwin and a 1970 25hp evinrude. the lowers will bolt up and match, but i was wondering if there is a difference in the gear ratios because there is a different part number on one of the gears. i have an 18hp lower i was going to plan on using for a short time while i rebuild my 25 hp lower.

 

[bspeth]

I think you can count the drive gear teeth and divide by the teeth on the pinion. Don't know about the thickness of the gears though.

 

[jbuote]

I looked it up a bit, and based on what I found, it looks like they're both 1.75:1
​Mind you, this is based on looking up the part numbers and counting teeth from an image.
​Both seemed to have 21 teeth on the FWD gear, and 12 on the pinion, so that's 21:12, reduced to 1.75:1. (If I did my math right.. haha)

​The brochures from those years I found online seem to indicate the same, but the 70 25hp brochure was blurry.. lol

​Can't say if they can interchange or not though..
​Driveshaft splines the same? Driveshaft lengths? Shifting mechanism? Water tube aligns up? etc. I don't know the answers to those....

Hope it helped with your original question though.. lol

 

[jimmbo]

The 18, 20, and 25hp of the 22 cu in block all had the 21:12 ratio

 

[Chinewalker]

They all use the same ratio. However, they do not all use the same gears. OMC changed gear teeth shapes and angles several times over the run of the 22-cubers. I'm guess it was partly to reduce gear noise, partly to make them last longer. As a unit, they'll interchange. Problems may pop up if you start mixing and matching innards.