It is a 17 mm bolt head on my 91 Civic LX manual transmission.

Also, I have a vague recollection that the last time I changed the transmission
oil I couldn't get a socket and ratchet in there and so had to go out and buy a
17 mm open end wrench.

If possible, bring both a combination (open end and box) 17 mm wrench and 17mm
socket+ratchet to the job.

"Chopface" <thename_is_slick@yahoo.com> wrote
> Hi,
>
> Can anyone remember what size the fill bolt is on 88-91 Civics? I think
> its right around 17mm, but I want to know for sure because I am away
> from my tools and want to just buy/borrow a wrench for only that bolt.
> I've got a transmission leaking due to a new driveshaft not fitting
> correctly in my transmission and I want to top it off before I put
> another ~120 miles on it on my way to where I work on the car.