I just put new Felpro Vitons on, and found I could press about half on by hand, pressing real hard. Seemed a bit too easy to me. The rest I had to tap on with a plastic mallet & socket. I'm pretty sure the variation is in the heads, not the seals, as a few of the prior Viton's came off easy on the same valves. They were brand new, too, but I had pulled the valves out so I replaced the seals during reassembly.

Is this ok?

 

Mine were stiff enough that you had to tap them to make sure their seated, but you pretty much could just muscle them on there by hand.

 

Good to know, thanks. I did give em a few good taps to be sure.

 

took mine off and replaced them with a GM install tool. They were tough both ways.