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It’s the idea to create something different, better, unique, less expensive and just because you can’t purchase the original parts anymore.
I recently saw on another site where people were searching parts suppliers parts books like megazip to find parts from other models that could fit their model.
I have done that for several years and YAMAHA has so many different models and variations of models depending on where the model is manufactured and/sold that you can find parts that interchange.
I have mostly focused on the 3 cylinder models., but I have noticed a few of the 2 cylinder 2 stroke and 30 to 50 hp 4 stroke models have similar gearcase models.
Now the challenge is finding these parts. New the parts especially new gearcases are very expensive and more than double what they use to cost. Plus the drive shaft usually is different and you likely will need to use a two piece drive shaft with a coupler or cut and weld provided the driveshaft is the correct diameter.
The good news is internal gearcase parts can be purchased aftermarket and those parts are not that costly. You need to shop because there is a wide difference in price among suppliers for exactly the same parts.

Now with gearcases you can cut and weld two different model midsections to get what you want. But that’s a lot of fabrication work and if your not a skilled DIY type. It is too costly when you depend on others.

So here are some direct bolt on switches you can do with a Yamaha triple.
- the 30 6J8 and the 40-50 6H4-5 gearcases will bolt on either model
- the power trim assembly are switchable, so you can put power trim on a early 30
- the 55 twin and the 60-70 6H2-3 gearcases are switchable..so you can put a nitro type gearcase on your 60-70..
- the 6H1 powertrim assembly fits other long shaft models.

I am currently looking into 4 stroke gearcases that will bolt on to the 55 twin and 70-75-80-85-90 hp midsection. It looks like the 30-40-50 hp 4 strokes could fit. And there is the possibility for the 63D gearcases to fit. These gearcases offer different gear ratios and some are smaller in size. I believe an adapter plate and maybe a 2 piece driveshaft will be required.

Lastly you need to consider the availability of prop selection when you are considering a gearcase switch. I like the 40 6H4 gearcase on a 30 6j8 because there are many props that fit that gearcase.
In Ontario we have a lot of OMC race props that fit the nitro case and we now have 3 different gear sets available for those cases. The triple midsection is costly but the twin midsection has the same lower portion measurement and those midsections are not expensive.

Quote: I am currently looking into 4 stroke gearcases that will bolt on to the 55 twin and 70-75-80-85-90 hp midsection. It looks like the 30-40-50 hp 4 strokes could fit. And there is the possibility for the 63D gearcases to fit. These gearcases offer different gear ratios and some are smaller in size. I believe an adapter plate and maybe a 2 piece driveshaft will be required.

With Tim Topping help and because Tim actually had these gearcases in stock he was able to test fit a 63D gearcase on to a 4 stroke Yamaha midsection and on a 90 Yamaha 2 stroke midsection.
The 63D gearcase is for the later model 40 & 50 hp 2 stroke Yamaha. I had tested fitted the early 30/40/50 versions on my 55 midsection and it was close to fitting, but would need a spacer. I thought the 63D gearcase looked larger and it looked similar to the 4 stroke models.
The surprise was it bolts right on the 55/90 midsection. It even has a 6H3 (70 hp) water pump impeller and housing. It also is a 1:84 gear ratio. The drive shaft spline as expected is not correct for my twin 55 hp, but, It is an easy cut and weld fix. The advantage is this gearcase.....it is much newer than 1980 to mid 1990 original gearcase and there is more of them on the market. Used they are not that costly and they are normally in better condition. For my application it’s a good fit. I don’t think the gearcase is strong enough for the 90 hp.

Excellent picture Matt. It’s a good example of being able to update your older Yamaha with the newer parts.
I believe this gearcase was the last model for the 2 stroke 70 hp. Last year 2017? But it fits the early models.
This is a long shaft version. There is a short shaft drive shaft still available that fits this gearcase and the older gearcase. That number is 6H3-45501-00-00

It is a small case from an F60..

Now that is a great option for the 70 hp Yamaha. Especially with the small gearcase the 1:84 gear set .The position of the water pickups on the gearcase should allow you to run the motor higher. Plus there is the possibility that you could install a small nose cone with water pickups on it leading to the factory location.

A couple of OMC guys are seeing new possibilities with these Yamaha parts. This small 1:84 gearcase could perform better than an OMC nitro case and it wouldn’t cost $1500. To $2000..
Interesting enough Matt was successful in 2017 to purchase new parts for his 70 Yamaha short shaft.
http://www.hpbc.ca/viewtopic.php?f=19&t=3672

I for one think this later model gearcase is better than the original 70CES gearcase because of the water pickup position.

Now can you adapt a OMC base plate onto a Yamaha 70 hp short shaft midsection? Would it be possible to have a Yamaha cone with an OMC powerhead? Maybe? And there is also the 55 hp Yamaha short shaft midsection you can still buy new and it has a much larger inside in size. A 90 Yamaha can mount on it.

Now this suggestion is interesting:
Why not cut and weld the top part of an OMC 56 long shaft to the lower part of a 70 Yamaha midsection to make a 15 inch . Then you have exactly what you need. Someone needs to see if it’s possible. Seems like an inexpensive idea to try.

Another good suggestion was to make an adapter plate that bolts to the bottom of your 15 inch OMC midsection that will allow you to bolt on the Yamaha gearcase.

In your search for Yamaha gearcases you should also check mercury 4 strokes. Yamaha does make parts for mercury.

The FWD bearing and gears in the newer style small Yamaha case (F50/F60) are larger and much nicer than what is in the OMC nitro.
I don't know about the prop shaft though.. Yamaha 19mm, OMC 20mm at the smallest diam. The OMC prop shaft looks noticeably beefier.

Quote: from boatracingfacts.com 2 days ago.
“anyone modding the 2 cylinder 40x or 40j?
Found out that Parsun still make a model that use the G series 1:85 small box from the old 70 CES”

I did look at some pictures today of the 40X twin and I looked at the parts book. Hard to tell by the pictures the size of the gearcase. I have a 70CES and it has a 1:71 ratio gearcase. Now I didn’t check all the models in the 40X & 40J series . Just like we have mentioned in this thread ...there could be a model that matches the quote. The 40X & 40J Models are not sold here so you would need to shop overseas and the low Canadian dollar will make the purchase expensive.

Update...more searching!
This model does have the 63D gearcase so it will fit and the 1:85 ratio.....the model is 40V.
https://www.megazip.net/zapchasti-dlya- ... 1-15834196

Gordon Bay Marine has a new one in stock. Just proves you need to keep searching!
https://www.ebay.ca/itm/183834826755

The 40X series has parts starting with the prefix 66T.....so far I have not been able to confirm those gearcases fit, but, they could fit.
https://www.megazip.net/zapchasti-dlya- ... 31-5935831

The 63D 2 strokes and the 4 strokes do fit.