How to swap transmission oil fill bung – JDM vs USDM

2 years ago I bought a s80 case (USDM) when I rebuilt my y21 JDM transmission, because of cracks in the y21 housing.
Then after 1 year of use, a metal plate on the freeway hit my transmission and caused a hole.
The aluminum was brittle and filled with stress cracks in the housing, so when I TIG welded the hole, it caused issues.

I decided to go to the junk yard and get a s80 case from an integra. I found a 1995 3 door RS 5spd and pulled the transmission. (read about it here)
I did not know it at the time, but after I got home, I found it impossible to find a large oil cap.
I tried Honda and Acura dealers, and even used JDM engine suppliers, but no one had any for sale.
The USDM 1994-1995 Honda Del Sol VTEC model is the only USDM car with that oil cap. The part is discontinued and not available.
I needed a way to cap my new rebuild, so I tried to change the breather vent tube (oil fill hole) from the USDM s80 case to the JDM s80 case.

JDM vs USDM S80 Y21 oil fill tube

This is the view of my old s80 case (USDM) -Vent cap PN:21396-P20-000.

So I can remove it, I clamped it in vice grips (gentle to not bend or deform the tube, but firm to not slip).

I made an animated GIF showing the back and forth motion I did to loosen the fitting.

After breaking it loose, I added a mini pry-bar under the vice grips and gently pushed it up as I moved the vice grips around.

Its almost out!

You can no longer see the shaft stick out on the inside.

Now that’s dirty… but its out!

This is the USDM oil fill tube.

The size of the transmission side of the tube is about 10mm.

I also measured about 10mm on the oil fill side.


I just put this transmission in last week, before I tried looking for the oil fill vent cap!
This is a JDM s80 case from the junk yard. The larger oil fill cap PN:21396-P80-000 – not available at Honda – or online!

You can see the size difference. The USDM fits inside the JDM one!

Another comparison.

An animated GIF of the back and forth movement with the locking pliers to loosen it up.

I placed the mini pry-bar between my mount bolts to keep it in place while I pulled the part up.

Its starting to come up!

Finally out.

That’s really dirty… so I cleaned all the dirt by the hole.

The size of the transmission side of the tube is about 12mm! Its larger then the USDM cases!

I take another measurement of the USDM one, and the ring around the tube is about 13mm.

Here is a side by side of the JDM and USDM oil fill tubes.

The fill side.

The transmission side.

I spent a while thinking, and going through my parts and getting ideas how I can make it fit and hold.
I decided that maybe a layer or 2 of heat shrink may work…

I placed the heat shrink over the tube, and picked where I wanted to make my cut.

That’s looking good.

You can use a lighter, or a heat gun to shrink the heat shrink. Heat shrink can shrink up to 50%.

Nice and tight wrap. But its just a bit too small still.

This is the measurements of the black heat shrink I used. About 18mm.

The measurements of the white heat shrink: about 30mm (but a bit smaller would have been better).

I added 1 layer of white heat shrink over the 1 layer of black heat shrink.

Then heated it and cleaned off the black residue from the lighter.

Looks really close… almost too big.

I added a bit of motor oil to the heat shrink tip that will be inside the transmission.

Then I gently placed it in correct position, and gently pushed it down.

I then tapped it a few times with my wrench (using a 1/2″ wrench extension to cap the fill hole)

It was not going in any further, so I pulled it out, and to my surprise the heat shrink stayed in the hole, and broke off in the perfect spot.
I now have a rubber ring mounted inside my housing fill hole.

This is the rest of the heat shrink still on the fill tube.

I cut and pulled off the heat shrink that is not needed.

Back to the bare tube.

I dip the end of the fill tube into motor oil and spread the oil around the tube.

I now gently push the fill tube back into the new rubber fill hole. I do not want to push the heat shrink into my transmission case!

It was not that hard to push it in. The oil really helped a lot.
The oil should go away with time and make this sturdy, and difficult to pull out (or hard for it to fall out).
I tried to push it side to side, and it was in there really snug.