TN 27 Gear Ratio

[51seahorsetn27]

Does any one know what the gear ratio on the lower unit of a Johnson TN 27 is, or a way to calculate the ratio?

Thanks in advance,
Malcolm
51SeaHorseTN27

[frankr]

Don’t know the answer, but you can determine it by counting the teeth on the pinion gear and prop shaft gear. Or you can get a rough estimate by comparing the number of turns of the flywheel it takes to turn the prop shaft one revolution. That’s about as accurate as you are–or not.

[51seahorsetn27]

Cool I got the engine apart still, so ill tear in the lower unit this weekened and count the teeth on the gears any idea how its set up big gear 50 teeth and say small gear 25 teeth would be 2:1 right i know there is a formula somewhere just dont remember it

[frankr]

In your example, the ratio would be 25:50, which could be stated as 2:1 (two turns of the pinion to one turn of the prop gear). However, that is seldom the case in the real world. A typical 6hp motor has 15 teeth on the pinion and 26 on the prop shaft gear. So that is stated as 15:26 because it cannot be divided and come up with whole numbers. On your example, a tooth on the big gear would meet with the same tooth on the pinion on every second turn of the pinion. Over and over again. Making the ratio like the 6hp example means the same teeth do not match up for many turns. In other words it spreads the wear.

[51seahorsetn27]

Thats pretty much what I gather from all of it, its been a long time since Ive dealt with gear ratios. I need to figure this out for a custom prop I wanna build for my motor and hull combo.

[phil-b]

Looking at the 1952 Johnson brochure at the old-omc.de site thru the magic of the "wayback machine":

https://web.archive.org/web/20120524010 … start.html

It’s a little blurry, but I get "14 to 25"

The 1949 brochure (TD) says 14 to 25 as well.

[51seahorsetn27]

So that simplified that now i just gotta count mine to be certain thats what i got, i know there werent too many lower units that interchanged those parts, at least one would think anyways

[51seahorsetn27]

And looking at the posted pages you put up it definitely says 14:25 thats a big help

[frankr]

quote 51SeaHorseTN27:

So that simplified that now i just gotta count mine to be certain thats what i got, i know there werent too many lower units that interchanged those parts, at least one would think anyways

The 1951-54 Gale 5D10 motors shared the same gearcase.

[51seahorsetn27]

Thanks to phil b and frankr i’ll be sure to add that info in my compiled chart for lower units as Im compiling my sources on my engine build, you guys are awesome for all the help, to the guys who contacted me on parts: I havent forgotten any of you either, moneys tight right now I wil be in buy mode again in a few weeks. Court payments, before life enjoyment. Its slavery I tell ya, slavery.

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