What size is the Transmission fill plug?

Hello everyone,

So I got ready to change my ATF in out 2008 Prius and realized that the fill plug has a standard bolt head (not a 10mm allen key head). The drain plug has the 10mm allen key.

When I went through my tools, I didn't have a socket that would fit (largest I have is 19mm).

Can someone...
1)confirm that the FILL plug on a 2008 Prius is NOT a 10mm allen key

2)Tell me what size socket I need for this (Metric I suspect, but maybe SAE?)

Thanks

-Steve

P.S. if the fill plug is a 10mm allen key then I'll keep looking for the fill plug.
P.S.S So my measuring calipers say the bolt head is 24.6mm. When I convert this to inches it is 0.965 inches. My 15/16th socket (0.9375) fits, it is a tiny bit lose but expected. Is this what people use or is there a metric socket that is used.

 

One more thing. If the fill plug is NOT a 10mm hex allen head, could it be replaced with one? Do the GenIII have 10mm hex allen heads on both the drain and the fill? Could I just order a fill plug for a gen III prius and swap?

 

OK so for those who want to know here are the answers provided by the one asking the question

I used Clear Vinly Tubing (home depot) with an O.D. of 7/16" and I.D. of 5/16" to attach to a funnel.
The Fill plug transmission size. 15/16th socket worked to get it off and back on.
Torqued to ~ 29ftlb. This seems pretty light so don't become the HULK.
Added 4 qts fluid from Toyota (ATF/WS) , but some drained out (~ 100-200ml)
-Steve

 

Thanks for the pict. Already got the job done and now have new clean ATF WS in my car. Thanks for the picture again.
Steve