Subaru 5MT drain plug thread?

[Neeraj Ahluwalia]

Hi all,

Making a transmission cooler for my 5 speed in my 818. Looking to see if anyone knows the thread pitch for the drain plug in the 5 speed. I heard from somewhere it's an M16x1.5, but I am not sure.

Also does anyone know how sealed the 5mt needs to be? If I put the drain back from the cooler into the rear cap plate in the trans, do I need to use a bulkhead connector? Or will just drooping the hose into it with a grommet suffice?

[cob427sc]

I don't know about the thread size but I would go with a fitting connection on the return line. Even though it would be low pressure you do have fluid flowing through that line which could cause the line to disconnect or allow leakage back out of the transmission.

[Dave 53]

The plug is M20.

What are you doing with the car that it needs a tranny cooler? I was talking to my engine tuner about tranny coolers. He said even on their hard-core Subaru rally cars, they didn't use a cooler, but they did "go up a weight" with the fluid. Not to say there aren't certain applications that a tranny cooler would be in order.

My muffler is just to the right of my tranny. I forget the exact numbers, but I measured the outside tranny case temp at about 120 on the left non-muffler side and 220 on the right muffler side. 100 degree difference! That radiant muffler heat had to be heating up the tranny fluid. After wrapping the muffler, the muffler right side is now only about 20 degrees hotter than the left non-muffler side.171276

[Sgt.Gator]

M26 x 1.5. I drilled my oem plug out and welded in a steel AN 90. For the return into the tranny I put it back in the speedo port. I use Legacy trannys so that's no problem because there's no speedo gear inside them. My 818R has the return back into the top above 5th gear now which I had installed when the tranny was rebuilt at ZF Design.:
171278..171279..171280

[Dave 53]

M26 x 1.5. I drilled my oem plug out and welded in a steel AN 90. For the return into the tranny I put it back in the speedo port. I use Legacy trannys so that's no problem because there's no speedo gear inside them. My 818R has the return back into the top above 5th gear now which I had installed when the tranny was rebuilt at ZF Design.:
171278..171279..171280

That's interesting about the M26 plug. I used my OEM plug on my Killer B pan and that pan is stamped M20. Why the difference?

Next day edit: I just answered my own question - We were talking about the tranny plug and I was thinking oil pan plug. My bad. Always take the wisdom of Sgt.Gator over mine! I sure do.

[Neeraj Ahluwalia]

M26 x 1.5. I drilled my oem plug out and welded in a steel AN 90. For the return into the tranny I put it back in the speedo port. I use Legacy trannys so that's no problem because there's no speedo gear inside them. My 818R has the return back into the top above 5th gear now which I had installed when the tranny was rebuilt at ZF Design.:
171278..171279..171280

Awesome, thanks for the information. That's a big plug - I might have to do something similar to you and modify the existing bolt. I could only find an M26 to AN12 adapter - (im running AN10 all around).

I decided to do the transmission cooler as I've heard that people have elevated temps on the track. I wanted to do the best I could to keep my 5MT lasting long. I was also planning on adding some braces to the outside of the case to help stiffen things up, and help prevent the gearsets from separating under load. Maybe it's a little unnecessary, but I've been scared by all the horror stories of the subaru 5MT.