1. Ive got a 2003 3500 dodge cummins.
2. im trying to fill the oil in the manual transmission but dont know what the hex bolt size is. I measured it at 16 mm is that right?

 

Could be. I don't have one, so don't have any idea. Does your socket fit tightly?

 

i dont know. I dont have a hex bolt that big. Im trying to figure that out so i can go buy it.
Nobody seems to know, not even the dodge dealer. Some guys are filling there tranny threw the shifter boot but that’s wrong. They have a fill hole for a reason but why they choose that for a plug will befuddle me.

 

You can get a hex kit at harbor freight with all big sizes for about $25

 

Its a 17mm

 

That's not that big....... As 95 posted, you can get everything you need at harbor freight.

 

A 17 mm allen key is pretty big.
you guys must be thinking of a common hex socket, in that case yes thats not very big.