Water pump Plug?

[4X4HIGH]

There is the very unlikely chance that it is a 5/16" square. http://buy1.snapon.com/catalog/item...8&group_ID=259&store=snapon-store&dir=catalog

Or even more unlikely that it is an 11/32" square. http://buy1.snapon.com/catalog/item.asp?item_ID=65529&group_ID=259

 

[gone huntin]

It is smaller than 3/8", I took a 3/8" short extension and ground it down to fit.

 

[Mountain_man]

I tried the exact same thing however simply stripped out the bold. It's still there and not coming out until I drill it. Sucks I say.

 

[Mike D]

Maybe it is a 3/8" allen head plug? Take a mirror and a flash light and check to see if it is a square or hexagonal slot.