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Guys, if your not going to go crazy with engine mods. I would look into using the lower section of a yamaha 90 hp mid with the top portion of the V4 mid. Then you could use a yamaha 1.71 small gear case.

I will have to find a long 90 Yamaha mid to do some measuring. I'm not to familiar with Yamaha specs, so what diameter would the 1.71 Yamaha gear case be? What motor does this gear case come from a stock 90, 70 or something smaller? I have seen a few 80's/90's mid sections with corrosion even some with a hole in them. Will this be strong enough?
Has this mod been done before?
Thanks

Tim, I don't think it has been tried before. You would need to check the measurements. You could use a 40 hp yamaha 4 stroke mid , 55 hp mariner/yamaha or 70 hp or 90 hp yamaha. the 55 hp and the 90 hp are similar. The 4 stroke is different as is the 70 hp. The gear case is off the 60 mariner/yamaha but you could use a 55 hp yamaha which has a little higher ratio. The lower unit is the size of a OMC nitro case. Years ago I was building a mod 90 hp yamaha. Steve Cooler was a FV racer in the states and he raced a 130 to 140 hp mod 90 yamaha with this same gear case.
His boat ran mid 80's on the oval. He had continual gasket problems with the power heads and no issues with the gear case. So it may? work.
The last 60 mariner complete motor I found cost less than $400.00.

Is it safe to say that a 55hp Yamaha gear case would bolt onto as large as a 90Hp Yamaha mid?

YES, bolts right on. I think the water pump would even have the capacity to cool the V4.

This is Steve Cooler's mod 130 to 140 hp mod yamaha.
The gear case took this punishment!

https://www.youtube.com/watch?v=SfsK2tPzVKc

Here is a 40 four stroke...the gear case on it could work.
The 55 case bolts on.
The mid is bigger and may be a better fit for the V4
https://www.kijiji.ca/v-boat-parts-trai ... nFlag=true

Top Gun has used the 40 hp four stroke long shaft mid to make 15 inch 70CES yamahas with power trim.

I was talking to Paul the other day about some other things, I know he has a lot of Yamaha
stuff and he would be a good one to through the idea around with. The only immediate stumble I just found about is the gear selector is a rotation style on Yamaha and push pull on omc?? may have to incorporate some Merc linkage parts into this...Lol

Keep in mind the 90 degree looper V4 OMC makes considerably more torque than these 3 cyl Yams; even more in modified form (a relatively mild pump-gas lake motor can make 200 hp).
Will these cases designed for much smaller motors hold up to lake use ?

The V4 lower unit is durable, smaller diameter than a V6, and multiple gear ratios are achievable with some work. Before you make things super complicated think about cost/effort/benefit......
Oh yeah, and potentially a wider selection of props since you can swing a 14” prop on a V4 case, what will these Yam cases spin ?

Matt

Have there been any informative/detailed posts on mods to this motor?

Thanks

There’s a post or two on S&F on the V4s. All the same principles that apply to mods to the V6 and V8 loopers apply to the smaller sibling also.

Matt

Over the years I have spend 100's of hours working on cars and outboard motors. For the person that doesn't have the patience to complete a complex project which always takes longer and ends up costing more money than you thought. This is my advice. Rare doesn't always mean costly. It is all about supply and demand. Right now there is little demand for a 15 inch V4 OMC. I bet there a few fellows out there that have one of these motors. I would post a WANTED thread on S & F for a 15 inch V4 OMC and see if you get any hits.