If it wasn’t for the fact that Japanese Sega Mega Drive games are slightly larger than Sega Genesis cartridges most of the  Japanese Mega Drive library would be playable on the Sega Genesis. The larger Mega Drive carts just don’t fit into the slot opening of a Genesis.
A lot of games didn’t have any form of region encoding. If a game wasn’t too language dependent (as many are) non-Japanese speaking gamer would have access to games that didn’t get translated. It would also increase your chances of finding a particular title if English (or another non-Japanese language) was not a requirement for you.

I still have my same Genesis that my parents bought me many years ago when it was new. Back then I had the store I bought it from modify it for me. Now as I’m getting into my Sega games again I’m enjoying learning about some of the mods you can perform on the Sega Genesis.
I would say that widening the slot opening to accommodate a larger cart is the simplest mod and well worth doing. The concept is simple. File or shave away plastic from the edge of the cartridge slot opening. My choice of tool for this is the Dremel.

Test Subject

Not Bad. Pretty close to the mark.

Since I have never used a Dremel before I decided to test it out first. I wanted to get a feel for how it works. I used the plastic cover from a spool of DVD’s as my test subject. I marked off a section that I wanted to cut out… then I cut it out. The Dremel is basically a file that spins. As it spins you press the head against the surface you want to file down and it does just that. It was very easy and quick. I noticed that it was important not to spend too much time on one spot. I was constantly moving the Dremel head along the surface I wanted to remove trying to keep each edge uniform as I worked my way to my black line. After that simple test I was already confident that I could do this.

Nice snug fit.

“Sunsoft!!” Why?!

As I said my Genesis had already been modified and it could accommodate most Japanese Mega Drive games already. But there was one company called Sunsoft that used a cartridge that was slightly larger and these carts would not fit into my Genesis.
You can see above how a regular Mega Drive carts fits well and how the Sunsoft cart doesn’t fit at all. So there wasn’t much work to do which helps because this is my first attempt.

Dremeled

Dust Cover re-Installed

Just like my test on the DVD’s spool cover the Genesis was also easy to Dremel. The plastic was much harder and it took longer but it still was not hard to do. The Sunsoft carts are not much wider but just more square around the corners. So most of my time was spent expanding the four corners of the slot opening.
With the dust cover installed again you can easily see the size difference of the Sunsoft games compared to the Genesis carts.

Sunsoft cartridge now fit!

Playing Sunsoft

And here is it. My North American Genesis which can now fit a larger Sunsoft cartridge and any other Japanese Mega Drive carts as well. The second image shows my Genesis playing the Sunsoft game Panorama Cotton. The only mod required was to widen the opening.
I know this will not work for all Japanese Mega Drives games. For Example Golden Axe 3 and Sonic 3 are region locked and will display a message on screen and not load. Click Japanese region locked games for a more complete list.