70 Yamaha - Smaller Lower Unit

[abwehrly42]

I have a 1988 two stroke 70hp Yamaha, with the 2.33:1 lower unit. I would like to run a smaller lower unit with higher gears.

I know that the lower unit from a 55hp Yamaha will bolt up, and has a 1.86:1 ratio. I have read that the lower unit from a four stroke 50 will bolt up, and has a 1.84:1 ratio.

Can anyone confirm that the gearcase from a F50 will fit a 70hp two stroke?

[hydrospeed77]

I was looking in to this too and from studying photos of the F50 driveshaft and the spline holder tools for the F50, it looks like the top of the F50 driveshaft is the smaller 40/50/Pro50 size. That doesn't mean the lower casing won't bolt up - would like to test/confirm what happens there. If the casing did fit, one would have to mod the top of the F50 driveshaft to make it work.

[Ron Hill]

Igot this gearcase with a bunch of stuff I bought, seems like it is almost new. It is a short shaft, and the guy had it to use on a Mod 30. It is a 13 spline shaft, not what year.

Is this gearcase what you guys are talking about?

[hydrospeed77]

That looks like a 3cyl 40/50 gearcase. Hounddog says that may also fit the Yamaha 3cyl 30. Also, it looks like it is pre-1995 (like my 1993).

The one we are talking about will have a rotating shift shaft, not push/pull. Also, the F50 lower has the 2 external bolts for the prop shaft bearing carrier instead of the large internal ring nut. We are tying to figure out if the F50 one mates with the mid of a 2 stroke 70. And then.. if the driveshaft is interchangeable on the F50 with something that will fit the 2 stroke 70 crank.. (sometimes the drive shaft bearing part numbers can hint at this).

[Roflhat]

i believe the F50 gearbox will fit the 70, it fits the 70ces, have a look here - http://www.thundercatinflatable.com.au/forum/showthread.php?3493-70CES-Yamaha-Infanta-Thundercat-Build

[Ron Hill]

$400 or offer....

[hydrospeed77]

i believe the F50 gearbox will fit the 70, it fits the 70ces, have a look here - http://www.thundercatinflatable.com.au/forum/showthread.php?3493-70CES-Yamaha-Infanta-Thundercat-Build

I was hoping there would be the possibility of using a 48/55/60 2Cyl driveshaft or a 70 driveshaft in a F50 gearcase to bolt right up to a 70, but it doesn't seem to be the case. The F50 drive shaft is too small and less splines than the 2stroke 70hp so it is not a simple bolt on.

From looking at various parts catalogs, it seems like the F50 driveshaft is the shorter type like the Pro50 shaft (shorter inside the lower casing from pinion to top of casing), meaning that you can't interchange in a 55hp 2Cyl shaft to get the splines needed to fit the 70 crank (the 48/55/60 2Cyls have the same pinion to casing top as the large 70 2.33 lower). So the only route is cut/weld/custom work of the F50 driveshaft to fit the 70 crank. Cool to see in that thread the photos of the F50 lower casing fitting the 70 mid.

The other issue I haven't looked in to yet is the water pump reaching the water tube when using an F50 lower on a 2 stroke 70..

If anyone can confirm any of this with real world trials, post up the results!

[Maasters]

I have a 15" 1:71 from a early 80s 60hp mariner 2cyl

[Hounddog]

This likely is the same as the 70CES case. It has to be the turn to shift gear case. The only difference is the length of the spline. The 70 hp is longer and round at the top. The 60 has a flat top. It will bolt right on the 70 mid.
Those with a long shaft 70 Yamaha can also use this case. Bay Mfg. make 5 inch drive shaft extensions. Use one and the case bolts on.

[88workcar]

You also have the option of using any case that bolts up. Cut the drive shaft, cut a 6 side hex on the top two inches. Cut the spline (TOP) of a 70 shaft, weld a impact socket onto the 70 end and just drop it over the new lower shaft that has been cut to match. The shift of any can be lock into forward, or adapted to shift.