Man am I glad you posted this. At least you got two arms that fit, all three of mine would not fit over the 3/8" shaft. After opening all three up, I realised that one arm was wrong, they had sent 2 of the 1st/2nd arms. It was a lot of work making them fit.
I contacted Hurst, what a pain going through Mr Gasket voice mail hell, and made them aware of the situation. They said they would check into it and send me the correct arm. When the new one came, it didn't fit either!
Next time when I called, I finally got through to the head Hurst guy (if this thread goes any farther I will try to dig up his name for you). Explained in no uncertain terms that Hurst has a problem with this, and exactly what the problem was. He understood the problem, said he would go back to the bin himself and find three correct arms and send them off to me. I also asked him to call or email me to aknowledge the problem. Guess what? Never did get the arms or aknowledgement.
Can you tell I'm still a little PO'ed? I always thought of Hurst as a first rate organisation with excellent products and company service. My faith has been severly shaken.
Sorry for the long reply, I had to get that off my chest.
Pat G.