When mine did this, one of the starter wires had a small hole and was grounding between the frame and longtubes. The fuse would blow as soon as I turn the key to start. I just repaired the wire and attached it at a different angle for clearance. Not sure if you have long tubes, but it's worth checking the wire.

 

yes I'm thinking that's what it is, I removed and tested the starter as good, re installed and re angled the wires to make sure there was no arching and it works fine now. I've only started it twice but it worries me that I never found the real problem. I'd like to have a reliable truck to drive.

 

I run an electrical contractor (commercial) and was googling some fuse information AND THIS thread was the first result! GO PT.NET!!!